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Paleontology investigates the whole history of life on Earth
Paleontologist at work, John Day Fossil Beds National Monument.
Preparation of the fossilized bones of Europasaurus holgeri

Paleontology (British: palaeontology)[derivation 1] is the study of prehistoric life, including organisms' evolution and interactions with each other and their environments (their paleoecology). As a "historical science" it tries to explain causes rather than conduct experiments to observe effects. Paleontological observations have been documented as far back as the 5th century BC. The science became established in the 18th century as a result of Georges Cuvier's work on comparative anatomy, and developed rapidly in the 19th century. Fossils found in China since the 1990s have provided new information about the earliest evolution of animals, early fish, dinosaurs and the evolution of birds and mammals. Paleontology lies on the border between biology and geology, and shares with archaeology a border that is difficult to define. It now uses techniques drawn from a wide range of sciences, including biochemistry, mathematics and engineering. As knowledge has increased, paleontology has developed specialized subdivisions, some of which focus on different types of fossil organisms while others study ecological and environmental history, such as ancient climates.

Body fossils and trace fossils are the principal types of evidence about ancient life, and geochemical evidence has helped to decipher the evolution of life before there were organisms large enough to leave fossils. Estimating the dates of these remains is essential but difficult: sometimes adjacent rock layers allow radiometric dating, which provides absolute dates that are accurate to within 0.5%, but more often paleontologists have to rely on relative dating by solving the "jigsaw puzzles" of biostratigraphy. Classifying ancient organisms is also difficult, as many do not fit well into the Linnean taxonomy that is commonly used for classifying living organisms, and paleontologists more often use cladistics to draw up evolutionary "family trees". The final quarter of the 20th century saw the development of molecular phylogenetics, which investigates how closely organisms are related by measuring how similar the DNA is in their genomes. Molecular phylogenetics has also been used to estimate the dates when species diverged, but there is controversy about the reliability of the molecular clock on which such estimates depend.

Use of all these techniques has enabled paleontologists to discover much of the evolutionary history of life, almost all the way back to when Earth became capable of supporting life, about 3,800 million years ago. For about half of that time the only life was single-celled micro-organisms, mostly in microbial mats that formed ecosystems only a few millimeters thick. Earth's atmosphere originally contained virtually no oxygen, and its oxygenation began about 2,400 million years ago. This may have caused an accelerating increase in the diversity and complexity of life, and early multicellular plants and fungi have been found in rocks dated from 1,700 to 1,200 million years ago. The earliest multicellular animal fossils are much later, from about 580 million years ago, but animals diversified very rapidly and there is a lively debate about whether most of this happened in a relatively short Cambrian explosion or started earlier but has been hidden by lack of fossils. All of these organisms lived in water, but plants and invertebrates started colonizing land from about 490 million years ago and vertebrates followed them about 370 million years ago. The first dinosaurs appeared about 230 million years ago and birds evolved from one dinosaur group about 150 million years ago. During the time of the dinosaurs, mammals' ancestors survived only as small, mainly nocturnal insectivores, but after the non-avian dinosaurs became extinct in the Cretaceous–Tertiary extinction event 65 million years ago mammals diversified rapidly. Flowering plants appeared and rapidly diversified between 130 million years ago and 90 million years ago, possibly helped by coevolution with pollinating insects. Social insects appeared around the same time and, although they have relatively few species, now form over 50% of the total mass of all insects. The upright-walking common ancestor of humans and chimpanzees Sahelanthropus tchadensis appeared around 6 to 7 million years ago, and anatomically modern humans appeared under 200,000 years ago. The course of evolution has been changed several times by mass extinctions that wiped out previously dominant groups and allowed other to rise from obscurity to become major components of ecosystems.

Contents

Definition

A paleontologist carefully chips rock from a column of dinosaur vertebrae.

The simplest definition is "the study of ancient life".[1] Paleontology seeks information about several aspects of past organisms: "their identity and origin, their environment and evolution, and what they can tell us about the Earth's organic and inorganic past".[2]

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A historical science

Paleontology is one of the historical sciences, along with archaeology, geology, biology, astronomy, cosmogony, philology and history itself.[3] This means that it aims to describe phenomena of the past and reconstruct their causes.[4] Hence it has three main elements: description of the phenomena; developing a general theory about the causes of various types of change; and applying those theories to specific facts.[3]

When trying to explain past phenomena, paleontologists and other historical scientists often construct a set of hypotheses about the causes and then look for a "smoking gun", a piece of evidence which indicates that one of the hypotheses is a better explanation than the others. Sometimes the "smoking gun" is discovered by a fortunate accident during other research, for example the discovery by Luis Alvarez and Walter Alvarez of an iridium-rich layer at the CretaceousTertiary boundary made asteroid impact and volcanism the most favored explanations for the Cretaceous–Tertiary extinction event.[4]

The other main type of science is experimental science, which is often said to work by conducting experiments to disprove hypotheses about the workings and causes of natural phenomena – note that this approach cannot prove a hypothesis is correct, since some later experiment may disprove it. However, when confronted with totally unexpected phenomena, such as the first evidence for invisible radiation, experimental scientists often use the same approach as historical scientists: construct a set of hypotheses about the causes and then look for a "smoking gun".[4]

Related sciences

Paleontology lies on the boundary between biology and geology since paleontology focuses on the record of past life but its main source of evidence is fossils, which are found in rocks.[5] For historical reasons paleontology is part of the geology departments of many universities, because in the 19th and early 20th centuries geology departments found paleontological evidence important for estimating the ages of rocks while biology departments showed little interest.[6]

Paleontology also has some overlap with archaeology, which primarily works with objects made by humans and with human remains, while paleontologists are interested in the characteristics and evolution of humans as organisms. When dealing with evidence about humans, archaeologists and paleontologists may work together – for example paleontologists might identify animal or plant fossils around an archaeological site, to discover what the people who lived there ate; or they might analyze the climate at the time when the site was inhabited by humans.[7]

Analyses using engineering techniques show that Tyrannosaurus had a devastating bite, but raise doubts about how fast it could move.

In addition paleontology often uses techniques derived from other sciences, including biology, ecology, chemistry, physics and mathematics.[1] For example geochemical signatures from rocks may help to discover when life first arose on Earth,[8] and analyses of carbon isotope ratios may help to identify climate changes and even to explain major transitions such as the Permian–Triassic extinction event.[9] A relatively recent discipline, molecular phylogenetics, often helps by using comparisons of different modern organisms' DNA and RNA to re-construct evolutionary "family trees"; it has also been used to estimate the dates of important evolutionary developments, although this approach is controversial because of doubts about the reliability of the "molecular clock".[10] Techniques developed in engineering have been used to analyse how ancient organisms might have worked, for example how fast Tyrannosaurus could move and how powerful its bite was.[11][12]

Paleontology even contributes to astrobiology, the investigation of possible life on other planets, by developing models of how life may have arisen and by providing techniques for detecting evidence of life.[13]

Subdivisions

As knowledge has increased, paleontology has developed specialised subdivisons.[14] Vertebrate paleontology concentrates on fossils of vertebrates, from the earliest fish to the immediate ancestors of modern mammals. Invertebrate paleontology deals with fossils of invertebrates such as molluscs, arthropods, annelid worms and echinoderms. Paleobotany focuses on the study of fossil plants, but traditionally includes the study of fossil algae and fungi. Palynology, the study of pollen and spores produced by land plants and protists, straddles the border between paleontology and botany, as it deals with both living and fossil organisms. Micropaleontology deals with all microscopic fossil organisms, regardless of the group to which they belong.[15]

In the Carboniferous period, the continents were not in the same places as they are today, and there was extensive glaciation.

Instead of focusing on individual organisms, paleoecology examines the interactions between different organisms, such as their places in food chains, and the two-way interaction between organisms and their environment[16] – for example the development of oxygenic photosynthesis by bacteria hugely increased the productivity and diversity of ecosystems,[17] and also caused the oxygenation of the atmosphere, which in turn was a prerequisite for the evolution of the most complex eucaryotic cells, from which all multicellular organisms are built.[18] Paleoclimatology, although sometimes treated as part of paleoecology,[15] focuses more on the history of Earth's climate and the mechanisms which have changed it[19] – which have sometimes included evolutionary developments, for example the rapid expansion of land plants in the Devonian period removed more carbon dioxide from the atmosphere, reducing the greenhouse effect and thus helping to cause an ice age in the Carboniferous period.[20]

Biostratigraphy, the use of fossils to work out the chronological order in which rocks were formed, is useful to both paleontologists and geologists.[21] Biogeography studies the spatial distribution of organisms, and is also linked to geology, which explains how Earth's geography has changed over time.[22]

Sources of evidence

Body fossils

This Marrella specimen illustrates how clear and detailed the fossils from the Burgess Shale lagerstätte are.

Fossils of organisms' bodies are usually the most informative type of evidence. The most common types are wood, bones, and shells.[23] Fossilisation is a rare event, and most fossils are destroyed by erosion or metamorphism before they can be observed. Hence the fossil record is very incomplete, increasingly so further back in time. Despite this, it is often adequate to illustrate the broader patterns of life's history.[24] There are also biases in the fossil record: different environments are more favorable to the preservation of different types of organism or parts of organisms.[25] Further, only the parts of organisms that were already mineralised are usually preserved, such as the shells of molluscs. Since most animal species are soft-bodied, they decay before they can become fossilised. As a result, although there are 30-plus phyla of living animals, two-thirds have never been found as fossils.[26]

Occasionally, unusual environments may preserve soft tissues. These lagerstätten allow paleontologists to examine the internal anatomy of animals that in other sediments are represented only by shells, spines, claws, etc – if they are preserved at all. However, even lagerstätten present an incomplete picture of life at the time. The majority of organisms living at the time are probably not represented because lagerstätten are restricted to a narrow range of environments, e.g. where soft-bodied organisms can be preserved very quickly by events such as mudslides; and the exceptional events that cause quick burial make it difficult to study the normal environments of the animals.[27] The sparseness of the fossil record means that organisms are expected to exist long before and after they are found in the fossil record – this is known as the Signor-Lipps effect.[28]

Trace fossils

Trace fossils consist mainly of tracks and burrows, but also include coprolites (fossil feces) and marks left by feeding.[23][29] Trace fossils are particularly significant because they represent a data source that is not limited to animals with easily-fossilized hard parts, and which reflects organisms' behaviour. Also many traces date from significantly earlier than the body fossils of animals that are thought to have been capable of making them.[30] Whilst exact assignment of trace fossils to their makers is generally impossible, traces may for example provide the earliest physical evidence of the appearance of moderately complex animals (comparable to earthworms).[29]

Geochemical observations

Geochemical observations may help to deduce the global level of biological activity, or the affinity of a certain fossil. For example geochemical features of rocks may reveal when life first arose on Earth,[8] and may provide evidence of the presence of eucaryotic cells, the type from which all multicellular organisms are built.[31] Analyses of carbon isotope ratios may help to explain major transitions such as the Permian–Triassic extinction event.[9]

Classifying ancient organisms

Tetrapods

Amphibians


Amniotes
Synapsids

Extinct Synapsids


   

Mammals



Reptiles

Extinct reptiles



Lizards and snakes


Archosaurs
 ? 

Extinct
Archosaurs



Crocodilians


Dinosaurs
 ? 

Extinct
Dinosaurs



 ? 

Birds







Simple example cladogram.
    Warm-bloodedness evolved somewhere in the
synapsid–mammal transition.
 ?  Warm-bloodedness must also have evolved at one of
these points – an example of convergent evolution.[32]
Levels in the Linnean taxonomy.

Naming groups of organisms in a way that is clear and widely agreed is important, as some disputes in palaeontology have been based just on misunderstandings over names.[33] Linnean taxonomy is commonly used for classifying living organisms, but runs into difficulties when dealing with newly-discovered organisms that are significantly different from known ones. For example: it is hard to decide at what level to place a new higher-level grouping, e.g. genus or family or order; this is important since the Linnean rules for naming groups are tied to their levels, and hence if a group is moved to a different level it has to be renamed.[34]

Paleontologists generally use approaches based on cladistics, a technique for working out the evolutionary "family tree" of a set of organisms.[33] It works by the logic that, if groups B and C have more similarities to each other than either has to group A, then B and C are more closely related to each other than either is to A. Characters that are compared may be anatomical, such as the presence of a notochord, or molecular, by comparing sequences of DNA or proteins. The result of a successful analysis is a hierarchy of clades – groups that share a common ancestor. Ideally the "family tree" has only two branches leading from each node ("junction"), but sometimes there is too little information to achieve this and paleontologists have to make do with junctions that have several branches. The cladistic technique is sometimes fallible, as some features, such as wings or camera eyes, evolved more than once, convergently – this must be taken into account in analyses.[32]

Evolutionary developmental biology, commonly abbreviated to "Evo Devo", also helps paleontologists to produce "family trees". For example the embryological development of some modern brachiopods suggests that brachiopods may be descendants of the halkieriids, which became extinct in the Cambrian period.[35]

Estimating the dates of organisms

Pecten gibbus
Calyptraphorus
velatus
Scaphites
hippocrepis
Perisphinctes
tiziani
Trophites
subbullatus
Leptodus
americanus
Cactocrinus
multibrachiatus
Dictyoclostus
americanus
Mucrospinifer
mucronatus
Cystiphyllum
niagarense
Bathyurus extans
Paradoxides pinus
Neptunea tabulata
Venericardia
planicosta
Inoceramus
labiatus
Nerinea trinodosa
Monotis
subcircularis
Parafusilina
bosei
Lophophyllidium
proliferum
Prolecanites gurleyi
Palmatolepus
unicornis
Hexamocaras hertzeri
Tetragraptus fructicosus
Billingsella corrugata
Common index fossils used to date rocks in North-East USA.

Paleontology seeks to map out how living things have changed through time. A substantial hurdle to this aim is the difficulty of working out how old fossils are. Beds which preserve fossils typically lack the radioactive elements needed for radiometric dating. This technique is our only means of giving rocks greater than about 50 million years old an absolute age, and can be accurate to within 0.5% or better.[36] Although radiometric dating requires very careful laboratory work, its basic principle is simple: the rates at which various radioactive elements decay are known, and so the ratio of the radioactive element to the element into which it decays shows how long ago the radioactive element was incorporated into the rock. Radioactive elements are common only in rocks with a volcanic origin, and so the only fossil-bearing rocks that can be dated radiometrically are a few volcanic ash layers.[36]

Consequently, paleontologists must usually rely on stratigraphy to date fossils. Stratigraphy is the science of deciphering the "layer-cake" that is the sedimentary record, and has been compared to a jigsaw puzzle.[37] Rocks normally form relatively horizontal layers, with each layer younger than the one underneath it. If a fossil is found between two layers whose ages are known, the fossil's age must lie between the two known ages.[38] Because rock sequences are not continuous, but may be broken up by faults or periods of erosion, it is very difficult to match up rock beds that are not directly next to one another. However, fossils of species that survived for a relatively short time can be used to link up isolated rocks: this technique is called biostratigraphy. For instance, the conodont Eoplacognathus pseudoplanus has a short range in the Middle Ordovician period.[39] If rocks of unknown age are found to have traces of E. pseudoplanus, they must have a mid-Ordovician age. Such index fossils must be distinctive, be globally distributed and have a short time range to be useful. However, misleading results are produced if the index fossils turn out to have longer fossil ranges than first thought.[40] Stratigraphy and biostratigraphy can in general provide only relative dating (A was before B), which is often sufficient for studying evolution. However, this is difficult for some time periods, because of the problems involved in matching up rocks of the same age across different continents.[41]

Family-tree relationships may also help to narrow down the date when lineages first appeared. For instance, if fossils of B or C date to X million years ago and the calculated "family tree" says A was an ancestor of B and C, then A must have evolved more than X million years ago.

It is also possible to estimate how long ago two living clades diverged – i.e. approximately how long ago their last common ancestor must have lived  – by assuming that DNA mutations accumulate at a constant rate. These "molecular clocks", however, are fallible, and provide only a very approximate timing: for example, they are not sufficiently precise and reliable for estimating when the groups that feature in the Cambrian explosion first evolved,[42] and estimates produced by different techniques may vary by a factor of two.[10]

Overview of the history of life

The evolutionary history of life stretches back to over 3,000 million years ago, possibly as far as 3,800 million years ago. Earth formed about 4,540 million years ago and, after a collision that formed the Moon about 40 million years later, may have cooled quickly enough to have oceans and an atmosphere about 4,440 million years ago.[43] However there is evidence on the Moon of a Late Heavy Bombardment from 4,000 to 3,800 million years ago. If, as seem likely, such a bombardment struck Earth at the same time, the first atmosphere and oceans may have been stripped away.[44] The oldest undisputed evidence of life on Earth dates to 3,000 million years ago, although there have been reports, often disputed, of fossil bacteria from 3,400 million years ago and of geochemical evidence for the presence of life 3,800 million years ago.[8][45] Even the simplest modern organisms are too complex to have emerged directly from non-living materials.[46] Some scientists have proposed that life on Earth was "seeded" from elsewhere,[47] but most research concentrates on various explanations of how life could have arisen independently on Earth.[48]

This wrinkled "elephant skin" texture is a trace fossil of a non-stromatolite microbial mat.
The image shows the location, in the Burgsvik beds of Sweden, where the texture was first identified as evidence of a microbial mat.[49]

For about 2,000 million years microbial mats, multi-layered colonies of different types of bacteria, were the dominant life on Earth.[50] The evolution of oxygenic photosynthesis enabled them to play the major role in the oxygenation of the atmosphere[51] from about 2,400 million years ago. This change in the atmosphere increased their effectiveness as nurseries of evolution.[52] While eukaryotes, cells with complex internal structures, may have been present earlier, their evolution speeded up when they acquired the ability to transform oxygen from a poison to a powerful source of energy in their metabolism. This innovation may have come from primitive eukaryotes capturing oxygen-powered bacteria as endosymbionts and transforming them into organelles called mitochondria.[53] The earliest evidence of complex eukaryotes with organelles such as mitochondria, dates from 1,850 million years ago.[18]

Multicellular life is composed only of eukaryotic cells, and the earliest evidence for it is from 1,700 million years ago, although specialization of cells for different functions first appears between 1,430 million years ago (a possible fungus) and 1,200 million years ago (a probable red alga). Sexual reproduction may be a prerequisite for specialization of cells, as an asexual multicellular organism might be at risk of being taken over by rogue cells that retain the ability to reproduce.[54][55]

Opabinia made the largest single contribution to modern interest in the Cambrian explosion.

The earliest known animals are cnidarians from about 580 million years ago, but these are so modern-looking that the earliest animals must have appeared before then.[56] Early fossils of animals are rare because they did not develop mineralized hard parts that fossilize easily until about 548 million years ago.[57] The earliest modern-looking bilaterian animals appear in the Early Cambrian, along with several "weird wonders" that bear little obvious resemblance to any modern animals. There is a long-running debate about whether this Cambrian explosion was truly a very rapid period of evolutionary experimentation; alternative views are that modern-looking animals began evolving earlier but fossils of their precursors have not yet been found, or that the "weird wonders" are evolutionary "aunts" and "cousins" of modern groups.[58] Vertebrates remained an obscure group until the first fish with jaws appeared in the Late Ordovician.[59][60]

The spread of life from water to land required organisms to solve several problems, including protection against drying out and supporting themselves against gravity.[61][62] The earliest evidence of land plants and land invertebrates date back to about 476 million years ago and 490 million years ago respectively.[62][63] The lineage that produced land vertebrates evolved later but very rapidly between 370 million years ago and 360 million years ago;[64] recent discoveries have overturned earlier ideas about the history and driving forces behind their evolution.[65] Land plants were so successful that they caused an ecological crisis in the Late Devonian, until the evolution and spread of fungi that could digest dead wood.[20]

The Early Cretaceous Yanoconodon was only about 13 centimetres (5.1 in) long, but was longer than the average mammal of its time.[66]
Birds are the last surviving dinosaurs.[67]

During the Permian period synapsids, including the ancestors of mammals, may have dominated land environments,[68] but the Permian–Triassic extinction event 251 million years ago came very close to wiping out complex life.[69] During the slow recovery from this catastrophe a previously obscure group, archosaurs, became the most abundant and diverse terrestrial vertebrates. One archosaur group, the dinosaurs, were the dominant land vertebrates for the rest of the Mesozoic,[70] and birds evolved from one group of dinosaurs.[67] During this time mammals' ancestors survived only as small, mainly nocturnal insectivores, but this apparent set-back may have accelerated the development of mammalian traits such as endothermy and hair.[71] After the Cretaceous–Tertiary extinction event 65 million years ago killed off the non-avian dinosaurs – birds are the only surviving dinosaurs – mammals increased rapidly in size and diversity, and some took to the air and the sea.[72][73][74]

A modern social insect collects pollen from a modern flowering plant.

Fossil evidence indicates that flowering plants appeared and rapidly diversified in the Early Cretaceous, between 130 million years ago and 90 million years ago.[75] Their rapid rise to dominance of terrestrial ecosystems is thought to have been propelled by coevolution with pollinating insects.[76] Social insects appeared around the same time and, although they account for only small parts of the insect "family tree", now form over 50% of the total mass of all insects.[77]

Humans evolved from a lineage of upright-walking apes whose earliest fossils date from over 6 million years ago.[78] Although early members of this lineage had chimp-sized brains, about 25% as big as modern humans', there are signs of a steady increase in brain size after about 3 million years ago.[79] There is a long-running debate about whether modern humans are descendants of a single small population in Africa, which then migrated all over the world less than 200,000 years ago and replaced previous hominine species, or arose worldwide at the same time as a result of interbreeding.[80]

Mass extinctions

Cambrian Ordovician Silurian Devonian Carboniferous Permian Triassic Jurassic Cretaceous Paleogene Neogene
Millions of years ago
Cambrian Ordovician Silurian Devonian Carboniferous Permian Triassic Jurassic Cretaceous Paleogene Neogene
Apparent extinction intensity, i.e. the fraction of genera going extinct at any given time, as reconstructed from the fossil record. (Graph not meant to include recent epoch of Holocene extinction event)

Life on earth has suffered occasional mass extinctions at least since 542 million years ago. Although they are disasters at the time, mass extinctions have sometimes accelerated the evolution of life on earth. When dominance of particular ecological niches passes from one group of organisms to another, it is rarely because the new dominant group is "superior" to the old and usually because an extinction event eliminates the old dominant group and makes way for the new one.[81][82]

The fossil record appears to show that the rate of extinction is slowing down, with both the gaps between mass extinctions becoming longer and the average and background rates of extinction decreasing. However, it is not certain whether the actual rate of extinction has altered, since both of these observations could be explained in several ways:[83]

  • The oceans may have become more hospitable to life over the last 500 million years and less vulnerable to mass extinctions: dissolved oxygen became more widespread and penetrated to greater depths; the development of life on land reduced the run-off of nutrients and hence the risk of eutrophication and anoxic events; marine ecosystems became more diversified so that food chains were less likely to be disrupted.[84][85]
  • Reasonably complete fossils are very rare, most extinct organisms are represented only by partial fossils, and complete fossils are rarest in the oldest rocks. So paleontologists have mistakenly assigned parts of the same organism to different genera which were often defined solely to accommodate these finds – the story of Anomalocaris is an example of this.[86] The risk of this mistake is higher for older fossils because these are often unlike parts of any living organism. Many of the "superfluous" genera are represented by fragments which are not found again and the "superfluous" genera appear to become extinct very quickly.[83]
All genera
"Well-defined" genera
Trend line
"Big Five" mass extinctions
Other mass extinctions
Million years ago
Thousands of genera
Phanerozoic biodiversity as shown by the fossil record

Biodiversity in the fossil record, which is

"the number of distinct genera alive at any given time; that is, those whose first occurrence predates and whose last occurrence postdates that time"[87]

shows a different trend: a fairly swift rise from 542 to 400 million years ago, a slight decline from 400 to 200 million years ago, in which the devastating Permian–Triassic extinction event is an important factor, and a swift rise from 200 million years ago to the present.[87]

This illustration of an Indian elephant jaw and a mammoth jaw (top) is from Cuvier's 1796 paper on living and fossil elephants.

History of paleontology

Although paleontology became established around 1800, earlier thinkers had noticed aspects of the fossil record. The ancient Greek philosopher Xenophanes (570–480 BC) concluded from fossil sea shells that some areas of land were once under water.[88] During the Middle Ages the Persian naturalist Ibn Sina, known as Avicenna in Europe, discussed fossils and proposed a theory of petrifying fluids on which Albert of Saxony elaborated in the 14th century.[89] The Chinese naturalist Shen Kuo (1031–1095) proposed a theory of climate change based on the presence of petrified bamboo in regions that in his time were too dry for bamboo.[90]

In early modern Europe, the systematic study of fossils emerged as an integral part of the changes in natural philosophy that occurred during the Age of Reason. At the end of the 18th century Georges Cuvier's work established comparative anatomy as a scientific discipline and, by proving that some fossil animals resembled no living ones, demonstrated that animals could become extinct, leading to the emergence of paleontology.[91] The expanding knowledge of the fossil record also played an increasing role in the development of geology, particularly stratigraphy.[92]

The first half of the 19th century saw geological and paleontological activity become increasingly well organized with the growth of geologic societies and museums[93][94] and an increasing number of professional geologists and fossil specialists. Interest increased for reasons that were not purely scientific, as geology and paleontology helped industrialists to find and exploit natural resources such as coal.[95]

This contributed to a rapid increase in knowledge about the history of life on Earth and to progress in the definition of the geologic time scale, largely based on fossil evidence. In 1822 Henri Marie Ducrotay de Blanville, editor of Journal de Phisique, coined the word "paleontology" to refer to the study of ancient living organisms through fossils.[96] As knowledge of life's history continued to improve, it became increasingly obvious that there had been some kind of successive order to the development of life. This encouraged early evolutionary theories on the transmutation of species.[97] After Charles Darwin published Origin of Species in 1859, much of the focus of paleontology shifted to understanding evolutionary paths, including human evolution, and evolutionary theory.[97]

Haikouichthys, from about 518 million years ago in China, may be the earliest known fish.[98]

The last half of the 19th century saw a tremendous expansion in paleontological activity, especially in North America.[99] The trend continued in the 20th century with additional regions of the Earth being opened to systematic fossil collection. Fossils found in China near the end of the 20th century have been particularly important as they have provided new information about the earliest evolution of animals, early fish, dinosaurs and the evolution of birds.[100] The last few decades of the 20th century saw a renewed interest in mass extinctions and their role in the evolution of life on Earth.[101] There was also a renewed interest in the Cambrian explosion that apparently saw the development of the body plans of most animal phyla. The discovery of fossils of the Ediacaran biota and developments in paleobiology extended knowledge about the history of life back far before the Cambrian.[58]

Increasing awareness of Gregor Mendel's pioneering work in genetics led first to the development of population genetics and then in the mid-20th century to the modern evolutionary synthesis, which explains evolution as the outcome of events such as mutations and horizontal gene transfer which provide genetic variation, with genetic drift and natural selection driving changes in this variation over time.[102] Within the next few years the role and operation of DNA in genetic inheritance were discovered, leading to what is now known as the "Central Dogma" of molecular biology.[103] In the 1960s molecular phylogenetics, the investigation of evolutionary "family trees" by techniques derived from biochemistry, began to make an impact, particularly when it was proposed that the human lineage had diverged from apes much more recently than was generally thought at the time.[104] Although this early study compared proteins from apes and humans, most molecular phylogenetics research is now based on comparisons of RNA and DNA.[105]

See also

Notes

  1. ^ from Greek: παλαιός (palaeos) "old, ancient", ὄν, ὀντ- (on, ont-) "being, creature", and λόγος (logos) "speech, thought"

References

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  2. ^ Laporte, L.F. (October 1988). "What, after All, Is Paleontology?". PALAIOS 3 (5): 453. doi:10.2307/3514718. http://www.jstor.org/pss/3514718. Retrieved September 17, 2008. 
  3. ^ a b Laudan, R. (1992). "What's so Special about the Past?". in Nitecki, M.H., and Nitecki, D.V.. History and Evolution. SUNY Press. p. 58. ISBN 0791412113. http://books.google.co.uk/books?hl=en&lr=&id=kyLRtsvLS2AC&oi=fnd&pg=PA55&dq=%22What%27s+so+Special+about+the+Past%22+laudan&ots=JlEDjDS50V&sig=IGw0RZCNeiEZ7cT3xlDX8Cgs8V0#v=onepage&q=%22What%27s%20so%20Special%20about%20the%20Past%22%20laudan&f=false. Retrieved 7 Feb 2010. 
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1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

PALAEONTOLOGY (Gr. iraAat6, ancient, neut. pl. i vra, beings, and Aayla, discourse, science), the science of extinct forms of life. Like many other natural sciences, this study dawned among the Greeks. It was retarded and took false directions until the revival of learning in Italy. It became established as a distinct branch in the beginning of the 19th century, and somewhat later received the appellation " palaeontology," which was given independently by De Blainville and by Fischer von Waldheim about 1834. In recent years the science of vegetable palaeontology has been given the distinct name of Palaeobotany, so that " palaeontology e' among biologists mainly refers to zoology; but historically the two cannot be disconnected.

Palaeontology both borrows from and sheds light upon geology and other branches of the physical history of the earth, each of which, such as palaeogeography or palaeometeorology, is the more fascinating because of the large element of the unknown, the need for constructive imagination, the appeal to other branches of biological and physical investigation for supplementary evidence, and the necessity of constant comparison with the present aspects of nature. The task of the palaeontologist thus begins with the appearance of life on the globe, and ends in close relation to the studies of the archaeologist and historian as well as of the zoologist and botanist. That wealth of evidence which the zoologist enjoys, including environment in all its aspects and anatomy in its perfection of organs and tissues, the palaeontologist finds partially or wholly destroyed, and his highest art is that of complete restoration of both the past forms and past environments of life (see Plates I. and II.; figs. 1, 2, 3, 4, 5). The degree of accuracy in such anatomical and physiographic restorations from relatively imperfect evidence will always represent the state of the science and the degree of its approach toward being exact or complete. Progress in the science also depends upon the pursuit of palaeontology as zoology and not as geology, because it was a mere accident of birth which connected palaeontology so closely with geology.

In order to illustrate the grateful services which palaeontology through restoration may render to the related earth sciences let us imagine a vast continent of the past wholly unknown in its physical features, elevation, climate, configuration, but richly represented by fossil remains. All the fossil plants and animals of every kind are brought from this continent into a great museum; the latitude, longitude and relative elevation of each specimen are precisely recorded; a corps of investigators, having the most exact and thorough training in zoology and botany, and gifted with imagination, will soon begin to restore the geographic and physiographic outlines of the continent, its fresh, brackish and salt-water confines, its seas, rivers and lakes, its forests, uplands, plains, meadows and swamps, also to a certain extent the cosmic relations of this continent, the amount and duration of its sunshine, as well as something of the chemical constitution of its atmosphere and the waters of its rivers and seas; they will trace the progressive changes which took place in the outlines of the continent and its surrounding oceans, following the invasion§ of the land by the sea and the re-emergence of the land and retreatal of the seashore; they will outline the shoals and deeps of its border seas, and trace the barriers which prevented intermingling of the inhabitants of the various provinces of the continent and the surrounding seas. From a study of remains of the mollusca, brachiopoda and other marine organisms they will determine the shallow water (littoral) and deep water (abyssal) regions of the surrounding oceans, and the clear or muddy, salt, brackish or fresh character of its inland and marginal seas; and even the physical conditions of the open sea at the time will be ascertained.

In such manner Johannes Walther (Die Fauna der Solnhofener Platten Kalke Bionomisch betrachtet. Festschrift zum 70ten Geburtstage von Ernst Haeckel, 19(34) has restored the conditions existing in the lagoons and atoll reefs of the Jurassic sea of Solnhofen in Bavaria; he has traced the process of gradual accumulation of the coral mud now constituting the fine lithographic stones in the inter-reef region, and has recognized the periodic laying bare of the mud surfaces thus formed; he has determined the winds which carried the dust particles from the not far distant land and brought the insects from the adjacent Jurassic forests. Finally the presence of the flying lizards (Pterydactylus, Rhamphorhynchus) and the ancient birds (Archaeopteryx) is determined from remains in a most wonderful state of preservation in these ancient deposits.

Still another example of restoration, relating to the surface of a continent, may be cited. It has been discovered that at the beginning of the Eocene the lake of Rilly occupied a vast area east of the present site of Paris; a water-course fell there in cascades, and Munier-Chalinas has reconstructed all the details of that singular locality; plants which loved moist places, such as Marchantia, Asplenium, the covered banks overshadowed by lindens, laurels, magnolias and palms; there also were found the vine and the ivy; mosses (Fontinalis) and Chara sheltered the crayfish (Astacus); insects and even flowers have left their delicate impressions in the travertine which formed the borders of this lake. The Oligocene lake basin of Florissant, Colorado, has been reconstructed similarly by Samuel Hubbard Scudder and T. D. A. Cockerell, including the plants of its shores, the insects which lived upon them, the fluctuations of its level, and many other characteristics of this extinct water body, now in the heart of the arid region of the Rocky Mountains.

Such restorations are possible because of the intimate fitness of animals and plants to their environment, and because such fitness has distinguished certain forms of life from the Cambrian to the present time; the species have altogether changed, but the laws governing the life of certain kinds of organisms have remained exactly the same for the whole period of time assigned to the duration of life; in fact, we read the conditions of the past in a mirror of adaptation, often sadly tarnished and incomplete owing to breaks in the palaeontological record, but constantly becoming more polished by discoveries which increase the understanding of life and its all-pervading relations to the non-life. Therefore adaptation is the central principle of modern palaeontology in its most comprehensive sense.

This conception of the science and its possibilities is the result of very gradual advances since the beginning of the 19th century in what is known as the method of palaeontology. The history of this science, like that of all physical sciences, covers two parallel lines of development which have acted and reacted upon each other - namely, progress in exploration, research and discovery, and progress in philosophic interpretation. Progress in these two lines is by no means uniform; while, for example, palaeontology enjoyed a sudden advance early in the 19th century through the discoveries and researches of Cuvier, guided by his genius as a comparative anatomist, it was checked by his failure as a natural philosopher. The great philosophical impulse was that given by Darwin in 1859 through his demonstration of the theory of descent, which gave tremendous zest to the search for pedigrees (phylogeny) of the existing and extinct types of animal and plant life. In future the philosophic method of palaeontology must continue to advance step by step with exploration; it would be a reproach to later generations if they did not progress as far beyond the philosophic status of Cuvier, Owen and even of Huxley and Cope, as the new materials represent an advance upon the material opportunities which came to them through exploration.

To set forth how best to do our thinking, rather than to follow the triumphs achieved in any particular line of exploration, and to present the point we have now reached in the method or principles of palaeontology, is the chief purpose of this article. The illustrations will be drawn both from vertebrate and invertebrate palaeontology. In the latter branch the author is wholly indebted to Professor Amadeus W. Grabau of Columbia University. The subject will be treated in its biological aspects, because the relations of palaeontology to historical and stratigraphic geology are more appropriately considered under the article Geology. See also, for botany, the article Palaeobotany. We may first trace in outline the history of the birth of palaeontological ideas, from the time of their first adumbration. But for full details reference must be made to the treatises on the history of the science cited in the bibliography at the end of the article.

I.-First Historic Period The scientific recognition of fossils as connected with the past history of the earth, from Aristotle (384-322 B.C.) to the beginning of the 19th century, in connexion with the rise of comparative anatomy and geology. - The dawn of the science covers the first observation of facts and the rudiments of true interpretation. Among the Greeks, Aristotle (384-322 B.C.) Xenophon (430-357 B.C.) and Strabo (63 B.C.-A.D. 24) knew of the existence of fossils and surmised in a crude way their relation to earth history. Similar prophetic views are found among certain Roman FIG, I. - :fin is ,. ,, .,. „,,r (I. quad .,rissus) Ong in the boc; the preserved si,: ,otons of sevc. young, proving that the young of the animal developed within the maternal body and were brought forth alive; i.e. that the ichthyosaur was a viviparous animal. (Specimen presented to the American Museum of Natural History by the Royal Museum of Stuttgart through Professor Eberhard Fraas.) FIG. 2. - A hypothetical pictorial restoration of the mother ichthyosaur accompanied by five of its newly born young, from the information furnished by actual fossils.

(From a drawing by Charles R. Knight made lender the direction of Professor Osborn.) FIG. 3. - One of the most perfect of the many specimens discovered and prepared by Herr Bernard Hauff, and showing the extraordinary preservation of the epidermis of the ichthyosaur, which gives the complete contour of the body in silhouette, the outlines of the paddles, of the remarkably fish-like tail, into the lower lobe of which the vertebral column extends, and the great integumentary dorsal fin.

Table of contents

Materials for the Restoration of Ichthyosaurs.

This plate illustrates the exceptional opportunity afforded the palaeontologist through the remarkably preserved remains of Ichthyosaurs in the quarries of Holzmaden near Stuttgart, Wurttemberg, excavated for many years by Herr Bernard Hauff. (Illustrations reproduced by permission from specimens in the American Museum of Natural History, New York.) XX. 580.

PLATE II.

FIG. 4. - Skeleton Of Allosaurus.

FIG. 5. - Restoration Of Allosaurus.

Materials for the Restoration of D inosaurs. - Carnivorous dinosaur Allosaurus o f the U mal closely related to the Me alosaurus (Allosaurus) pper Jurassic period of North America, an anit type of England. The skeleton (fig. 4) was found nearly complete in the beds of the Morrison formation, Upper Jurassic of central Wyoming, U.S.A. Near it was discovered the posterior portion of the skeleton of a giant herbivorous dinosaur (Brontosaurus Marsh). It was observed that ten of the caudal vertebrae of the latter skeleton bore tooth marks and grooves corresponding exactly with the sharp pointed teeth in the jaw of the carnivorous dinosaur. This proved that the great herbivorous dinosaur had been preyed upon by its smaller carnivorous contemporary. Teeth of the carnivorous dinosaur scattered among the bones of the herbivorous dinosaur completed the line of circumstantial evidence. Upon this testimony the restoration (fig. 5) of the Megalosaur has been drawn by Charles R. Knight under the direction of Professor Osborn.

(Originals reproduced by permission of the American Museum of Natural History.) writers. The pioneers of the science in the 16th and 17th centuries put forth anticipations of some of the well-known modern principles, often followed by recantations, through deference to prevailing religious or traditional beliefs. There were the retarding influences of the Mosaic account of sudden creation, and the belief that fossils represented relics of a universal deluge. There were crude medieval notions that fossils were " freaks " or " sports " of nature (lusus naturae), or that they represented failures of a creative force within the earth (a notion of Greek and Arabic origin), or that larger and smaller fossils represented the remains of races of giants or of pygmies (the mythical idea).

As early as the middle of the 15th century Leonardo da Vinci (1452-1519) recognized in seashells as well as in the teeth of marine fishes proofs of ancient sea-levels on what are now the summits of the Apennines. Successive observers in Italy, notably Fracastoro (1483-1553), Fabio Colonna (1567-1640 or 1650) and Nicolaus Steno (1638 - c. 1687), a Danish anatomist, professor in Padua, advanced the still embryonic science and set forth the principle of comparison of fossil with living forms. Near the end of the 17th century Martin Lister (1638-1712), examining the Mesozoic shell types of England, recognized the great similarity as well as the differences between these and modern species, and insisted on the need of close comparison of fossil and living shells, yet he clung to the old view that fossils were sports of nature. In Italy, where shells of the subApennine formations were discovered in the extensive quarrying for the fortifications of cities, the close similarity between these Tertiary and the modern species soon led to the established recognition of their organic origin. In England Robert Hooke (1635-1703) held to the theory of extinction of fossil forms, and advanced the two most fertile ideas of deriving from fossils a chronology, or series of time intervals in the earth's history, and of primary changes of climate, to account for the former existence of tropical species in England.

The 18th century witnessed the development of these suggestions and the birth of many additional theories. Sir A. Geikie assigns high rank to Jean Etienne Guettard (1715-1786) for his treatises on fossils, although admitting that he had no clear idea of the sequence of formations. The theory of successive formations was soundly developing in the treatises of John Woodward (1665-1728) in England, of Antonio Vallisnieri (1661-1730) in Italy, and of Johann Gottlob Lehmann (d. 1767) in Germany, who distinguished between the primary, or unfossiliferous, and secondary or fossiliferous, formations. The beginnings of palaeogeography followed those of palaeometeorology. The Italian geologist Soldani distinguished (1758) between the fossil fauna of the deep sea and of the shore-lines. In the same year Johann Gesner (1709-1790) set forth the theory of a great period of time, which he estimated at 80,000 years, for the elevation of the shell-bearing levels of the Apennines to their present height above the sea. The brilliant French naturalist Georges Louis Leclerc, comte de Buffon (1707-1788), in Les Epoques de la nature, included in his vast speculations the theory of alternate submergence and emergence of the continents. Abraham Gottlob Werner (1750-1817), the famous exponent of the aqueous theory of earth formation, observed in successive geological formations the gradual approach to the forms of existing species.

II.-Second Historic Period Invertebrate palaeontology founded by Lamarck, vertebrate palaeontology by Cuvier. Palaeontology connected with comparative anatomy by Cuvier. Invertebrate fossils employed for the definite division of all the great periods of time. - Although preevolutionary, this was the heroic period of the science, extending from the close of the 18th century to the publication of Darwin's Origin of Species in 1859. Among the pioneers of this period were the vertebrate zoologists and comparative anatomists Peter Simon Pallas, Pieter Camper and Johann Friedrich Blumenbach. Pallas (1741-1811) in his great journey (1768-1 77 4) through Siberia discovered the vast deposits of extinct mammoths and rhinoceroses. Camper (1722-1789) contrasted (1777) the Pleistocene and recent species of elephants and Blumenbach (1752-1840) separated (1780) the mammoth from the existing species as Elephas primigenius. In 1793 Thomas Pennant (1726-1798) distinguished the American mastodon as Elephas americanus. Political troubles and the dominating influence of Werner's speculations checked palaeontology in Germany, while under the leadership of Lamarck and Cuvier France came to the fore. J. B. Lamarck (1744-1829) was the founder of invertebrate palaeontology. The treatise which laid the foundation for all subsequent invertebrate palaeontology was his memoir, Sur les fossiles des environs de Paris.. . (1802-1806). Beginning in 1793 he boldly advocated evolution, and further elaborated five great principles--namely, the method of comparison of extinct and existing forms, the broad sequence of formations and succession of epochs, the correlation of geological horizons by means of fossils, the climatic or environmental changes as influencing the development of species, the inheritance of the bodily modifications caused by change of habit and habitat. As a natural philosopher he radically opposed Cuvier and was distinctly a precursor of uniformitarianism, advocating the hypothesis of slow changes and variations, both in living forms and in their environment. His speculations on phylogeny, or the descent of invertebrates and vertebrates, were, however, most fantastic and bore no relation to palaeontological evidence.

It is most interesting to note that William Smith (1769-1839), now known as the " father of historical geology," was born in the same year as Cuvier. Observing for himself (1794-1800) the stratigraphic value of fossils, he began to distinguish the great Mesozoic formations of England (1801). Cuvier (1769-1832) is famous as the founder of vertebrate palaeontology, and with Alexandre Brongniart (1770-1847) as the author of the first exact contribution to stratigraphic geology. Early trained as a comparative anatomist, the discovery of Upper Eocene mammals in the gypsum quarries of Montmartre found him fully prepared (1798), and in 1812 appeared his Recherches sur les ossemens fossiles, brilliantly written and constituting the foundation of the modern study of the extinct vertebrates. Invulnerable in exact anatomical description and comparison, he failed in all his philosophical generalizations, even in those strictly within the domain of anatomy. His famous " law of correlation," which by its apparent brilliancy added enormously to his prestige, is not supported by modern philosophical anatomy, and his services to stratigraphy were diminished by his generalizations as to a succession of sudden extinctions and renovations of life. His joint memoirs with Brongniart, Essai sur la geographie des environs de Paris avec une carte geognostique et des coupes de terrain (1808) and Description geologique des environs de Paris (1835) were based on the wonderful succession of Tertiary faunas in the rocks of the Paris basin. In Cuvier's defence Charles Deperet maintains that the extreme theory of successive extinctions followed by a succession of creations is attributable to Cuvier's followers rather than to the master himself. Deperet points also that we owe to Cuvier the first clear expression of the idea of the increasing organic perfection of all forms of life from the lower to the higher horizons, and that, while he believed that extinctions were due to sudden revolutions on the surface of the earth, he also set forth the pregnant ideas that the renewals of animal life were by migration from other regions unknown, and that these migrations were favoured by alternate elevations and depressions which formed various land routes between great continents and islands. Thus Cuvier, following Buffon, clearly anticipated the modern doctrine of faunal migrations. His reactionary and retarding ideas as a special creationist and his advocacy of the cataclysmic theory of change exerted a baneful influence until overthrown by the uniformitarianism of James Hutton (1726-1797) and Charles Lyell (1797-1875) and the evolutionism of Darwin.

The chief contributions of Cuvier's great philosophical opponent, Etienne Geoffroy St Hilaire (1772-1844), are to be found in his maintenance with Lamarck of the doctrine of the mutability of species. In this connexion he developed his special theory of saltations, or of sudden modifications of structure through changes of environment, especially through the direct influences of temperature and atmosphere. He clearly set forth also the phenomena of analogous or parallel adaptation.

It was Alcide Dessalines d'Orbigny (1802-1857) who pushed to an extreme Cuvier's ideas of the fixity of species and of successive extinctions, and finally developed the wild hypothesis of twenty-seven distinct creations. While these views were current in France, exaggerating and surpassing the thought of Cuvier, they were strongly opposed in Germany by such authors as Ernst Friedrich von Schlotheim (1764-1832) and Heinrich Georg Bronn (1800-1862); and the latter demonstrated that certain species actually pass from one formation to another.

In the meantime the foundations of palaeobotany were being laid (1804) by Ernst Friedrich von Schlotheim (1764-1832), (1811) by Kaspar Maria Sternberg (1761-1838) and (1838) by Theophile Brongniart (1801-1876).

Following Cuvier's Recherches sur les ossemens fossiles, the rich succession of Tertiary mammalian life was gradually revealed to France through the explorations and descriptions of such authors as Croizet, Jobert, de Christol, Eymar, Pomel and Lartet, during a period of rather dry, systematic work, which included, however, the broader generalizations of Henri Marie Ducrotay de Blainville (1778-1850), and culminated in the comprehensive treatises on Tertiary palaeontology of Paul Gervais (1816-1879). Extending the knowledge of the extinct mammals of Germany, the principal contributors were Georg August Goldfuss (1782-1848), Georg Friedrich von Jaegar (1785-1866), Felix F. Plieninger (1807-1873) and Johann Jacob Kaup (1803-1873). As Cuvier founded the palaeontology of mammals and reptiles, so Louis Agassiz's epoch-making works Recherches sur les poissons fossiles (1833-1845) laid the secure foundations of palaeichthyology, and were followed by Christian Heinrich Pander's (1794-1865) classic memoirs on the fossil fishes of Russia. In philosophy Agassiz was distinctly a disciple of Cuvier and supporter of the doctrine of special creation, and to a more limited extent of cataclysmic extinctions. Animals of the next higher order, the amphibians of the coal measures and the Permian, were first comprehensively treated in the masterly memoirs of Christian Erich Hermann von Meyer (1801-1869) beginning in 1829, especially in his Beitrage zur Petrefactenkunde (1829-1830) and his Zur Fauna der Vorwelt (4 vols., 1845-1860). Successive discoveries gradually revealed the world of extinct Reptilia; in 1821 Charles Konig (1784-1851), the first keeper of the mineralogical collection in the British Museum, described Ichthyosaurus from the Jurassic; in the same year William Daniel Conybeare (1787-1857) described Plesiosaurus; and a year later (1822) Mosasaurus; in 1824 William Buckland described the great carnivorous dinosaur Megalosaurus; while Gideon Algernon Mantell (1790--1852) in 1848 announced the discovery of Iguanodon. Some of the fossil Reptilia of France were made known through St Hilaire's researches on the Crocodilia (1831), and those of J. A. Deslongchamps (1794-1867) and his son on the teleosaurs, or longsnouted crocodiles. Materials accumulated far more rapidly, however, than the power of generalization and classification. Able as von Meyer was, his classification of the Reptilia failed because based upon the single adaptive characters of foot structure. The reptiles awaited a great classifier, and such a one appeared in England in the person of Sir Richard Owen (1804-1892), the direct successor of Cuvier and a comparative anatomist of the first rank. Non-committal as regards evolution, he vastly broadened the field of vertebrate palaeontology by his descriptions of the extinct fauna of England, of South America (including especially the great edentates revealed by the voyage of the " Beagle "), of Australia (the ancient and modern marsupials) and of New Zealand (the great struthious birds). His contributions on the Mesozoic reptiles of Great Britain culminated in his complete rearrangement and classification of this group, one of his greatest services to palaeontology. Meanwhile the researches of Hugh Falconer (1808-1865) and of Proby Thomas Cautley (1802-1871) in the sub-Himalayas brought to light the marvellous fauna of the Siwalik hills of India, published in Fauna antiqua Sivalensis (London, 1845) and in the volumes of Falconer's individual researches. The ancient life of the Atlantic border of North America was also becoming known through the work of the pioneer verte.rate palaeontologists Thomas Jefferson (1743-1826), Richard Harlan (1796-1843), Jeffries Wyman (1814-1874) and Joseph Leidy (1823-1891). This was followed by the revelation of the vast ancient life of the western half of the American continent, which was destined to revolutionize the science. The master works of Joseph Leidy began with the first-fruits of western exploration in 1847 and extended through a series of grand memoirs, culminating in 1874. Leidy adhered strictly to Cuvier's exact descriptive methods, and while an evolutionist and recognizing clearly the genetic relationships of the horses and other groups, he never indulged in speculation.

The history of invertebrate palaeontology during the second period is more closely connected with the rise of historic geology and stratigraphy, especially with the settlement of the great and minor time divisions of the earth's history. The pathbreaking works of Lamarck were soon followed by the monumental treatise of Gerard Paul Deshayes (1795-1875) entitled Descriptions des coquilles fossiles des environs de Paris (1824-1837), the first of a series of great contributions by this and other authors. These and other early monographs on the Tertiary shells of the Paris basin, of the environs of Bordeaux, and of the sub-Apennine formations of Italy, brought out the striking distinctness of these faunas from each other and from other molluscan faunas. Recognition of this threefold character led Deshayes to establish a threefold division of the Tertiary based on the percentage of molluscs belonging to types now living found in each. To these divisions Lyell gave in 1833 ,the names Eocene, Miocene and Pliocene.

James Hutton (1726-1797) had set forth (1788) the principle that during all geological time there has been no essential change in the character of events, and that uniformity of law is perfectly consistent with mutability in the results. Lyell marshalled all the observations he could collect in support of this principle, teaching that the present is the key to the past, and arraying all obtainable evidence against the cataclysmic theories of Cuvier. He thus exerted a potent influence on palaeontology through his persistent advocacy of uniformitarianism, a doctrine with which Lamarck should also be credited. As among the vertebrates, materials were accumulating rapidly for the great generalizations which were to follow in the third period. De Blainville added to the knowledge of the shells of the Paris basin; Giovanni Battista Brocchi (1772-1826) in 1814, and Luigi Bellardi (1818-1889) and Giovanni Michelotti (born 1812) in 1840, described the Pliocene molluscs of the subApennine formation of Italy; from Germany and Austria appeared the epoch-making works of Heinrich Ernst Beyrich (1815-1896) and of Moritz Hoernes (1815-1868).

We shall pass over here the labours of Adam Sedgwick (1785-1873) and Sir Roderick Murchison (1792-1871) in the Palaeozoic of England, which because of their close relation to stratigraphy more properly concern geology; but must mention the grand contributions of Joachim Barrande (1799-1883), published in his Systeme silurien du centre de la Boheme, the first volume of which appeared in 1852. While establishing the historic divisions of the Silurian in Bohemia, Barrande also propounded his famous theory of " colonies," by which he attempted to explain the aberrant occurrence of strata containing animals of a more advanced stage among strata containing earlier and more primitive faunas; his assumption was that the second fauna had migrated from an unknown neighbouring region. It is proved that the specific instances on which Barrande's generalizations were founded were due to his misinterpretation of the overturned and faulted strata, but his conception of the simultaneous existence of two faunas, one of more ancient and one of more modern type, and of their alternation in a given area, was based on sound philosophical principles and has been confirmed by more recent work.

The greatest generalization of this second period, however, was that partly prepared for by d'Orbigny, as will be more fully explained later in this article, and clearly expressed by Agassiz - namely, the law of repetition of ancestral stages of life in the course of the successive stages of individual development. This law of recapitulation, subsequently termed the " biogenetic law " by Ernest Haeckel, was the greatest philosophic contribution of this period, and proved to be not only one of the bulwarks of the evolution theory but one of the most important principles in the method of palaeontology.

On the whole, as in the case of vertebrate palaeontology, the pre-Darwinian period of invertebrate palaeontology was one of rather dry systematic description, in which, however, the applications of the science gradually extended to many regions of the world and to all divisions of the kingdom of invertebrates.

III. - Third Historic Period Beginning with the publication of Darwin's great works, " Narrative of the Surveying Voyages of H.M.S. `Adventure' and Beagle' " (1839), and " On the Origin of Species by Means of Natural Selection " (1859). - A review of the two first classic works of Charles Robert Darwin (1809-1882) and of their influence proves that he was the founder of modern palaeontology. Principles of descent and other applications of uniformitarianism which had been struggling for expression in the writings of Lamarck, St Hilaire and de Blainville here found their true interpretation, because the geological succession, the rise, the migrations, the extinctions, were all connected with the grand central idea of evolution from primordial forms.

A close study of the exact modes of evolution and of the philosophy of evolution is the distinguishing feature of this period. It appears from comparison of the work in the two great divisions of vertebrate and invertebrate palaeontology made for the first time in this article that in accuracy of observation and in close philosophical analysis of facts the students of invertebrate palaeontology led the way. This was due to the much greater completeness and abundance of material afforded among invertebrate fossils, and it was manifested in the demonstration of two great principles or laws: first, the law of recapitulation, which is found in its most ideal expression in the shells of invertebrates; second, in the law of direct genetic succession through very gradual modification. It is singular that the second law is still ignored by many zoologists. Both laws were of paramount importance, as direct evidence of Darwin's theory of descent, which, it will be remembered, was at the time regarded merely as an hypothesis. Nevertheless, the tracing of phylogeny, or direct lines of descent, suddenly began to attract far more interest than the naming and description of species.

The Law of Recapitulation. Acceleration. Retardation

This law, that in the stages of growth of individual development (ontogeny), an animal repeats the stages of its ancestral evolution (phylogeny) was, as we have stated, anticipated by d'Orbigny. He recognized the fact that the shells of molluscs, which grow by successive additions, preserve unchanged the whole series of stages of their individual development, so that each shell of a Cretaceous ammonite, for example, represents five stages of progressive modification as follows: the first is the periode embryonnaire, during which the shell is smooth; the second and third represent periods of elaboration and ornamentation; the fourth is a period of initial degeneration; the fifth and last a period of degeneration when ornamentation becomes obsolete and the exterior smooth again, as in the young. D'Orbigny, being a special creationist, failed to recognize the bearing of these individual stages on evolution. Alpheus Hyatt (1838-1902) was the first to discover (1866) that these changes in the form of the ammonite shell agreed closely with those which had been passed through in the ancestral history of the ammonites. In an epoch-making essay, On the Parallelism between the Different stages of Life in the individual and those in the entire group of the Molluscous Order Tetrabranchiata (1866), and in a number of subsequent memoirs, among which Genesis of the Arietidae (1889) and Phylogeny of Characteristic (1894) should be mentioned, he laid the foundations, by methods of the most exact analysis, for all future recapitulation work of invertebrate palaeontologists. He showed that from each individual shell of an ammonite the entire ancestral series may be reconstructed, and that, while the earlier shell-whorls retain the characters of the adults of preceding members of the series, a shell in its own adult stage adds a new character, which in turn becomes the pre-adult character of the types which will succeed it; finally, that this comparison between the revolutions of the life of an individual and the life of the entire order of ammonites is wonderfully harmonious and precise. Moreover, the last stages of individual life are prophetic not only of future rising and progressing derivatives, but in the case of senile individuals of future declining and degradational series.

Thus the recapitulation law, which had been built up independently from the observations and speculations on vertebrates by Lorenz Ofen (1779-1851), Johann Friedrich Meckel (1781-1833), St Hilaire, Karl Ernst von Baer (1;92-1876) and others, and had been applied (1842-1843) by Karl Vogt (1817-1895) and Agassiz, in their respective fields of observation, to comparison of individual stages with the adults of the same group in preceding geological periods, furnished the key to the determination of the ancestry of the invertebrates generally.

Hyatt went further and demonstrated that ancestral characters are passed through by successive descendants at a more and more accelerated rate in each generation, thus giving time for the appearance of new characters in the adult. His " law of acceleration " together with the complementary " law of retardation," or the slowing up in the development of certain characters (first propounded by E. D. Cope), was also a philo -Ya ( -lb. l 2b - Ic ?+ -- 2c 36 I - Id - 2d -- - -- 3d (5d --{ - le -12e 3e - 1 f +2f - 31. ---} - 4f --- 5f 6f. --I-7f --?

lg-1-2g`1.3g - I-4g ---I 5g { - 6g? -- 7g+----I -1h-- f - 2h-13h --F - 4h (7h 7, - ?-- - 81 ----i (From the American Naturalist.) FIG. 6.

sophic contribution of the first importance (see fig. 6 and Plate III., fig. 7).

In the same year, 1866, Franz Martin Hilgendorf (1839 - studied the shells of Planorbis from the Miocene lake basin underlying the present village of Steinheim in Wurttemberg, and introduced the method of examination of large numbers of individual specimens, a method which has become of prime importance in the science. He discovered the actual transmutations in direct genetic series of species on the successive deposition levels of the old lake basin. This study of direct genetic series marked another great advance, and became possible in invertebrate palaeontology long before it was introduced among the vertebrates. Hyatt, in a re-examination of the Steinheim deposits, proved that successive modifications occur at the same level as well as in vertical succession. Melchior Neumayr (1845-1890) and C. M. Paul similarly demonstrated genetic series of Paludina (Vivipara) in the Pliocene lakes of Slavonia (1875).

The Mutations of Waagen. Orthogenesis

In 1869 Wilhelm Heinrich Waagen (1841-1900) entered the field with the study of Ammonites subradiatus. He proposed the term " mutations " for the minute progressive changes of single characters in definite directions as observed in successive stratigraphic levels. Even when seen in minute features only he recognized them as constant progressive characters or " chronologic varieties " in 3b --i C D E F G H I -14-21 -I-31 1 - I - 41 contrast with contemporaneous or " geographic varieties," which he considered inconstant and of slight systematic value. More recent analysis has shown, however, that certain modifications observed within the same stratigraphic level are really grades of mutations which show divergences comparable to those found in successive levels. The collective term " mutation," as now employed by palaeontologists, signifies a type modified to a slight degree in one or more of its characters along a progressive or definite line of phyletic development. The term " mutation " also applies to a single new character and for distinction 1 may be known as " the mutation of Waagen." This definitely directed evolution, or development in a few determinable directions, has since been termed " orthogenetic evolution," and is recognized by all workers in invertebrate palaeontology and phylogeny as fundamental because the facts of invertebrate palaeontology admit of no other interpretation.

Among the many who followed the method of attack first outlined by Hyatt, or who independently discovered his method, only a few can be mentioned here - namely, Waagen (1869), Neumayr (1871), Wiirttemberger (1880), Branco (1880), Mojsisovics (1882), Buckman (1887), Karpinsky (1889), Jackson (1890), Beecher (1890), Perrin-Smith (1897), Clarke (1898) and Grabau (1904). Melchior Neumayr, the great Austrian palaeontologist, especially extended the philosophic foundations of modern invertebrate palaeontology, and traced a number of continuous genetic series (formenreihe) in successive horizons. He also demonstrated that mutations have this special or distinctive character, that they repeat in the same direction without oscillation or retrogression. He expressed great reserve as to the causes of these mutations. He was the first to attempt a comprehensive treatment of all invertebrates from the genetic point of view; but unfortunately his great work, entitled Die Stcimme des Thierreichs (Vienna and Prague, 1889), was uncompleted.

The absolute agreement in the results independently obtained by these various investigators, the interpretation of individual development as the guide to phyletic development, the demonstration of continuous genetic series, each mutation falling into its proper place and all showing a definite direction, constitute contributions to biological philosophy of the first importance, which have been little known or appreciated by zoologists because of their publication in monographs of very special character.

Vertebrate Palaeontology after Darwin

The impulse which Darwin gave to vertebrate palaeontology was immediate and unbounded, finding expression especially in the writings of Thomas Henry Huxley (1825-1895) in England, of Jean Albert Gaudry (b. 1827) in France, in America of Edward Drinker Cope (1840-1897) and Othniel Charles Marsh (1831-1899). Fine examples of the spirit of the period as applied to extinct Mammalia are Gaudry's Animaux fossiles et geologie de l'Attique (1862) on the Upper Miocene fauna of Pikermi near Athens, and the remarkable memoirs of Vladimir Onufrievich Kowalevsky (1842-1883), published in 1873. These works swept aside the dry traditional fossil lore which had been accumulating in France and Germany. They breathed the new spirit of the recognition of adaptation and descent. In1867-1872Milne Edwards published his memoirs en the Miocene birds of central France. Huxley's development of the method of palaeontology should be studied in his collected memoirs (Scientific Memoirs of Thomas Henry Huxley, 4 vols., 1898). In Kowalevsky's Versuch einer natiirlichen Classification der Fossilen Hufthiere (1873) we find a model union of detailed inductive study with theory and working hypothesis. All these writers attacked the problem of descent, and published preliminary phylogenies of such animals as the horse, rhinoceros and elephant, which time has proved to be of only general value and not at all comparable to the exact phylogenetic series which were being established by invertebrate palaeontologists. Phyletic gaps began to be filled in this general way, however, by discovery, especially through remarkable 1 The Dutch botanist, De Vries, has employed the term in another sense, to mean a slight jump or saltation.

discoveries in North America by Leidy, Cope and Marsh, and the ensuing phylogenies gave enormous prestige to palaeontology. Cope's philosophic contributions to palaeontology began in 1868 (see essays in The Origin of the Fittest, New York, 1887, and The Primary Factors of Organic Evolution, Chicago, 1896) with the independent discovery and demonstration among vertebrates of the laws of acceleration and retardation. To the law of " recapitulation " he unfortunately applied Hyatt's term " parallelism," a term which is used now in another sense. He especially pointed out the laws of the " extinction of the specialized " and " survival of the non-specialized " forms of life, and challenged Darwin's principle of selection as an explanation of the origin of adaptations by saying that the " survival of the fittest " does not explain the " origin of the fittest." He personally sought to demonstrate such origin, first, in the existence of a specific internal growth force, which he termed bathmic force, and second in the direct inheritance of acquired mechanical modifications of, the teeth and feet. He thus revived Lamarck's views and helped to found the so-called neoLamarckian school in America. To this school A. Hyatt, W. H. Dall and many other invertebrate palaeontologists subscribed. History of Discovery. Vertebrates. - In discovery the theatre of interest has shifted from continent to continent, often in a sensational manner. After a long period of gradual revelation of the ancient life of Europe, extending eastward to Greece, eastern Asia and to Australia, attention became centred on North America, especially on Rocky Mountain exploration. New and unheard-of orders of amphibians, reptiles and mammals came to the surface of knowledge, revolutionizing thought, demonstrating the evolution theory, and solving some of the most important problems of descent. Especially noteworthy was the discovery of birds with teeth both in Europe (Archaeopteryx) and in North America (Hesperornis), of Eocene stages in the history of the horse, and of the giant dinosauria of the Jurassic and Cretaceous in North America. Then the stage of novelty suddenly shifted to South America, where after the pioneer labours of Darwin, Owen and Burmeister, the field of our knowledge was suddenly and vastly extended by explorations by the brothers Ameghino (Carlos and Florentino). We were in the midst of more thorough examination of the ancient world of Patagonia, of the Pampean region and of its submerged sister continent Antarctica, when the scene shifted to North Africa through the discoveries of Hugh J. L. Beadnell and Charles W. Andrews. These latter discoveries supply us with the ancestry of the elephants and many other forms. They round out our knowledge of Tertiary history, but leave the problems of the Cretaceous mammals and of their relations to Tertiary mammals still unsolved. Similarly, the Mesozoic reptiles have been traced successively to various parts of the world from France, Germany, England, to North America and South America, to Australia and New Zealand and to northern Russia, from Cretaceous times back into the Permian, and by latest reports into the Carboniferous.

Discovery of Invertebrates

The most striking feature of exploration for invertebrates, next to the world-wide extent to which exploration has been carried on and results applied, is the early appearance of life. Until comparatively recent times the molluscs were considered as appearing on the limits of the Cambrian and Ordovician; but Charles D. Walcott has described a tiny lamellibranch (Modioloides) from the inferior Cambrian, and he reports the gastropod (?) genus Chuaria from the preCambrian. Cephalopod molluscs have been traced back to the straight-shelled nautiloids of the genus Volborthella, while true ammonites have been found in the inferior Permian of the Continent and by American palaeontologists in the true coal measures. Similarly, early forms of the crustacean sub-class Merostomata have been traced to the pre-Cambrian of North America.

Recent discoveries of vertebrates are of the same significance, the most primitive fishes being traced to the Ordovician or base of the Silurian, 2 which proves that we shall discover more 2 Professor Bashford Dean doubts the fish characters of these Ordovic Rocky Mountain forms. Frech admits their fish character but considers the rocks infaulted Devonic.

This series of feet represents the evolutionary succession from the Eocene Hypohippus (I) to the modern Equus (6) seen in front and in side view. The top bone is the os calcis, or hock bone, to which the tendon Achilles is attached. The bottom bone is the terminal phalanx which is inserted in the heart of the hoof.

PLATE III.

Equus Modern caballus. horse.

Merychippusl sp. (milk molar) .

Parahippus pawniensis. Miocene.

I

2

3

4

The stages are as follows: I. Hypohippus, Lower Eocene. 4. Protohippus, Upper Miocene. 2. Mesohippus, Lower Oligocene. 5. Neohipparion, Upper Miocene.

3 Parahippus, Lower Miocene. 6. Equus, Pleistocene and recent.

The evolution consists first in progressive increase in size; second, in the acceleration of the median digit and retardation of the lateral digits, the latter becoming more and more elevated from the ground until finally in Equus (6) they are the lateral splints, which in the embryonic condition have vestigial cartilages attached representing the last traces of the lateral phalanges.

Mesohippus intermedius.

Upper Oligocene (White river for- mation).

Mesohippus bairdi ? Oligocene (White river formation).

Mesohippus bairdi. Middle Eocene (Bridger for mation).

I 2 3 "4 5 6 FIG. 7. - LAW OF Acceleration And Retardation Illustrated In The Evolution Of The Hind Feet Of The Horse.

(From photos lent by the American Museum of Natural History.) XX. 584.

Orohippus sp. Lower Eocene (Wind river formation). (Wasatch formation).

FIG. 8. - TEN Stages In The Evolution Of The Second Upper Molar Tooth Of The Right Side, Arranged According To Geological Level.

(Nos. 1-9 from "American Equidae.") Eohippus sp. Eohippus sp.

FIG. 12. - Hypohippus, a forestFIG. living horse, rear view, showing large lateral digits on the fore and hind feet, adapted to prevent the animal from sinking into the soft soil.

FIG. 14. - Restoration of Hypohippus. (From a drawing by Charles R. Knight, made under the direction of Professor Osborn.) Laws of Local Adaptive Radiation and Polyphyletic Evolution, illustrated by two Upper Miocene Horses of the Plains Region of North America. These horses are of the same geologic age (Upper Miocene) and 'were found in the same geographic region (South Dakota, U.S.A.). One is supposed to have lived in the forests along the stream borders, and the other in the open plains.

(Illustrations reproduced by permission of the Ameri- can Museum of Natural FIG. 15. History, New York.) oration of Neohipparion. (From a drawing! liarles R. Knight, made under the direction of Professor Osborn.) 13. - Neohipparion, a plains-living horse with very slender limbs and lateral digits small and well raised from the ground, adapted to a dry, hard soil.

ancient chordates in the Cambrian or even pre-Cambrian. Thus all recent discovery tends to carry the centres of origin and of dispersal of all animal types farther and farther back in geological time.

IV.-Relations Of Palaeontology To Other Physical Earth Sciences Geology and Palaeophysiography. - Fossils are not absolute timekeepers, because we have little idea of the rate of evolution; they are only relative timekeepers, which enable us to check off the period of deposition of one formation with that of another. Huxley questioned the time value of fossils, but recent research has tended to show that identity of species and of mutations is, on the whole, a guide to synchroneity, though the general range of vertebrate and invertebrate life as well as of plant life is generally necessary for the establishment of approximate synchronism. Since fossils afford an immediate and generally a decisive clue to the mode of deposition of rocks, whether marine, lacustrine, fluviatile, flood plain or aeolian, they lead us naturally into palaeophysiography. Instances of marine and lacustrine analysis have been cited above. The analysis of continental faunas into those inhabiting rivers, lowlands, forests, plains or uplands, affords a key to physiographic conditions all through the Tertiary. For example, the famous bone-beds of the Oligocene of South Dakota have been analysed by W. D. Matthew, and are shown to contain fluviatile or channel beds with water and river-living forms, and neighbouring flood-plain sediments containing remains of plains-living forms. Thus we may complete the former physiographic picture of a vast flood plain east of the Rocky Mountains, traversed by slowly meandering streams.

As already intimated, our knowledge of palaeometeorology, or of past climates, is derivable chiefly from fossils. Suggested two centuries ago by Robert Hooke, this use of fossils has in the hands of Barrande, Neumayr, the marquis de Saporta (1895), Oswald Heer (1809-1883), and an army of followers developed into a sub-science of vast importance and interest. It is true that a great variety of evidence is afforded by the composition of the rocks, that glaciers have left their traces in glacial scratchings and transported boulders, also that proofs of arid or semiarid conditions are found in the reddish colour of rocks in certain portions of the Palaeozoic, Trias and Eocene; but fossils afford the most precise and conclusive evidence as to the past history of climate, because of the fact that adaptations to temperature have remained constant for millions of years. All conclusions derived from the various forms of animal and plant life should be scrutinized closely and compared. The brilliant theories of the palaeobotanist, Oswald Heer, as to the extension of a sub-tropical climate to Europe and even to extreme northern latitudes in Tertiary time, which have appealed to the imagination and found their way so widely into literature, are now challenged by J. W. Gregory (Climatic Variations, their Extent and Causes, International Geological Congress, Mexico, 1906), who holds that the extent of climatic changes in past times has been greatly exaggerated.

' It is to palaeogeography and zoogeography in their reciprocal relations that palaeontology has rendered the most unique services. Geographers are practically helpless as historians, and problems of the former elevation and distribution of the land and sea masses depend for their solution chiefly upon the palaeontologist. With good reason geographers have given reluctant consent to some of the bold restorations of ancient continental outlines by palaeontologists; yet some of the greatest achievements of recent science have been in this field. The concurrence of botanical (Hooker, 1847), zoological, and finally of palaeontological evidence for the reconstruction of the continent of Antarctica, is one of the greatest triumphs of biological investigation. To the evidence advanced by a great number of authors comes the clinching testimony of the existence of a number of varieties of Australian marsupials in Patagonia, as originally discovered by Ameghino and more exactly described by members of the Princeton Patagonian expedition staff; while the fossil shells of the Eocene of Patagonia as analysed by Ortmann give evidence of the existence of a continuous shoreline, or at least of shallow-water areas, between Australia, New Zealand and South America. This line of hypothesis and demonstration is typical of the palaeogeographic methods generally - namely, that vertebrate palaeontologists, impressed by the sudden appearance of extinct forms of continental life, demand land connexion or migration tracts from common centres of origin and dispersal, while the invertebrate palaeontologist alone is able to restore ancient coast-lines and determine the extent and width of these tracts. Thus has been built up a distinct and most important branch. The great contributors to the palaeogeography of Europe are Neumayr and Eduard Suess (b. 1831), followed by Frech, Canu, de Lapparent and others. Neumayr was the first to attempt to restore the grander earth outlines of the earth as a whole in Jurassic times. Suess outlined the ancient relations of Africa and Asia through his " Gondwana Land," a land mass practically identical with the " Lemuria " of zoologists. South American palaeogeography has been traced by von Ihring into a northern land mass, " Archelenis," and a southern mass, " Archiplata," the latter at times united with an antarctic continent. Following the pioneer studies of Dana, the American palaeontologists and stratographers Bailey Willis, John M. Clarke, Charles Schuchert and others have re-entered the study of the Palaeozoic geography of the North American continent with work of astonishing precision.

Zoogeography

Closely connected with palaeogeography is zoogeography, the animal distribution of past periods. The science of zoogeography, founded by Humboldt, Edward Forbes, Huxley, P. L. Sclater, Alfred Russel Wallace and others, largely upon the present distribution of animal life, is now encountering through palaeontology a new and fascinating series of problems. In brief, it must connect living distribution with distribution in past time, and develop a system which will be in harmony with the main facts of zoology and palaeontology. The theory of past migrations from continent to continent, suggested by Cuvier to explain the replacement of the animal life which had become extinct through sudden geologic changes, was prophetic of one of the chief features of modern method - namely, the tracing of migrations. With this has been connected the theory of " centres of origin " or of the geographic regions *here the chief characters of great groups have been established. Among invertebrates Barrande's doctrine of centres of origin was applied by Hyatt to the genesis of the Arietidae (1889); after studying thousands of individuals from the principal deposits of Europe he decided that the cradles of the various branches of this family were the basins of the CSte d'Or and southern Germany. Ortmann has traced the centre of dispersal of the fresh-water Crawfish genera Cambarus, Potamobius and Cambaroides to eastern Asia, where their common ancestors lived in Cretaceous time. Similarly, among vertebrates the method of restoring past centres of origin, largely originating with Edward Forbes, has developed into a most distinct and important branch of historical work. This branch of the science has reached the highest development in its application to the history of the extinct mammalia of the Tertiary through the original work o Cope and Henri Filhol, which has been brought to a much higher degree of exactness recently through the studies of H. F. Osborn, Charles Deperet, W. D. Matthew and H. G. Stehlin.

V.-Relations Of Palaeontology To Other Zoological Methods Systematic Zoology. - It is obvious that the Linnaean binomial terminology and its subsequent trinomial refinement for species, sub-species, and varieties was adapted to express the differences between animals as they exist to-day, distributed contemporaneously over the surface of the earth, and that it is wholly inadapted to express either the minute gradations of successive generic series or the branchings of a genetically connected chain of life. Such gradations, termed " mutations " by Waagen, are distinguished, as observed, in single characters; they are the xx. 19 a nuances, or grades of difference, which are the more gradual the more finely we dissect the geologic column, while the terms species, sub-species and variety are generally based upon a sum of changes in several characters. Thus palaeontology has brought to light an entirely new nomenclatural problem, which can only be solved by resolutely adopting an entirely different principle.

which is essentially based on a theory of interrupted or discontinuous characters, is inapplicable.

_

Formations in Western United Stales and Characteristic Type of Horse in Each

Fore Foot

Hind Foot

Teeth

Quaternary Recent

_ _ _

One Toe

S of

Ih

1 1

=1

Cement-

covered

?...

?

giinnaualM U' E

_

?? Protohippus

?t r?? MQSOhippus

Pliocene

Miocen

Three Toes

1

- Ulan-F

_ _ _ _ _ _

Side tr

Three Toes

Side toes

spllnl o(S l digif

I

Three Toes Side toes

_ g

1

? ??without

Cement

°

"W}

/

;, = ^

_Dir

___ - _ - ___

-

-

_-_" _'"-

=

- _ - - -

Four Toe

ai-N

_ - Protorohipnus

__ _-__

__

Four Toes

11

Toes

Hyracotherlum

Splint of 1'-' elg r

:'

Splint of 5' 1, digit.

-

Hypothetical Ancestors with Five Toes on Each Foot

and Teeth like those of Monkeys etc.

Reproduced by permission of the American Museum of Natural History

Age

1

Triassic

__

= __

-

_ c _ - _ ? S _ ,?

__

Embryology and Ontogeny

In following the discovery of the law of recapitulation among palaeontologists we have clearly stated the chief contribution of palaeontology to the science of ontogeny - namely, the correspondences and differences between FIG. 9.

This revolution may be accomplished by adding the term " mutation ascending " or " mutation descending " for the minute steps of transformation, and the term phylum, as employed in Germany, for the minor and major branches of genetic series. Bit by bit mutations are added to each other in different single characters until a sum or degree of mutations is reached which no zoologist would hesitate to place in a separate species or in a separate genus.

The minute gradations observed by Hyatt, Waagen and all invertebrate palaeontologists, in the hard parts (shells) of molluscs, &c., are analogous to the equally minute gradations observed by vertebrate palaeontologists in the hard parts of reptiles and mammals. The mutations of Waagen may possibly, in fact, prove to be identical with the " definite variations " or " rectigradations " observed by Osborn in the teeth of mammals. For example, in the grinders of Eocene horses (see Plate III., fig. 8; also fig. 9) in a lower horizon a cusp is adumbrated in shadowy form, in a slightly higher horizon it is visible, in a still higher horizon it is full-grown; and we honour this final stage by assigning to the animal which bears it a new specific name. When a number of such characters accumulate, we further honour them by assigning a new generic name. This is exactly the nomenclature system laid down by Owen, Cope, Marsh and others, although established without any understanding of the law of mutation. But besides the innumerable characters which are visible and measurable, there are probably thousands which we cannot measure or which have not been discovered, since every part of the organism enjoys its gradual and independent evolution. In the face of the continuous series of characters and types revealed by palaeontology, the Linnaean terminology, the individual order of development and the ancestral order of evolution. The mutual relations of palaeontology and embryology and comparative anatomy as means of determining the ancestry of animals are most interesting. In tracing the phylogeny, or ancestral history of organs, palaeontology affords the only absolute criterion on the successive evolution of organs in time as well as of (progressive) evolution in form. From comparative anatomy alone it is possible to arrange a series of living forms which, although structurally a convincing array because placed in a graded series, may be, nevertheless, in an order inverse to that of the actual historical succession. The most marked case of such inversion in comparative anatomy is that of Carl Gegenbaur (5826-5903), who in arranging the fins of fishes in support of his theory that the fin of the Australian. lung-fish (Ceratodus) was the most primitive (or Archipterygium), placed as the primordial type a fin which palaeontology has proved to be one of the latest types if not the last. It is equally true that palaeontological evidence has frequently failed where we most sorely needed it. The student must therefore resort to what may be called a tripod of evidence, derived from the available facts of embryology, comparative anatomy and palaeontology.

VI

THE Palaeontologist As Historian The modes of change among animals, and methods of analysing them. - As historian the palaeontologist always has before him as one of his most fascinating problems phylogeny, or the restoration of the great tree of animal descent. Were the geologic record complete he would be able to trace the ancestry of man and of all other animals back to their very beginnings in the' primordial protoplasm. Dealing with interrupted evidence, however, it becomes necessary to exercise the closest analysis and synthesis as part of his general art as a restorer.

The most fundamental distinction in analysis is that which must be made between homogeny, or true hereditary resemblance, and those multiple forms of adaptive resemblance which are variously known as cases of " analogy," " parallelism," " convergence " and " homoplasy." Of these two kinds of genetic and adaptive resemblance, homogeny is the warp composed of the vertical, hereditary strands, which connect animals with their ancestors and their successors, while analogy is the woof, composed of the horizontal strands which tie animals together by their superficial resemblances. This wide distinction between similarity of descent and similarity of adaptation applies to every organ, to all groups of organs, to animals as a whole, and to all groups of animals. It is the old distinction between homology and analogy on a grand scale.

Analogy, in its power of transforming unlike and unrelated animals or unlike and unrelated parts of animals into likeness, has done such miracles that the inference of kinship is often almost irresistible. During the past century it was and even now is the very " will-o'-the-wisp " of evolution, always tending to lead the phylogenist astray. It is the first characteristic of analogy that it is superficial. Thus the shark, the ichthyosaur, (After a drawing by Charles R. Knight, made under the direction of Professor Osborn.) FIG. i o. - Analogous or convergent evolution in Fish, Reptile and Mammal.

The external similarity in the fore paddle and back fin of these three marine animals is absolute, although they are totally unrelated to each other, and have a totally different internal or skeletal structure. It is one of the most striking cases known of the law of analagous evolution.

A, Shark (Lamna cornubica), with long lobe of tail upturned.

B, Ichthyosaur (Ichthyosaurus quadricissus), with fin-like paddles, long lobe of tail down-turned.

C, Dolphin (Sotalia fluviatilis), with horizontal tail, fin or fluke.

and the dolphin (fig. 10) superficially resemble each other, but if the outer form be removed this resemblance proves to be a mere veneer of adaptation, because their internal skeletal parts are as radically different as are their genetic relations, founded on heredity. Analogy also produces equally remarkable internal or skeletal transformations. The ingenuity of nature, however, in adapting animals is not infinite, because the same devices are repeatedly employed by her to accomplish the same adaptive ends whether in fishes, reptiles, birds or mammals; thus she has repeated herself at least twenty-four times in the evolution of long-snouted rapacious swimming types of animals. The grandest application of analogy is that observed in the adaptations of groups of animals evolving on different continents, by which their various divisions tend to mimic those on other continents. Thus the collective fauna of ancient South America mimics the independently evolved collective fauna of North America, the collective fauna of modern Australia mimics the collective fauna of the Lower Eocene of North America. Exactly the same principles have developed on even a vaster scale among the Invertebrata. Among the ammonites of the Jurassic and Cretaceous periods types occur which in their external appearance so closely resemble each other that they could be taken for members of a single series, and not infrequently have been taken for species of the same genus and even for the same species; but their early stages of development and, in fact, their entire individual history prove them to be distinct and not infrequently to belong to widely separated genetic series.

Homogeny, in contrast, the " special homology " of Owen, is the supreme test of kinship or of hereditary relationship, and thus the basis of all sound reasoning in phylogeny. The two joints of the thumb, for example, are homogenous throughout the whole series of the pentadactylate, or five-fingered animals, from the most primitive amphibian to man.

The conclusion is that the sum of homogenous parts, which may be similar or dissimilar in external form according to their similarity or diversity of function, and the recognition of former similarities of adaptation (see below) are the true bases for the critical determination of kinship and phylogeny.

Adaptation and the Independent Evolution of Parts

Step by step there have been established in palaeontology a number of laws relating to the evolution of the parts of animals which closely coincide with similar laws discovered by zoologists. All are contained in the broad generalization that every part of an animal, however minute, has its separate and independent basis in the hereditary substance of the germ cells from which it is derived and may enjoy consequently a separate and independent history. The consequences of this principle when applied to the adaptations of animals bring us to the very antithesis of Cuvier's supposed "law of correlation," for we find that, while the end results of adaptation are such that all parts of an animal conspire to make the whole adaptive, there is no fixed correlation either in the form or rate of development of parts, and that it is therefore impossible for the palaeontologist to predict the anatomy of an unknown animal from one of its parts only, unless the animal happens to belong to a type generally familiar. For example, among the land vertebrates the feet (associated with the structure of the limbs and trunk) may take one of many lines of adaptation to different media or habitat, either aquatic, terrestrial, arboreal or aerial; while the teeth (associated with the structure of the skull and jaws) also may take one of many lines of adaptation to different kinds of food, whether herbivorous, insectivorous or carnivorous. Through this independent adaptation of different parts to their specific ends there have arisen among vertebrates an almost unlimited number of combinations of foot and tooth structure, the possibilities of which are illustrated in the accompanying diagram (see fig. II; also Plate III., fig. 8). As instances of such combinations, some of the (probably herbivorous) Eocene monkeys with arboreal limbs have teeth so difficult to distinguish from those of the herbivorous ground-living Eocene horses with cursorial limbs that at first in France and also in America they were both classed with the hoofed animals. Again, directly opposed to Cuvier's principle, we have discovered carnivores with hoofs, such as Mesonyx, and herbivores with sloth-like claws, such as Chalicotherium. This latter animal is closely related to one which Cuvier termed Pangolin gigantesque, and had he restored it according to his " law of correlation " he would have pictured a giant " scaly anteater," a type as wide as the poles from the actual form of Chalicotherium, which in body, limbs and teeth is a modified ungulate herbivore, related remotely to the tapirs. In its claws alone does it resemble the giant sloths.

This independence of adaptation applies to every detail of structure; the six cusps of a grinding tooth may all evolve alike, or each may evolve independently and differently. Independent evolution of parts is well shown among invertebrates, where the shell of an ammonite, for example, may change markedly in form without a corresponding change in suture, or vice versa.

Similarly, there is no correlation in the rate of evolution either of adjoining or of separated parts; the middle digit of the foot of the three-toed horse is accelerated in development, while the lateral digits on either side are retarded. Many examples might be cited among invertebrates also.

Aquatic

Unguligrade

[[Adaptive Types Of Limbs And Feet Volant]] Short-limbed, plantigrade, Ambulatory pentadactyl, unguicuOR late Stem [[Terrestrial Adaptive Types Of Teeth]] Stem Insectivorous Law Of The Independent Adaptive Evolution Of Parts. Fig. Ii. - Diagram demonstrating that there are an indefinite number of combinations of various adaptive types of limbs and feet with various adaptive types of teeth, and that there is no fixed law of correlation between the two series of adaptations.

All these principles are consistent with Francis Galton's law of particulate inheritance in heredity, and with the modern doctrine of " unity of characters " held by students of Mendelian phenomena.

Sudden versus Gradual Evolution of Parts. - There is a broad and most interesting analogy between the evolution of parts of animals and of groups of animals studied as a whole. Thus we observe persistent organs and persistent types of animals, analogous organs and analogous types of animals, and this analogy applies still further to the rival and more or less contradictory hypotheses of the sudden as distinguished from the gradual appearance of new parts or organs of animals, and the sudden appearance of new types of animals. The first exponent of the theory of sudden appearance of new parts and new types, to our knowledge, was Geoffroy St Hilaire, who suggested saltatory evolution through the direct action of the environment on development, as explaining the abrupt transitions in the Mesozoic Crocodilia and the origin of the birds from the reptiles.

Waagen's law of mutation, or the appearance of new parts or organs so gradually that they can be perceived only by following them through successive geologic time stages, appears to be directly contradictory to the saltation principle; it is certainly one of the most firmly established principles of palaeontology, and it constitutes the contribution par excellence of this branch of zoology to the law of evolution, since it is obvious that it could not possibly have been deduced from comparison of living animals but only through the long perspective gained by comparison of animals succeeding each other in time. The essence of Waagen's law is orthogenesis, or evolution in a definite direction, and, if there does exist an internal hereditary principle controlling such orthogenetic evolution, there does not appear to be any essential contradiction between its gradual operation in the " mutations of Waagen " and its occasional hurried operation in the " mutations of de Vries," which are by their definition discontinuous or saltatory (Osborn, 1907).

VII.-Modes Of Change In Animals As A Whole Or In Groups Of Animals, And Methods Of Analysing Them.

I. Origin from Primitive or Stem Forms. - As already observed, the same principles apply to groups of animals as to organs and groups of organs; an organ originates in a primitive and unspecialized stage, a group of animals originates in a primitive or stem form. It was early perceived by Huxley, Cope and many others that Cuvier's broad belief in a universal progression was erroneous, and there developed the distinction between " persistent primitive types " (Huxley) and " progressive types." The theoretical existence of primitive or stem forms was clearly perceived by Darwin, but the steps by which the stem form might be restored were first clearly enunciated by Huxley in 1880 (" On the Application of Evolution to the Arrangement of the Vertebrata and more particularly of the Mammalia," Scient. Mem. iv. 457) namely, by sharp separation of the primary or stem characters from the secondary or adaptive characters in all the known descendants or branches of a theoretical original form. The sum of the primitive characters approximately restores the primitive form; and the gaps in palaeontological evidence are supplied by analysis of the available zoological, embryological and anatomical evidence. Thus Huxley, with true prophetic instinct, found that the sum of primitive characters of all the higher placental mammals points to a stem form of a generalized insectivore type, a prophecy which has been fully confirmed by the latest research. On the other hand, Huxley's summation of the primitive characters of all the mammals led him to an amphibian stem type, a prophecy which has proved faulty because based on erroneous analysis and comparison. More or less independently, Huxley, Kowalevsky and Cope restored the stem ancestor of the hoofed animals, or ungulates, a restoration which has been nearly fulfilled by the discovery, in 1873, of the generalized type Phenacodus of northern Wyoming. Similar anticipations and verifications among the invertebrates have been made by Hyatt, Beecher, Jackson and others.

In certain cases the character stem forms actually survive in unspecialized types. Thus the analysis of George Baur of the ancestral form of the lizards, mosasaurs, dinosaurs, crocodiles and phytosaurs led both to the generalized Palaeohatteria of the Permian and indirectly to the surviving Tuatera lizard of New Zealand.

2. Adaptations to Alternations of Habitat. Law of Irreversibility of Evolution. - In the long vicissitudes of time and procession of continental changes, animals have been subjected to alternations of habitat either through their own migrations or through the " migration of the environment itself," to employ Van den Broeck's epigrammatic description of the profound and sometimes sudden environmental changes which may take place in a single locality. The traces of alternations of adaptations corresponding to these alternations of habitat are recorded both in palaeontology and anatomy, although often after the obscure analogy of the earlier and later writings of a palimpsest. Huxley in 1880 briefly suggested the arboreal origin, or primordial treehabitat of all the marsupials, a suggestion abundantly confirmed by the detailed studies of Dollo and of Bensley, according to which we may imagine the marsupials to have passed through (r) a former terrestrial phase, followed by (2) a primary arboreal phase - illustrated in the tree phalangers - followed by (3) a secondary terrestrial phase - illustrated in the kangaroos and wallabies - followed by (4) a secondary arboreal phase - illustrated in the tree kangaroos. Louis Dollo especially has Fossorial Amphibious Digitigrade Grass Herb Herbivorous Shrub Fruit Root Dentition reduced Omnivorous Fish Carnivorous-{Flesh Carrion contributed most brilliant discussions of the theory of alternations of habitat as applied to the interpretation of the anatomy of the marsupials, of many kinds of fishes, of such reptiles as the herbivorous dinosaurs of the Upper Cretaceous. He has applied the theory with especial ingenuity to the interpretation of the circular bony plates in the carapace of the aberrant leather-back sea-turtles (Sphargidae) by prefacing an initial land phase, in which the typical armature of land tortoises was acquired, a first marine or pelagic phase, in which this armature was lost, a third littoral or seashore phase, in which a new polygonal armature was acquired, and a fourth resumed or secondary marine phase, in which this polygonal armature began to degenerate.

Each of these alternate life phases may leave some profound modification, which is partially obscured but seldom wholly lost; thus the tracing of the evidences of former adaptations is of great importance in phylogenetic study.

A very important evolutionary principle is that in such secondary returns to primary phases lost organs are never recovered, but new organs are acquired; hence the force of Dollo's dictum that evolution is irreversible from the point of view of structure, while frequently reversible, or recurrent, in point of view of the conditions of environment and adaptation.

3. Adaptive Radiations of Groups, Continental and Local

Starting with the stem forms the descendants of which have passed through either persistent or changed habitats, we reach the underlying idea of the branching law of Lamarck or the law of divergence of Darwin, and find it perhaps most clearly expressed in the words "adaptive radiation" (Osborn), which convey the idea of radii in many directions. Among extinct Tertiary mammals we can actually trace the giving off of these radii in all directions, for taking advantage of every possibility to secure food, to escape enemies and to reproduce kind; further, among such well-known quadrupeds as the horses, rhinoceroses and titanotheres, the modifications involved in these radiations can be clearly traced. Thus the history of continental life presents a picture of contemporaneous radiations in different parts of the world and of a succession of radiations in the same parts. We observe the contemporaneous and largely independent radiations of the hoofed animals in South America, in Africa and in the great ancient continent comprising Europe, Asia and North America; we observe the Cretaceous radiation of hoofed animals in the northern hemisphere, followed by a second radiation of hoofed animals in the same region, in some cases one surviving spur of an old radiation becoming the centre of a new one. As a rule, the larger the geographic theatre the grander the radiation. Successive discoveries have revealed certain grand centres, such as (1) the marsupial radiation of Australia, (2) the littleknown Cretaceous radiation of placental mammals in the northern hemisphere, which was probably connected in part with the peopling of South America, (3) the Tertiary placental radiation in the northern hemisphere, partly connected with Africa, (4) the main Tertiary radiation in South America. Each of these radiations produced a greater or less number of analogous groups, and while originally independent the animals thus evolving as autochthonous types finally mingled together as migrant or invading types. We are thus working out gradually the separate contributions of the land masses of North America, South America, Europe, Asia, Africa, and of Australia to the mammalian fauna of the world, a result which can be obtained through palaeontology only.

4. Adaptive Local Radiation

On a smaller scale are the local adaptive radiations which occur through segregation of habit and local isolation in the same general geographic region wherever physiographic and climatic differences are sufficient to produce local differences in food supply or other local factors of change. This local divergence may proceed as rapidly as through wide geographical segregation or isolation. This principle has been demonstrated recently among Tertiary rhinoceroses and titanotheres, in which remains of four or five genetic series in the same geologic deposits have been discovered. We have proof that in the Upper Miocene of Colorado there existed a forest-living horse, or more persistent primitive type, which was contemporaneous with and is found in the same deposits with the plains-living horse (Neohipparion) of the most advanced or specialized desert type (see Plate IV., figs. 12, 13, 14, 15). In times of drought these animals undoubtedly resorted to the same water-courses for drink, and thus their fossilized remains are found associated.

5. The Law of Polyphyletic Evolution. The Sequence of Phyla or Genetic Series

There results from continental and local adaptive radiations the presence in the same geographical region of numerous distinct lines in a given group of animals. The polyphyletic law was early demonstrated among invertebrates by Neumayr (1889) when he showed that the ammonite genus Phylloceras follows not one but five distinct lines of evolution of unequal duration. The brachiopods, generally classed collectively as Spirifer mucronatus, follow at least five distinct lines of evolution in the Middle Devonian of North America, while more than twenty divergent lines have been observed by Grabau among the species of the gastropod genus Fusus in Tertiary and recent times. Vertebrate palaeontologists were slow to grasp this principle; while the early speculative phylogenies of the horse of Huxley and Marsh, for example, were mostly displayed monophyletically, or in single lines of descent, it is now recognized that the horses which were placed by Marsh in a single series are really to be ranged in a great number of contemporaneous but separate series, each but partially known, and that the direct phylum which leads to the modern horse has become a matter of far more difficult search. As early as 1862 Gaudry set forth this very polyphyletic principle in his tabular phylogenies, but failed to carry it to its logical application. It is now applied throughout the Vertebrata of both Mesozoic and Cenozoic times. Among marine Mesozoic reptiles, each of the groups broadly known as ichthyosaurs, plesiosaurs, mosasaurs and crocodiles were polyphyletic in a marked degree. Among land animals striking illustrations of this local polyphyletic law are found in the existence of seven or eight contemporary series of rhinoceroses, five or six contemporary series of horses, and an equally numerous contemporary series of American Miocene and Pliocene camels; in short, the polyphyletic condition is the rule rather than the exception. It is displayed to-day among the antelopes and to a limited degree among the zebras and rhinoceroses of Africa, a continent which exhibits a survival of the Miocene and Pliocene conditions of the northern hemisphere.

6. Development of Analogous Progressive and Retrogressive Groups

Because of the repetition of analogous physiographic and climatic conditions in regions widely separated both in time and in space, we discover that continental and local adaptive radiations result in the creation of analogous groups of radii among all the vertebrates and invertebrates. Illustrations of this law were set forth by Cope as early as 1861 (see " Origin of Genera," reprinted in the Origin of the Fittest, pp. 95 -106) in pointing out the extraordinary parallelisms between unrelated groups of amphibians, reptiles and mammals. In the Jurassic period there were no less than six orders of reptiles which independently abandoned terrestrial life and acquired more or less perfect adaptation to sea life. Nature, limited in her resources for adaptation, fashioned so many of these animals in like form that we have learned only recently to distinguish similarities cf analogous habit from the similitudes of real kinship. From whatever order of Mammalia or Reptilia an animal may be derived, prolonged aquatic adaptation will model its outer, and finally its inner, structure according to certain advantageous designs. The requirements of an elongate body moving through the resistant medium of water are met by the evolution of similar entrant and exit curves, and the bodies of most swiftly moving aquatic animals evolve into forms resembling the hulls of modern sailing yachts (Bashford Dean). We owe especially to Willy Ki kenthal, Eberhard Fraas, S.W. Williston and R. C. Osburn a summary of those modifications of form to which aquatic life invariably leads.

The law of analogy also operates in retrogression. A. Smith Woodward has observed that the decline of many groups of fishes is heralded by the tendency to assume elongate and finally eel-shaped forms, as seen independently, for example, among the declining Acanthodians or palaeozoic sharks, among the modern crossopterygian Polypterus and Calamoichthys of the Nile, in the modern dipneustan Lepidosiren and Protopterus, in the Triassic chondrostean Belonorhynchus, as well as in the bow-fin (A7nia) and the garpike (Lepidosteus). Among invertebrates similar analogous groups also develop. This is especially marked in retrogressive, though also wellknown in progressive series. The loss of the power to coil, observed in the terminals of many declining series of gastropods from the Cambrian to the present time, and the similar loss of power among Natiloidea and Ammonoidea of many genetic series, as well as the ostraean form assumed by various declining series of pelecypods and by some brachiopods, may be cited as examples.

7. Periods of Gradual Evolution of Groups

It is certainly a very striking fact that wherever we have been able to trace genetic series, either of invertebrates or vertebrates, in closely sequent geological horizons, or life zones, we find strong proof of evolution through extremely gradual mutation simultaneously affecting many parts of each organism, as set forth above. This proof has been reached quite independently by a very large number of observers studying a still greater variety of animals. Such diverse organisms as brachiopods, ammonites, horses and rhinoceroses absolutely conform to this law in all those rare localities where we have been able to observe closely sequent stages. The inference is almost irresistible that the law of gradual transformation through minute continuous change is by far the most universal; but many palaeontologists as well as zoologists and botanists hold a contrary opinion.

8. Periods of Rapid Evolution of Groups

The above law of gradual evolution is perfectly consistent with a second principle, namely, that at certain times evolution is much more rapid than at others, and that organisms are accelerated or retarded in development in a manner broadly analogous to the acceleration or retardation of separate organs. Thus H. S. Williams observes (Geological Biology, p. 268) that the evolution of those fundamental characters which mark differences between separate classes, orders, sub-orders, and even families of organisms, took place in relatively short periods of time. Among the brachiopods the chief expansion of each type is at a relatively early period in their life-history. Hyatt (1883) observed of the ammonites that each group originated suddenly and spread out with great rapidity. Deperet notes that the genus Neumayria, an ammonite of the Kimmeridgian, suddenly branches out into an explosion" of forms. Deperet also observes the contrast between periods of quiescence and limited variability and periods of sudden efflorescence. A. Smith Woodward (" Relations of Palaeontology to Biology," Annals and Mag. Natural Hist., 1906, p. 317) notes that the fundamental advances in the growth of fish life have always been sudden, beginning with excessive vigour at the end of long periods of apparent stagnation; while each advance has been marked by the fixed and definite acquisition of some new anatomical character or " expression point," a term first used by Cope. One of the causes of these sudden advances is undoubtedly to be found in the acquisition of a new and extremely useful character., Thus the perfect jaw and the perfect pair of lateral fins when first acquired among the fishes favoured a very rapid and for a time unchecked development. It by no means follows, however, from this incontrovertible evidence that the acquisition either of the jaw or of the lateral fins had not been in itself an extremely gradual process.

Thus both invertebrate and vertebrate palaeontologists have reached independently the conclusion that the evolution of groups is not continuously at a uniform rate, but that there are, especially in the beginnings of new phyla or at the time of acquisition of new organs, sudden variations in the rate of evolution which have been termed variously " rhythmic," "pulsating," " efflorescent," "intermittent " and even " explosive " (Deperet).

This varying rate of evolution has (illogically, we believe) been compared with and advanced in support of the "mutation law of De Vries,"or the theory of saltatory evolution, which we may next consider.

9. Hypothesis of the Sudden Appearance of New Parts or Organs

The rarity of really continuous series has naturally led palaeontologists to support the hypothesis Of brusque transitions of structure. As we have seen, this hypothesis was fathered by Geoffroy St Hilaire in 1830 from his studies of Mesozoic Crocodilia, was sustained by Haldemann, and quite recently has been revived by such eminent palaeontologists as Louis Dollo and A. Smith Woodward. The evidence for it is not to be confused with that for the law of rapid efflorescence of groups just considered. It should be remembered that palaeontology is the most unfavourable field of all for observation and demonstration of sudden saltations or mutations of character, because of the limited materials available for comparison and the rarity of genetic series. It should be borne in mind, first, that wherever a new animal suddenly appears or a new character suddenly arises in a fossil horizon we must consider whether such appearance may be due to the non-discovery of transitional links with older forms, or to the sudden invasion of a new type or new organ which has gradually evolved elsewhere. The rapid variation of certain groups of animals or the acceleration of certain organs is also not evidence of the sudden appearance of new adaptive characters. Such sudden appearances may be demonstrated possibly in zoology and embryology but never can be demonstrated by palaeontology, because of the incompleteness of the geological record.

10. Decline or Senescence of Groups. - Periods of gradual evolution and of efflorescence may be followed by stationary or senescent conditions. In his history of the Arietidae Hyatt points out that toward the close of the Cretaceous this entire group of ammonites appears to have been affected with some malady; the unrolled forms multiply, the septa are simplified, the ornamentation becomes heavy, thick, and finally disappears in the adult; the entire group ends by dying out and leaving no descendants. This is not due to environmental conditions solely, because senescent branches of normal progressive groups are found in all geologic horizons, beginning, for gastropods, in the Lower Cambrian. Among the ammonites the loss of power to coil the shell is one feature of racial old age, and in others old age is accompanied by closer coiling and loss of surface ornamentation, such as spines, ribs, spirals; while in other forms an arresting of variability precedes extinction. Thus Williams has observed that if we find a species breeding perfectly true we can conceive it to have reached the end of its racial life period. Brocchi and Daniel Rosa (1899) have developed the hypothesis of the progressive reduction of variability. Such decline is by no means a universal law of life, however, because among many of the continental vertebrates at least we observe extinctions repeatedly occurring during the expression of maximum variability. Whereas among many ammonites and gastropods smooth ness of the shell, following upon an ornamental youthful Condition, is generally a symptom of decline, among many other invertebrates and vertebrates, as C. E. Beecher (1856-1905) has pointed out (1898), many animals possessing hard parts tend toward the close of their racial history to produce a superfluity of dead matter, which accumulates in the form of spines among invertebrates, and of horns among the land vertebrates, reaching a maximum when the animals are really on the down-grade of development.

I 1. The Extinction of Groups. - We have seen that different lines vary in vitality and in longevity, that from the earliest times senescent branches are given off, that different lines vary in the rate of evolution, that extinction is often heralded by symptoms of racial old age, which, however, vary widely in different groups. In general we find an analogy between the development of groups and of organs; we discover that each phyletic branch of certain organisms traverses a geologic career comparable to the life of an individual, that we may often distinguish, especially among invertebrates, a phase of youth, a phase of maturity, a phase of senility or degeneration foreshadowing the extinction of a type.

Internal causes of extinction are to be found in exaggeration of body size, in the hypertrophy or over-specialization of certain organs, in the irreversibility of evolution, and possibly, although this has not been demonstrated, in a progressive reduction of variability. In a full analysis of this problem of internal and external causes in relation to the Tertiary Mammalia, H. F. Osborn (" Causes of Extinction of the Mammalia," Amer. Naturalist, 1906, pp. 7 6 9-795, 82 9- 8 59) finds that foremost in the long series of causes which lead to extinction are the grander environmental changes, such as physiographic changes, diminished or contracted land areas, substitution of insular for continental conditions; changes of climate and secular lowering of temperature accompanied by deforestation and checking of the food supply; changes influencing the mating period as well as fertility; changes causing increased humidity, which in turn favours enemies among insect life. Similarly secular elevations of temperature, either accompanied by moisture or desiccation, by increasing droughts or by disturbance of the balance of nature, have been followed by great waves of extinction of the Mammalia. In the sphere of living environment, the varied evolution of plant life, the periods of forestation and deforestation, the introduction of deleterious plants simultaneously with harsh conditions of life and enforced migration, as well as of mechanically dangerous plants, are among the well-ascertained causes of diminution and extinction. The evolution of insect life in driving animals from feeding ranges and in the spread of disease probably has been a prime cause of extinction. Food competition among mammals, especially intensified on islands, and the introduction of Carnivora constitute another class of causes. Great waves of extinction have followed the long periods of the slow evolution of relatively inadaptive types of tooth and foot structure, as first demonstrated by Waldemar Kowalevsky; thus mammals are repeatedly observed in a cul-de-sac of structure from which there is no escape in an adaptive direction. Among still other causes are great bulk, which proves fatal under certain new conditions; relatively slow breeding; extreme specialization and development of dominant organs, such as horns and tusks, on which for a time selection centres to the detriment of more useful characters. Little proof is afforded among the mammals of extinction through arrested evolution or through the limiting of variation, although such laws undoubtedly exist. One of the chief deductions is that there are special dangers in numerical diminution of herds, which may arise from a chief or original cause and be followed by a conspiracy of other causes which are cumulative in effect. This survey of the phenomena of extinction in one great class of animals certainly establishes the existence of an almost infinite variety of causes, some of which are internal, some external in origin, operating on animals of different kinds.

Viii. - Underlying Biological Principles As They Appear To The Palaeontologist It follows from the above brief summary that palaeontology affords a distinct and highly suggestive field of purely biological research; that is, of the causes of evolution underlying the observable modes which we have been describing. The net result of observation is not favourable to the essentially Darwinian view that the adaptive arises out of the fortuitous by selection, but is rather favourable to the hypothesis of the existence of some quite unknown intrinsic law of life which we are at present totally unable to comprehend or even conceive. We have shown that the direct observation of the origin of new characters in palaeontology brings them within that domain of natural law and order to which the evolution of the physical universe conforms. The nature of this law, which, upon the whole, appears to be purposive or teleological in its operations, is altogether a mystery which may or may not be illumined by future research. In other words, the origin, or first appearance of new characters, which is the essence of evolution, is an orderly process so far as the vertebrate and invertebrate palaeontologist observes it. The selection of organisms through the crucial test of fitness and the shaping of the organic world is an orderly process when contemplated on a grand scale, but of another kind; here the test of fitne s is supreme. The only inkling of possible underlying principles in this orderly process is that there appears to be in respect to certain characters a potentiality or a predisposition through hereditary kinship to evolve in certain definite directions. Yet there is strong evidence against the existence of any law in the nature of an internal perfecting tendency which would operate independently of external conditions. In other words, a balance appears to be always sustained between the internal (hereditary and ontogenetic) and the external (environmental and selectional) factors of evolution.

Bibliography

Among the older works on the history of palaeontology are the treatises of Giovanni Battista Brocchi (1772-1826), Conchiologia fossile Subappenina ... Disc. sui progressi dello studio ... 1843 (Milan); of Etienne Jules d'Archiac, Histoire du progres de la geologie de 1834 1862 (Paris, Soc. Geol. de France, 1847-1860); of Charles Lyell in his Principles of Geology. A clear narrative of the work of many of the earlier contributors is found in Founders of Geology, by Sir Archibald Geikie (London, 1897-1905). The most comprehensive and up-to-date reference work on the history of geology and palaeontology is Geschichte der Geologie and Paldontologie, by Karl Alfred von Zittel (Munich and Leipzig, 1899), the final life-work of this great authority, translated into English in part by Maria M. Ogilvie-Gordon, entitled " History of Geology and Palaeontology to the end of the 19th Century." The succession of life from the earliest times as it was known at the close of the last century was treated by the same author in his Handbuch der Palciontologie (5 vols., Munich and Leipzig, 1876-1893). Abbreviated editions of this work have appeared from the author, Grundziige der Pal¢ontologie (Palaeozoologie) (Munich and Leipzig, 1895, 2nd ed., 1903), and in English form in Charles R. Eastman's TextBook of Palaeontology (1900-1902). A classic but unfinished work describing the methods of invertebrate palaeontology is Die Stamme des Thierreichs (Vienna, 1889), by Melchior Neumayr. In France admirable recent works are Elements de Paleontologie, by Felix Bernard (Paris, 1895), and the still more recent philosophical treatise by Charles Deperet, Les Transformations du monde animal (Paris, 1907). Huxley's researches, and especially his share in the development of the philosophy of palaeontology, will be found in his essays, The Scientific Memoirs of Thomas Henry Huxley (4 vols., London, 1898-1902). The whole subject is treated systematically in Nicholson and Lydekker's A Manual of Palaeontology (2 vols., Edinburgh and London, 1889), and A. Smith Woodward's Outlines of Vertebrate Palaeontology (Cambridge, 1898).

Among American contributions to vertebrate palaeontology, 'the development of Cope's theories is to be found in the volumes of his collected essays, The Origin of the Fittest (New York, 1887), and The Primary Factors of Organic Evolution (Chicago, 1896). A brief summary of the rise of vertebrate palaeontology is found in the address of O. Marsh, entitled " History and Methods of Palaeontological Discovery " (American Association for the Advancement of Science, 1879). The chief presentations of the methods of the American school of invertebrate palaeontologists are to be found in A. Hyatt's great memoir " Genesis of the Arietidae " (Smithsonian Contr. to Knowledge, 673, 1889), in Hyatt's Phylogeny of an Acquired Characteristic " (Philosophical Soc. Proc., vol. xxxii. 1894), and in Geological Biology, by H. S. Williams (New York, 1895).

In preparing the present article the author has drawn freely on his own addresses: see H. F. Osborn, " The Rise of the Mammalia in North America " (Proc. Amer. Assn. Adv. Science, vol. xlii., 1893), " Ten Years' Progress in the Mammalian Palaeontology of North America " (Comptes rendus du 6 e Congres intern. de zoologie, session de Bern, 1904), " The Present Problems of Palaeontology " (Address before Section of Zool. International Congress of Arts and Science, St Louis, Sept. 1904), " The Causes of Extinction of Mammalia " (Amer. Naturalist, xl. 769-795,829-859,1906).

(H. F. 0.)


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