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)1 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 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.2 The bold conception of the connection between these thirteen highly diversified orders and a simple ancestral form of diaptosaur, typified by the Permian Palæohatteria or the surviving Hatteria (tuatera of New Zealand), we owe chiefly to the genius of Baur,3 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 is 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 1 H. F. Osborn, Reclassification of the Reptilia, American Naturalist, Feb. 1904, pp. 93-115. For the Diaptosauria, see Osborn on The Reptilian Subclasses Diapsida and Synapsida, Memoirs, American Museum of Natural History, vol. 1, pt. VIII, Nov. 1903, p. 467 et seq. 2 H. F. Osborn, Reconsideration of the Evidence for a Common Dinosaur-Avian Stem in the Permian, American Naturalist, vol. XXXIV, no. 406, Oct. 1900, pp. 777-799. G. Baur, On the Phylogenetic Arrangement of the Sauropsida, Journal of Morphology, vol. 1, Sept. 1887, pp. 99-100. E. Fraas, Die Meer-Crocodilier (Thalattosuchia) des oberen Jura, Paleontographica, Bd. XLIX, Stuttgart, 1902. 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.2 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 centres 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 phase, in fact, is that obtained from the two surviving monotremes phases is fragmentary in the extreme. Our sole light on the first of the Australian region; from this extremely reptilian and egglaying 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- vol. x, no. 465, March, 1903, pp. 119-121. S. W. Williston, The Relationships and Habits of the Mosasaurs, Journal of Geology, vol. XII, no. 1, Jan.-Feb. 1904. See note 2, p. 581. 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 3), through the later Cretaceous and entire Tertiary a richly diversified fauna evolves, partly imitating the placentals and partly inventing new and more or less peculiar forms of mammals, such as the kangaroo. 4 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 it 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, T. H. Huxley, On the Application of the Laws of Evolution to the Arrangement of the Vertebrata, and more particularly of the Mammalia, Proceedings of the Zoological Society, London, pp. 649–662. 2 L. Dollo, Les Ancêtres des Marsupiaux étaient-ils arboricoles, Miscellanées Biologiques, 1899, Paris, pp. 188-203; Le Pied du Diprotodon et l'Origine arboricole des Marsupiaux, Bulletin Scientifique de la France et de la Belgique, 1900, pp. 275-280. B. A. Bensley, On the Evolution of the Australian Marsupialia, Transactions of the Linnæan Society, London, 2d ser. Zoology, vol. Ix, pt. 3, 4to, London, 1903. That is, the Multituberculata, Creodonta, Tillodontia, Condylarthra, Amblypoda. 5 Fl. Ameghino, Mammifères crétacés de l'Argentine, Bollettino del Instituto Geografico Argentina, tomo XVIII, 1897, p. 117; Notices Préliminaires sur des Mammifères Nouveaux des Terrains Crétacés de Patagonie, Boletin de l'Academia Nacional de Ciencias de Cordoba, tomo XVII, 1902, pp. 5-68. H. F. Osborn, Ten Years' Progress in the Mammalian Paleontology of North America, Comptes Rendus, Congrès Internationale de Zoölogie, Bâle, 1905. 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 fauna 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 halfway 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.1 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 feature of recent of the Princeton University Expeditions to Patagonia, 1896-99, 4to, Princeton, For a series of monographs on the South American fossil faunas, see Reports N. J. For the Australian element in the South American faunas see Moreno, Note on the Discovery of Miolania. in Patagonia, Nature, Aug. 24, 1899, p. 396; H. x1, April 13, 1900, pp. 564-566. Sinclair, M.J., The Marsupial fauna of the Santa Cruz Beds, Proceedings of the American Philosophical Society, vol. XLIX, 1905, pp. 73-81. paleontological progress has been the revelation of what was taking place in Africa at the same time (Andrews 1 and Beadnell). This discovery came with its quota of unthought-of forms, also with the representatives 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. 1 C. W. Andrews, in Geological Magazine for 1900, 1901, 1902, 1903, 1904, in Annals and Magazine of Natural History, 1903, p. 115; in Proceedings of the Zoological Society, London, 1902, p. 228; in Proceedings of the Royal Society, vol. 71, p. 443; in Philosophical Transactions, ser. B, vol. 196, 1903, p. 99; in Publications of the Survey Department, Cairo, Egypt. 2 H. F. Osborn, The Geological and Faunal Relations of Europe and America.... and the Theory of the Successive Invasions of an African Fauna, Science, N. S. vol. XI, no. 276, pp. 561-574, April 13, 1900. |