Popular Science Monthly/Volume 66/January 1905/The Present Problems of Paleontology
| THE PRESENT PROBLEMS OF PALEONTOLOGY.[1] |
By HENRY FAIRFIELD OSBORN,
DA COSTA PROFESSOR OF ZOOLOGY, COLUMBIA UNIVERSITY. CURATOR OF VERTEBRATE PALEONTOLOGY, AMERICAN MUSEUM OF NATURAL HISTORY.
GEOLOGIST AND PALEONTOLOGIST, UNITED STATES GEOLOGICAL SURVEY.
I CONGRATULATE myself that it has fallen to my lot to set forth some of the chief contemporary problems of paleontology, as well as to make an exposition of the prevailing methods of thought in this branch of biology. At the same time I regret that I can cover only one-half of the field, namely, that of the paleontology of the vertebrates. From lack of time and of the special knowledge required to do a great subject justice I am compelled to omit the science of invertebrate fossils and the important biological inductions made by the many able workers in this field. There is positively much in common between the inductions derived from vertebrate and invertebrate evolution and I believe a great service would be rendered to biology by a philosophical comparison and contrast of the methods and results of vertebrate and invertebrate paleontology.
The science of vertebrate fossils is in an extremely healthy state at present. The devotees of the science were never more numerous, never more inspired and certainly never so united in aim as at present. We have suffered some heavy personal losses, not only among the chiefs, but among the younger leaders of the science in recent years; Cope, Marsh, Zittel, Kowalevsky, Baur and Hatcher have gone, but they live in their works and their influence, which vary with the peculiar or characteristic genius of each.
As in every other branch of science, problems multiply like the heads of hydra; no sooner is one laid low than a number of new ones appear; yet we stand on the shoulders of preceding generations, so that if our philosophical vision be correct we gain a wider horizon, while the horizon itself is constantly expanding by discovery.
In discovery the chief theater of interest shifts from continent to continent in an unexpected and almost sensational manner. In 1870, all eyes were centered on North America and especially on Rocky Mountain exploration; for many ensuing years, new and even unthought of orders of beings came to the surface of knowledge, revolutionizing our thought, firmly establishing the evolution theory and appearing to solve some of the most important problems of descent. Then the stage shifted to South America, where an equally surprising revelation of unthought of life was made. We were in the very midst of the more thorough examination of this Patagonian and Pampæan world when the scene of new discovery suddenly changed to North Africa—previously the 'dark continent' of paleontology—and again a complete series of surprises was forthcoming. Each continent has solved its quota of problems and has aroused its quota of new ones. Now we look to central and South Africa, to the practically unknown eastern Asia, and possibly to a portion of the half sunken continent of Antarctica for a future stock of answers and new queries.
Rapid exploration and discovery, however, are not the only symptoms of health in a science; we do not aim to pass down to history as great collectors; we must accumulate conceptions and ideas as rapidly a? we accumulate materials; it will be a reproach to our generation if we do not advance as far beyond the intellectual status of Cuvier, Owen, Huxley and Cope as we advance beyond their material status in the way of collections of fossils. We must thoroughly understand where we are in the science, how we are doing our thinking, what we are aiming to accomplish; we must grasp, as the political leader, Tilden, observed, the most important things and do them first.
Paleontology a Branch of Biology.
Let us first cut away any remaining brushwood of misconception as to the position of paleontology among the sciences. I do not wish to quarrel with my superior officers, but I must first record a protest against the fact that in the classification scheme of this congress, in the year of our Lord 1904, paleontology is bracketed as a division of geology. It is chiefly an accident of birth which has connected paleontology with geology; because fossils were first found in the rocks, geology the foster mother was mistaken for the true mother, zoology—a confusion in the birth records which Huxley did his best to correct. The preservation of extinct animals and plants in the rocks is one of the fortunate accidents of time, but to mistake this position as indicative of scientific affinity is about as logical as it would be to bracket the Protozoa, which are principally aquatic organisms, under hydrology, or the Insecta, because of their aerial life, under meteorology. No, this is emphatically a misconception which is still working harm in some museums and institutions of learning. Paleontology is not geology, it is zoology; it succeeds only in so far as it is pursued in the zoological j and biological spirit.
In order to make clear the special rôle of paleontology among the biological sciences and at the same time the grateful services which it is enabled to render to its foster science, geology, as well as to geography, when pursued in a purely biological spirit, let us employ an imaginary problem. Figure to yourselves a continent absolutely unknown in any of its physical features of earth, climate or configuration; let us imagine that from such an unknown continent all the animals and all the plants could be brought into a vast museum, the only condition being that the latitude and longitude of each specimen should be precisely recorded, and let us further imagine a vast number of investigators of the most thorough zoological and botanical training and with a due share of scientific imagination, set to work on this collection. Such an army of investigators would soon begin to restore the geography of this unknown continent, its fresh, brackish and salt-water confines, its seas, rivers and lakes, its snow peaks, its glaciers, its forests, uplands, plains, meadows and swamps; also even the cosmic relations of this unknown continent, the amount and duration of sunshine as well as something of the chemical constitution of the atmosphere and of the rivers and seas. Such a restoration or series of restorations would be possible only because of the wonderful fitness or adaptation of plants and animals to their environment, for it is not too much to say that they mirror their environment.
At the historic period commemorated by this great exposition of St. Louis when Napoleon concluded to sell half a continent to strengthen his armies, it is true that such a solution of a physical problem by biological analysis might have been conceived by the pupils of Buffon, by Napoleon's great contemporaries Cuvier, Lamarck or St. Hilaire, but the solution itself would not have been possible. It has been rendered possible only by the wonderful advance in the understanding of the adaptation of the living to the lifeless forces of the planet. Finally, it is obvious in such a projection of the physical from the purely biological that the degree of accuracy reached will represent the present state of the science and the extent of its approach toward the final goal of being an exact or complete science. The illustrative figure need not be changed when the words paleozoology and paleobotany are substituted for zoology and botany. We still read with equal clearness the physical or environmental changes of past times in the biological mirror, a mirror often unburnished and incomplete, owing to the interruptions in the paleontological records, but constantly becoming more polished as our knowledge of life and its all pervading relations to the non-life becomes more extensive and more profound.
Such an achievement as the reconstruction of a continent would be impossible in paleontology pursued as geology or as a logical subdivision of geology. The importance of the services which paleontology may render geology as time-keeper of the rocks, or which geology may render paleontology, are so familiar that we need not stop to enumerate them. Te emphasize the relation I have elsewhere suggested the phrase, Non paleontologia sine geologic. With other physical sciences paleontology is hardly less intimate; from the physicist it demands time for the evolution of successive waves of organisms, from the geographer it demands continental connections or even whole continents for the passage of land animals and plants. As with geology, what it receives it is ever ready to return in gifts; the new branch of geography, for example, entitled paleogeography, appeals quite as often to the paleontologist as to the geologist for its data.
Problem of the Origin of Fitness.
Naturally the central thought of paleontology as biology is the origin of fitness as the property which above all others distinguishes the living from the non-living. Here the paleontologist enjoys the peculiar advantage of being present at the birth of new characters and watching the course of their development; and to this advantage is attached the peculiar responsibility of observing the birth and course of development of such characters with the utmost accuracy and a mind free from prejudices in favor of any particular hypothesis, with full acquaintance with the phenomena of evolution as they present themselves to the zoologist, the botanist and the experimentalist, and with the philosophical temper which will put every hypothesis to the test of every fact. The laughing remark of Cope on seeing a newly discovered specimen which controverted one of his hypotheses, 'if no one were watching I should be glad to throw that fossil out of the window,' has a serious reality in our often unconscious protection of our own opinions.
The birth of new characters is the crucial point in the origin of fitness. With Darwin himself, with Cope, with Bateson, we do not regard the Darwinian law of selection as the creative or birth factor; by its very terms it operates after there is something of value to select. Forgetting this distinction, some naturalists are so blind as to fail to see that selection is still the supreme factor in evolution in the sense that it produces the most grand and sweeping results as well as the most inconspicuous results in the organic world. Certain of the creative factors can not be seen at all by paleontologists; others, in my opinion, can not be seen by zoologists.
Before looking further into the creation of fitness, let us clear away another misconception, which happens to be of paleontological origin, although paleontologists are not responsible for it. It concerns the history of one of the great theories of the day. Many years ago, Waagen, a German paleontologist, observed that the varieties or minor changes in time (chronological varieties) differ from varieties in space (geographical varieties); that the latter have a variable value and are of small systematic importance, while the former are very constant and. though seen only in minute features, may always be recognized again. These varieties in time Waagen termed mutations. In 1891 Scott unearthed this distinction of Waagen's and clearly defined it as the hereditary or phylogenetic change of animals in time. Previous to this Osborn, without knowing of Waagen's statement, had discussed the same facts of the birth of new characters, describing them as 'definite variations.' Cope, it happens, did not follow this line of thought at all; but many other paleontologists did, notably Hyatt, whose peculiar style and multiplicity of terms obscured his depth of thought and extent of observation. Thus the term mutation acquired a definite significance among paleontologists.
It happened that De Vries, the eminent Dutch botanist, reading Scott's paper, mistakenly identified these new characters succeeding each other in time with those which he was observing as occurring contemporaneously in plants, and he adopted Waagen's term for the 'mutation theory,' which he has so brilliantly set forth, of the sudden production of new and stable varieties, from which nature proceeds to select those which are fit.
If paleontologists are correct in their observation, mutations may be figured graphically as an inclined plane, whereas De Vries's phenomena in plants represent a series of steps more or less extensive. Scott expressly excluded the element of discontinuity; and I believe there is no ground whatever for the assertion that the phenomena first named mutations by Waagen and independently observed by many paleontologists, are identical with the phenomena observed by De Vries in plants.
On the contrary, De Vries's facts accord with the favorite hypothesis of St. Hilaire. They demonstrate the law of saltation. This is the inevitable interpretation of the expositions of De Vries himself, of Hubrecht, and of the more recent references of Bateson in his British Association address. That saltation is a constant phenomenon in nature, a vera causa of evolution, no one can longer deny. Bateson shows that it harmonizes with Mendel's conceptions of heredity, and it may be regarded as par excellence the contribution of the experimental method.
Similarly, I regard mutation as a quite distinct phenomenon, and as par excellence the contribution of the paleontological method; it is the gradual rise of new adaptive characters neither by the selection of accidental variations nor by saltation, but by origin in an obscure and almost invisible form, followed by "direct increase and development in successive generations until a stage of actual usefulness is reached, where perhaps selection may begin to operate. While clearly setting forth the difficulties, I at one time attributed definite variation or mutation to Lamarck's principle of the inherited effects of habit as the only assignable cause; subsequently I realized that it was not explainable by the Lamarckian hypothesis.
I then attributed it to an unknown law of evolution, and there I believe it rests to-day, namely: as a process of which we do not know the cause. Still more recently, however, comes the discovery that original kinship is partly at least a control-principle. For example, in the descent of independent stocks of hornless animals arising from a common stock, rudimentary horn cores are found to appear independently in exactly the same region of the skull, indicating a kind of predetermination in the stock, or potential of similar evolution. The facts on which this law of mutation, properly called, rests have been misunderstood, totally denied, or explained away by selectionists as survivals of favorable out of indiscriminate variations. Even my colleague, Scott, has identified these phenomena with the saltations of De S Vries. Nevertheless, I regard the genesis of new adaptive characters from almost imperceptible beginnings as a vera causa, and as one of the greatest problems we have to solve.
That a natural solution will be found goes without saying, although this principle, as stated, is undoubtedly of a teleological nature. Its philosophical bearings are of far reaching importance. Just as we demand a continent to transfer land animals from Australia to South America, so we demand a natural law to explain these facts.
The creative factors of fitness cooperating with selection, which, in my judgment, are now well demonstrated, reside either primarily in the environment, in the bodies of animals, or in the germinal cells—they all ultimately find their way into the germinal cells. They may be summarized as follows:
1. Segregation.—Besides the familiar geographical segregation of animals, which reaches its highest expression in insular forms, such as the pygmy fossil elephants of Malta and those recently discovered in Cyprus (Wade), there is the no less effective segregation of habit among animals existing in the same geographical regions and under the same climatic conditions, but seeking different varieties of food on different kinds of soil. These give rise to what I have called local adaptive radiations, a principle which explains the occurrence in the same country, and almost side by side, of very conservative as well as very progressive forms.
2. Adaptive Modification.—This is a plastic principle which tends in the course of life to an increasing fitness of the bodies of individuals to their special environments and habits, well illustrated among men in the influence of various trades and occupations and operating both in active and in passive structures. Consistent with the adaptive modification principle is the fact that every individual requires habit and environment to model it into its parental form; and in every change of environment or habit every individual is carried an infinitesimal degree beyond the parental form; the wonderful phenomena of correlated development which puzzled Spencer so much are chiefly attributable to this principle.
These adaptive modifications are not directly inherited, as Lamarck
(supposed, but acting through long periods of time there results the organic selection (Morgan, Baldwin, Osborn), of those individuals in which hereditary predisposition happens most closely to coincide with adaptive modification, and there thus finally comes about an apparent, but not real, inheritance of acquired characters, as Lamarck, Spencer and Cope supposed.
3. Variations of Degree.—We should by no means exclude as true causes of evolution associated with both the above factors, the selection of those variations of degree or around a mean which conform to Quetelet's curve, the subject of the chief investigations of the Galton school, of Pearson and of Weldon, and which form the strongest remaining ground for Darwin's theory of selection in connection with fortuitous variation. For example, I regard the appearance of long-necked giraffes, of slender-limbed ruminants and horses, of long-snouted aquatic vertebrates, as instances of the selection of variations around a mean rather than of the selection of saltations. The selection of such variations where they happen to be adaptive has been an incessant cause of evolution.
4. Saltation.—Although Geoffroy St. Hilaire argued for paleontological evolution by saltation, I do not think we have much evidence in paleontology for the saltation theory. In the nature of the case, we can not expect to recognize such evidence even where it may exist, because wherever a new form appears or a new character arises, as it were, suddenly, we must suspect that this appearance is due to absence of the connecting transitional links to an older form. The whole tendency of paleontological discovery is to resolve what are apparently saltations or discontinuities into processes of continuous change. This, however, by no means precludes saltation from being a vera causa in past time, as rising from 'unknown' causes in the germ cells and as forming the materials from which nature may select the saltations which are adaptive from those which are inadaptive. The paleontologist has every reason to believe that he finds saltations in the sudden variations in the number of vertebra? of the neck, of the back, of the sacral region, for example. In the many familiar cases of the abbreviation or elongation of the vertebral column in adaptation to certain habits, a vertebra in the middle of a series can not dwindle out of existence, it must suddenly drop out or suddenly appear.
5. Mutation.—These new characters are also germinal in origin, because they appear in the teeth, which are structures fully formed beneath the surface before they pierce the gum, and therefore not subsequently modeled by adaptive modification, as the bones, muscles and all the other tissues of the body are. Mutations are found arising according to partly known influences of kinship. They do not, so far as we observe, possess adaptive value when they first appear, but then frequently, if not always, develop into a stage of usefulness.
Fitness is, therefore, the central thought of modern paleontology in its most comprehensive sense, as embracing fitness in the very remote past, in its evolution toward the present and in its tendencies for the future. Just as the uniformitarian method of Lyell transformed geology, so the uniformitarian method is penetrating paleontology and making observations of animal and plant life as it is to-day the basis of the understanding of animal and plant life as it was from the beginning. Here again paleontology is not merely an auxiliary to zoology; it is chief of a division and enjoys certain unique advantages. We pass in review with the pedigrees and the prodigies of fitness, the entirely unreasonable, irrational, paradoxical extremes of structure, such, for example, as the pterosaurs, which far surpass in boldness and ingenuity of design any of the creations of the modern yacht builder which are mistakenly regarded by some as having reached an absurd extreme.
Problem of Historical Study.
The paleontologist must also be a historian; he has to deal with lineage, with ancestors, he comes directly upon the problem of kinship or relationship, and he has to determine the various means of distinguishing the true from the apparent relationships. It happens that fitness, while fascinating in itself, has led even the most faithful and skilful into the most devious paths away from the truth. The explanation of this apparent contradiction is in this wise. The ingenuity of nature in adapting animals is astounding, but it is not infinite; the same devices are resorted to repeatedly to accomplish the same purposes. In the evolution of long-snouted rapacious swimming forms, for example, we have already discovered that nature has repeated herself twenty-four times in employing the same processes to accomplish the same ends in entirely different families of animals.
This introduces us to one of the two great ideas which we must employ in the interpretation of facts, namely, the idea of analogy. We see far more clearly than Huxley did the force of this idea. Owen, Cope, Scott, Fraas and many others', under the terms 'parallelism' 'convergence,' 'homoplasy' have developed the force of the old Aristotelian notion that analogy is a similarity of habit, and that in the course of evolution a similarity of habit finally results in a close or exact similarity of structure; this similarity of structure is mistaken as an evidence of kinship. Analogous evolution does not stop in its far reaching consequences with analogies in organs; it moulds animals as a whole into similar form, as, for example, the ichthyosaurs, sharks and dolphins; still more it moulds similar and larger groups of animals into similar lines or radii of specialization. Thus we reach the grand idea of analogy as operating in the divergencies or adaptive radiations of groups, according to which great orders of animals tend in their families and suborders to minic other orders, and the faunæ or collective orders of continents to mimic the faunæ of other continents.
Amid this repetition on a grand scale of similar adaptations, which is altogether comparable to what we know as having occurred over and over again in human history, the paleontologist as a historian must keep constantly before him the second great idea of homogeny, of real ancestral kinship, of direct blood descent and hereditary relationship. The shark and the ichthyosaur superficially look alike, but their germ cells are radically different, their external resemblances are a mere veneer of adaptation so deceptive, however, that it may be a matter of half a century before we recognize the wolf beneath the clothing of the sheep, or the ass in the lion's skin.
These two great ideas of analogy or similarity of habit, and homogeny or similarity of descent, do not run on the same lines; they are the woof and the warp of animal history. Analogy corresponds to the woof or horizontal strands which tie animals together by their superficial resemblances in the present, homogenies are the warp, or the fundamental vertical strands which connect animals with their ancestors and their successors. The far reaching extent of analogous revolution was only dimly perceived by Huxley, and constituted his one great defect as a philosophical anatomist. Its power of transforming unlike and unrelated animals has accomplished miracles in the way of producing a likeness so exact that the inference of kinship is almost irresistible.
The paleontologist who would succeed as historian must first, therefore, render himself immune to the misguiding influences of analogy by taking certain further precautions which will now be explained by watching his procedure as historian.
Paleontology as the history of life takes its place in the background of recorded history and archeology, and simply from the standpoint of the human pedigree is of transcendent interest. Although it has progressed far beyond the dreams of Darwin and Huxley, the first general statement which must be made is that the actual points of contact between the grand divisions of the animal and plant kingdom, as well as between the lesser and even many of the minor divisions, have yet to be discovered. You recall that the older grand divisions of the Vertebrata, to which we must confine our attention, were suggested by the so-called Ages of Fishes, of Amphibians, of Reptiles and of Mammals. Even within these grand divisions we observe a succession of more or less closely analogous groups. Each of these groups has its more or less central starting point in a smaller and older group which contains a large number of primitive or generalized characters.
The search for the primitive central form is always made by the same method of reasoning, a method which was first clearly outlined by Huxley, namely, by the more or less ideal reconstruction of the primitive central form from which radiation has occurred. This is a very difficult matter where the primitive central form is not preserved either living or as a fossil. In such instances we may by analysis of all the existing forms prophesy the structure of the primitive central form, as Huxley, Kowalevsky and Cope did in the case of the hoofed animals, a prophecy which was nearly fulfilled by the discovery in northern Wyoming of Phenacodus. In other more fortunate cases the primitive central form survives both living and fossil, as in the remarkable instance of Paleohatteria of the Permian and the Tuatera lizard (Hatteria) of New Zealand, which gave rise to the grand adaptive radiation of the lizards, mosasaurs, dinosaurs, crocodiles, phytosaurs and probably of the ichthyosaurs.
In the reconstruction of these primitive central forms, we must naturally discriminate between analogy and homogeny, and paleontologists are not agreed in all cases on such discrimination. On the border region, in fact, where the primitive central forms are still unknown, where analogy has reached its most perfect climaxes and imitations, are found the great paleontological controversies of to-day. For example, among the paleozoic fishes, the armored ostracoderms (Pteraspis, Cephalaspis, Pterichthys) and the arthognaths (Coccosteus, Dinichthys) by some authors (Hay, Regan, Jaekel) are placed in the single group of placoderms, while by other authors (Smith Woodward and Dean) they are regarded as entirely independent and superficially analogous groups. The dipnoi or lung fishes (Ceratodus, Protopterus) present so many analogies with the Amphibians (salamanders and frogs) that they were long regarded as ancestors of the latter; but more searching anatomical and paleontological analyses and recent embryological discoveries have proved that the dipnoi and amphibia are parallel analogous groups descended alike from the crossopterygian fishes, fishes which are now represented only by the bichir (Polypterus) of Africa. It is interesting to recall parenthetically that two naturalists, Harrington, an American, and Budgett, an Englishman, have given their lives to the solution of this problem in searching for the embryology of Polypterus. The latter explorer only was successful.
Missing Links between the Great Classes of Vertebrates.
Among the varied fins of the crossopterygians we have nearly, but not actually, discovered the prototype of the hand and the foot, the fingers and toes of the primordial amphibian. Volumes upon volumes have been written by embryologists and comparative anatomists on the hypothetical transformation of the fin into the hand. Considering the supreme value of the hand and foot in vertebrate history, this was certainly the most momentous transformation of all and worthy of volumes of speculation; but as a matter of fact, the speculation has been a total failure, and this problem of problems will only be settled by the future discovery in Devonian rocks of the actual connecting link, which will be a partly air-breathing fish, capable of emerging upon land, in which the cartilages of the fin will be found disposed very much as in the limbs of the earliest Carboniferous amphibians. The unity of composition in the hand and the foot points to an original similarity of habit in the use of these organs.
This missing point of contact, or of the actual link between amphibians and fishes, is equally characteristic of paleontology as history from the top to the bottom of the animal scale. We are positive that Amphibians descended from fishes, probably of the crossopterygian kind, but the link still eludes us; we have brought the reptiles within close reach of the amphibians, but the direct link is still to be found; mammals are in close proximity to a certain order of reptiles, but the connecting form is still undiscovered; man himself is not far from the various types of anthropoid apes, but his actual connecting relationship is unknown.
We are no longer content, however, with these approaches to actual contact and genetic kinship, we have toiled so long both by discovery and by the elimination of one error after another, and are so near the promised land, we can hardly restrain our impatience. I venture to predict that the contact of the Amphibia with the fishes will be found either in America or Europe. No such prediction could be safely made regarding the connecting form between the amphibians and reptiles, because America, Eurasia and Africa all show in contemporaneous deposits evidence that such connection may be discovered at any time. The transformation from reptiles to birds will probably be -found in the Permian of America or Eurasia; chances of connecting the mammals with the reptiles are decidedly brightest in South Africa; while in Europe, or more probably in Asia, we shall connect man with generalized catarhine primates.
Passing from these larger questions of the relations of the great classes of vertebrates to each other, let us review the problems arising in the individual evolution of the classes themselves.
Geographical Problems.
The primordial, solid-skulled or stegocephalian amphibia of the Permian diverged into a great variety of forms which wandered over Eurasia and North America so freely that, for example, we find as close a resemblance between certain Würtemberg and New Mexican genera (Metopias) as between the existing stag of Europe and the wapiti deer. Which branch of these primordial amphibians gave rise to the modern frogs and salamanders we do not know. This and hundreds of similar facts suggest the vital importance of paleogeography.
As regards paleogeography, the great induction can be made that, throughout the whole period of vertebrate evolution and until comparatively recent times, Europe, Asia and North America constituted one continent and one life region, or Arctogæa (Huxley 1868, Blanford 1890), with which the continents of the southern hemisphere, namely, Africa, South America and Australia, were intermittently, but not continuously connected by land. A great southerly continent, Notogæa (Huxley 1868), connected with a south polar Antarctica, now submerged, is a theory very widely supported by zoologists and, I believe, by botanists, although its existence is still denied by certain geographers (Murray). We find Permian, Jurassic, late Cretaceous and early Tertiary proofs of Antarctica in the fresh-water crustaceans (Ortmann), in fresh-water fishes (Gill), in littoral mollusca (Ortmann), in reptiles (Smith Woodward and Osborn), in birds (Forbes and Milne Edwards), in worms (Beddard), in the Australian animals (Spencer), in the fossil mollusca of Patagonia (Ortmann) and in the fossil mammals of Patagonia (Ameghino). To marshal and critically examine all this evidence and convert this most convenient Antarctic hypothesis into an established working theory I consider one of the most pressing problems of the day.
Problem of the Source of the Reptiles and Mammals.
Returning from this geographical detour to paleontology as history, we should first note that already in the Permian there was developed such an astonishing variety and differentiation of the reptiles that we must look to future discoveries in the Carboniferous to find the actual points of descent of reptiles from the amphibia. These Permian and Lower Triassic reptiles are of three kinds, comparable to a parent (Cotylosauria) and two offspring (Anomodontia and Diaptosauria). In the parent group (the Cotylosauria, or solid-skulled reptiles,) we find so many fundamental similarities to the Stegocephalia, or solid-skulled amphibia, that only by the possession of many parts of the body can we surely ditinguish reptile from amphibian remains. The primordial reptile was probably altogether a land animal continuously using its limbs in awkward progression, bringing forth its young by land-laid eggs and probably possessing gills only as vestiges. These cotylosaurs show very wide geographical distribution, South Africa, Siberia, Great Britain and North America, and equally remarkable adaptive radiations of habit into small and large, horned and hornless types, some of which were certainly dying out branches, while others led into the two offspring groups.
Leaving this parental order, in the Permian and Lower Trias, we first see in the older offspring, the Anomodontia, reptiles of varied size and description, carnivorous and herbivorous in habit, most abundantly found in South Africa, in Asia and in Europe, and not at all as yet in America, either North or South. The high degree of fitness for different habits, or radiation, of the Anomodonts is distinguished from that of any other reptiles at any time by its numerous analogies to the radiation of the mammals, namely, into very large and very small forms, into carnivorous and herbivorous, into terrestrial and possibly into aquatic types; in fact, some of these animals, if seen on land to-day might readily be mistaken for mammals.
The second offspring of the Cotylosauria, on the contrary, the Diaptosauria, are essentially and unmistakably saurians; that is, if seen about us to-day they would undoubtedly at first be described as lizards. They were still more broadly cosmopolitan in range, being scattered over both Americas (Pelycosauria, Proganosauria), Europe (Protorosauria, Rhynchosauria), Asia (Rhynchosauria) and Africa (Proganosauria, Rhynchosauria). They are also found highly diversified in type, but all their analogies of fitness are with the reptiles and not with the mammals. It is of prime importance that more of these diaptosaurs be found, and that those already known in the museums should be more critically examined. What we already know, however, enables us to establish the following facts: first, that the parentage of these animals is more probably among the cotylosaurs than among the anomodonts, and second that already in the Permian they had formed a sufficiently large number of branches to be regarded as a fully evolved radiation.
Problem of the Adaptation of the Mesozoic Reptiles.
In the Triassic the offspring of the anomodonts and of the diaptosaurs appear as the third generation from the cotylosaurs.
The recurrent difficulty arises that the actual points of contact or transition from the anomodonts are wanting, and we must continue to reason by the ideal reconstruction of the hypothetical linking forms. Such reasoning connects the Testudinata (turtles and tortoises), the Sauropterygia, or marine plesiosaurs, and, singularly enough, our own ancestors, the primordial mammals, with the group of anomodonts and not at all with that of the diaptosaurs. Here in the Upper Permian and Lower Trias we must await both discovery and the closest critical analysis, but if this still hypothetical affiliation be confirmed by discovery, as I personally am sanguine it will be, then it will be true to say that the mammals, and hence man, are much more nearly affiliated to the anomodonts than to either the lizards or snakes, which are both on the great Diaptosaur branch. Our presence on the great Anomodont branch and remoteness from the creeping and crawling reptiles will perhaps afford some consolation to those who still shrink from the ultimate consequences of Darwin's 'Descent of Man' As regards degrees of probability, it must be said that while the affiliation of the Plesiosaurs and Testudinata with the Anomodont group still requires confirmation, the connection of the mammals with certain Anomodonts (Theriodontia) is not only probable but is almost on the verge of actual demonstration, and at present it seems likely that the Karoo Desert of South Africa will enjoy the honor of yielding the final answer to the problem of the origin of mammals, which has has stirred comparative anatomists for the last sixty years.
Turning to the progeny of the other branch, the Permian diaptosaurs, we find them embracing (with the exception of the Testudinata and plesiosaurs) not only vast reptilian armies, marshaling into thirteen orders, mastering the distinctive Age of Reptiles (Triassic, Jurassic and Cretaceous), and surviving in the four existing orders of lizards, snakes, crocodiles and tuateras, but we also find them giving off the birds as their most aristocratic descendants. The bold conception of the connection between these thirteen highly diversified orders and a simple ancestral form of diaptosaur, typified by the Permian Palæoliatteria or the surviving Hatteria (tuatera of New Zealand) we owe chiefly to the genius of Baur, a Bavarian by birth, an American by adoption. Absolutely diverse as these modern and extinct orders are, whatever material for analysis we adopt, whether paleontological, anatomical or embryological, the result is always the same—the reconstructed primordial central form is always the little diaptosaurian lizard. The actual lines of connection, however, are still to be traced into the great radiations of the Mesozoic.
The chief impression derived from the survey of this second branch of the Reptiles in the Mesozoic as a whole is again of radiations and subradiations from central forms and the frequent independent evolution of analogous types. The aquatic life had been already chosen by the plesiosaurs and by some of the turtles, as well as by members of three diaptosaur orders (Proganosauria, Choristodera, certain Rhynchocephalia), two of which were surviving in Jurassic times. Yet it in independently again chosen by four distinct Triassic orders, always beginning with a fresh-water phase (Parasuchia, Crocodilia), and sometimes terminating in a high sea phase (Ichthyosauria, Mosasauria, Crocodilia). In the Jurassic period there were altogether no less than six orders of reptiles which had independently abandoned terrestrial life and acquired more or less perfect adaptation to aquatic life. Nature, limited in her resources of outfitting for aquatic life, fashioned so many of these animals into like form, it is small wonder that only within the last two years have we finally distinguished all the similarities of analogous habit from the similarities of real kinship.
The most conservative members of this second branch are the terrestrial, four-footed, persistently saurian or lizard-like forms, the tuateras and the true lizards; but from these types again there radiated off one of the marine orders (Mosasauria), the limbless snakes (Ophidia), while the lizards themselves have in recent times diverged almost to the point of true ordinal separation.
The most highly specialized members of this second branch are of course the flying pterosaurs, of whose ancestry we know nothing. Also in a grand division by themselves there evolved the dinosaurs, distinctively terrestrial, ambulatory, originally carnivorous, and probably more or less bipedal animals. Not far from the stem of the dinosaurs was also the source of the birds, also distinguished by bipedalism.
The working plan of creation becomes day by day more clear; it is, that each group, given time and space, will not only be fruitful and multiply, but will diversify in the search for every form of food by every possible method. Specialization in the long run proves fatal; the most specialized branches die out; the members of the least specialized branches become the centers or stem forms of new radiations.
The Mammals of Four Continents.
So it is among the mammals, in which these principles find new and beautiful illustrations, although our knowledge of the early phases is fragmentary in the extreme. Our sole light on the first phase, in fact, is that obtained from the two surviving monotremes of the Australian region; from this extremely reptilian and egg-laying monotreme phase it appears, although opinion is divided on this point, that before the Jurassic period (i. e., already in the Trias) two branches were given off, the placental, from which sprang all the modernized mammals and the marsupial.
The marsupials appear to have passed through an arboreal or tree life condition, something similar to that seen in the modern opossum. The marsupials found their opportunity for unchecked adaptive radiation in Australia and despite the disadvantage of starting from a specialized arboreal type (Huxley, Dollo, Bensley), through the later Cretaceous and entire Tertiary a richly diversified fauna evolves, partly imitating the placentals and partly inventing entirely new and very peculiar forms of mammals, such as the kangaroo.
The oldest placental radiation which is fully known is that which was first perceived in Europe and fully recognized by the discovery in 1880 of the basal Eocene mammals of North America—it may be called the Cretaceous radiation. These mammals are distinctly antique, small-brained, clumsily built, diversified, imitative both of the marsupial and of the subsequent placental radiations; and our fuller knowledge of them after twenty-five years of research is at once satisfying and disappointing, satisfying because it gives us prototypes of the higher or modern mammals, disappointing because few if any of these prototypes connect with the modern mammals. This fauna is found in the Cretaceous and basal Eocene of Europe, North America, and possibly in Patagonian beds of South America (Ameghino), and while giving rise to many dying-out branches, by theory furnished the original spring from which the great radiations of modern mammals flowed. But practically again we await the direct connections and the removal of many difficulties in this theory. In fact, one of the great problems of the present day is to ascertain whether this radiation of Cretaceous mammals actually furnished the stock from which the modern mammals sprang, or whether there was also some other generalized source.
The Tertiary, or Age of Mammals, presents the picture of the dying out of these Cretaceous mammals in competition with the direct ancestors of the modern mammals. I use the word modern advisedly, because even the small horses, tapirs, rhinoceroses, wolves, foxes and other mammals of the early Tertiary are essentially modern in brain development and in the mechanics of the skeleton as compared with the small brained, ill-formed and awkward Cretaceous mammals.
Whatever the origin, two great facts have been established: first, the modern mammals suddenly appear in the Lower Eocene (as distinguished from the basal Eocene, in which the Cretaceous mammals are found), and second, they enjoy a more or less independent evolution and radiation on each of the four great continents. There thus arose the four peculiar or indigenous continental faunæ of South America, of North America, of Europe and Asia or Eurasia and of Africa. Of these South America was by far the most isolated and unique in its animal life. North America and Eurasia were much the closest, and Africa acquired a half way position between isolation and companionship with Eurasia.
South America.—The most surprising result of recent discovery is that the foreign element mingled with the early indigenous South American fauna is not at all North American but Australian. The wonderful variety of eight orders of indigenous rodents, hoofed animals, edentates and other herbivores were preyed upon by carnivores of the marsupial radiation from Australia, which apparently came overland by way of Antarctica. There are possibly here also some South African foreigners. The South American radiation more or less closely imitated that of the northern hemisphere. Late in Tertiary times North America exchanged its animal products with South America, practically to the elimination of the latter.
Eurasia and North America.—Each of these continents contained four orders of mammals in common with South America, namely, the Primates (monkeys), the Insectivores (moles and shrews), the Rodents (porcupines, mice, etc.), and the Edentates (armadillos, etc.). From some early Tertiary source North America, Eurasia and Africa also acquired in common four great orders of mammals which are not found at all in the indigenous fauna of South America. These are the Carnivores (dogs, cats, etc.), the Artiodactyls (deer, bovines, camels, and pigs), the Perissodactyla (horses, rhinoceroses and tapirs), and the Cheiroptera (bats). Migration and animal intercommunication between North America and Eurasia was very frequent. The history of these nine orders of mammals in North America and Eurasia developed as follows: Certain families indigenous to North America both evolved and remained here, others finally migrated into Europe and South America. Similarly Eurasia had its continuous evolution into forms which remained at home as well as into those which finally migrated into North America and even into South America.
Africa.—The most astonishing and gratifying features of recent paleontological progress has been the revelation of what was taking place in Africa at the same time (Andrews and Beadnell). This discovery came with its quota of unthought of forms, also with the representative of three orders which it had been prophesied would be found there, namely, the Proboscidea (elephants and mastodons), the Sirenia (manatees and dugongs), and the Hyracoidea (conies). The basis of this prophecy was the anomalous fact that these animals suddenly appeared in Europe in the Miocene and Pliocene fully formed and without any ancestral bearings; it was certain that they had evolved somewhere, and Africa seemed the most probable home, rather than the currently accepted unknown regions of Asia. Thus by a sudden bound paleontology gains the early Tertiary pedigree of the elephants and of two if not three other orders.
Africa in the early Tertiary, whether from the absence of land connections or from climatic barriers, was a very independent zoological region. Some predatory Cretaceous mammals (Creodonta or primitive Carnivores) found their way in there, also certain peculiar artiodactyls (Hyopotamids). Here also were two remarkable types of mammals (Arsinoitherium, Barytherium) which have no known affinities elsewhere, as well as the extremely aberrant Cetaceans or Zeuglodonts.
The Outlook.
From all these continents we have, therefore, finally gathered the main history during the Tertiary period of eighteen orders of mammals. We have still to solve the origin of the cetaceans or whales, still to connect many of these orders which we call 'modern' with their sources in the basal Eocene and Upper Cretaceous, still to follow the routes of travel which they took from continent to continent. Encouraged by the prodigious progress of the past twenty-five years, we are confident that twenty-five years more will see all the present problems of history solved, and judging by past experience we may look for the addition of as many new and no less important ones.
- ↑ Address delivered before the Section of Zoology of the International Congress of Arts and Science, September 22, 1904.