Sept. 27, 1877] 



NA TURE 



471 



from the chalk of Kansas. The two genera Hesperoritis and 

 Ichthyortiis are types of distinct orders, and differ from each 

 other and from Archicopteyyx much more than do any exis'ing 

 birJs among themselves, thus showing that birds are now a 

 closed type, and that the key to the history of the class must be 

 sought for in the distant past. 



In Hcsperoniis we have a large aquatic bird, nearly six feet in 

 length, with a strange combination of characters. The jaws are 

 provided with teeth, set in grooves ; the wings were rudimentary, 

 and useless, while the legs were very simdar to those of modern 

 diving birds. This last feature was merely an adaptation, as the 

 more important chaiacters are struthious, showing that Hesper- 

 ornts was essentially a carnivorous swimming ostrich. Ichthy- 

 oi-ms, a small flying bird, was stranger still, as the teeth were in 

 sockets, and the vt rtebras biconcave, as in fishes and a few rep- 

 tiles. Apafornis and all other allied forms occur in the same 

 beds, and probably all were provided with teeth. It is strange 

 that the companions of these ancient toothed birds shnuld have 

 been pterodactyls without teeth. In the lattr ere aceous beds 

 of the Atlantic coast various remains of aquatic birds have been 

 found, but all are apparently distinct from those of the west. 



During the tertiary period birds were numerous in this country, 

 and all yet discovered appear to have belonged to modern types. 



It is now generally admitted by biologists who have made a 

 study of the vertebrates, that biri^s have come down to us 

 through the dinosaurs, and the close affinity of the latter wdth 

 recent struthious birds will hardly be questioned. The case 

 amounts almost to a demonstration, if we compare, with dino- 

 saurs, their contemporaries, the mesozoic birds. The classes of 

 birds and reptiles, as now living, are separated by a gulf so pro- 

 found, that a few years since it was cited by the opponents of 

 evolution as the most important break in the animal series, and 

 one which that doctrine could not bridge over. Since then, as 

 Huxley has clearly shown, this gap has been virtually filled by the 

 discoverv of bird-like reptdes and reptilian birds. Compsogtialhus 

 and Arcluroptoyx of the old world, ^x\AIchthyornis and Hesperornis 

 of the ne^v, are the stepping-stones by which the evolutionist of 

 to-day leads the doubting brother across the shallow remnant of 

 th gulf once thought impassable. 



It remains now to consider the highest group of the animal 

 kingdom, the class Mammalia, which includes Man. Of the 

 existence of this class before the trias we have no evidence, 

 either in this country or in the old world, and it is a signficant 

 fact that at essentially the same horizon in each hemisphere, 

 similar low forms of mammals make tfeir appearance. Although 

 only a few incomple'e S' ecinicns have been di-covered, they are 

 characteristic and well preserved, and all are apparently marsu- 

 pials, the lowe-t mamma ian group which we know in tiiis coun- 

 try, living or fossd. The American triassic mammals are known 

 at present only from two small lower jaws, on which is based 

 the genus Z); wOTP/ferK/w, supprjsed to be related to the insect- 

 eating Myrmccobiiis, now living in Aus'ralia. 



Although the Jura of Europe has yielded other similar 

 mammals, we have as yet none of this class from that forma- 

 tion ; while, from rocks of cretaceous age, no mammals are 

 known in any part of the world. 



In the lowest tertiary beds of this country a rich mammalian 

 fauna suddenly makes its appearance, and from that time through 

 the age of mammals to the present, America has been constanily 

 occupied by this type of liie in the greatest diversity of form. 

 Fortunately, a nearly continuous record of this life, as preserved, 

 is now accessible to U';, and ensures great additions to our know- 

 ledge of the genealogy of mammals, and perhaps the solution of 

 more profountl problems. 



The boundary line between the cretaceous and tertiary in the 

 region of the Rocky Mountains has been much in dispute during 

 the last few years, mainly in consequence of the uncertain geolo- 

 gical bearings of the fossil plants found near this horizon. The 

 accompanying invertebra.e fossils have thrown little light on the 

 question, which is essentially whether the great lignite series of 

 the West is uppermost cretaceous, or lowest eocene. The 

 evidence of the numerous vertebrate remains is, in my judgment, 

 decisive, and in favour of the former view. 



This brings up an important point in palaeontology, one to 

 which my attention was drawn several years since, namely, the 

 comparative value of different groups of fossils in marking geolo- 

 gical time. In examining the subject with some care, I found 

 that for this purpose plants, as their nature indicates, are most 

 unsatisfactory witnesses ; that invertebrate animals are much 

 better; and that vertebrates afford the most reliable evidence of 



climatic and other geological changes. The sub-divisir.ns of the 

 latter group, moreover, and in fact all forms of animal life, are of 

 value in this respect, mainly ac<-ording to the perfection of their 

 organisation, or zoological rank. Fishes, for example, are but 

 slightly affected by changes that would destroy reptiles or birds, 

 and the higher mammals succumb under influences that the lower 

 forms pass through in safety. The more special applications of 

 this general law, and its value in geology, will readily suggest 

 themselves. 



The evilence offered by fossil remains is, in the light of this 

 law, conclusive, that the line, if line there be, separating our 

 cretaceous from the tertiary, must at present be rirawn where 

 the dinosaurs and other mesozoic vertebrates disappear, and are 

 replaced by the mammals, henceforth the dominant type. 



It is frequenily asserted, and very generally believed, that the 

 large number of huge Edentata which lived in North America 

 during the post-pliocene, were the results of an extensive migra- 

 tion from South America soon after the elevation of the Isthmus 

 of Panama, near the close of the tertiary. No conclusive proof 

 of such migration has been offered, and the evidence it seems to 

 me, so far as we now have it, is directly opposed to this view. 

 No undoubted tertiary edentates liave yet been discovered in 

 South America, while we have at least two species in our miocene, 

 and during the deposition of our lower pliocene large individuals 

 of this group were not uncommon as far north as the forty-third 

 parallel of latitude, on both sides of the Rocky Mountains. In 

 view of these facts and others which I shall lay before you, it 

 seems more natural to conclude from our present knowledge that 

 the migration which no doubt took place was from north to south. 

 The edentates finding thus in .South America a congenial home 

 flourished greatly for a time, and, although the larger forms are 

 now all extinct, diminutive representatives of the group still 

 inhabit the same region. 



The ungulates are the most abundant mammals in the tertiary, 

 and the most important, since they include a great variety of 

 types, some of which we can trace through their various changes 

 down to the modified forms that represent them to-day. Of the 

 various divisions in this comprehensive group, the perissodactyle, 

 or odd-toed ungulates, are evidently the oldest, and throughout 

 the eocene are the prevailing forms. Although all of the 

 peiissodactyles of the earlier tertiary are more or less generalised, 

 they are still quite distinct from the artiodactyles, even at the 

 base of the eocene. One family, however, the CoryphoJoHlid^e, 

 which is well represented at this horizon, both in America and 

 Europe, although essentially Perissodactyle, possesses some 

 characters which point to a primitive ungulate type from which 

 the present orders have been evolved. Among these characters 

 are the diminutive brain, which in size and form approaches that 

 of the reptiles, and also the five-toed feet from which all the 

 various forms of the mammalian foot have been derived. Of 

 this family, only a single genus, Coryphodon [Bathmodon], is 

 known, but there were several distinct species. They were the 

 largest mammals of the lower eocene, some exceeding in size 

 the existing tapirs. 



In the middle eocene, west of the Rocky Mountains, a 

 remarkable group of ungulates makes its appearance. These 

 animals nearly equalled the elephant in size, but had shorter 

 limbs. The skull was armed with two or three pairs of horn- 

 cores, and with enormous canine tusks. The brain was propor- 

 tionally smaller than in any other land mammal. The feet had 

 five toes, and resembled in their general structure those of 

 Coryphodon, thus indicating some affinity with that genus. 

 These mammals resemble in some respects the perissodactyles, 

 and in others the proboscidians, yet differ so widely from any 

 known ungulates, recent or fossil, that they must be regarded as 

 forming a distinct order, the Duiocerata. 



Besides these peculiar mammals which are extinct, and mainly 

 of interest to the biologist, there were othtrs in the early tertiary 

 which remind us of those at present living around us. When a 

 student in Germany some twelve years ago, I heard a world- 

 renowned professor of zoology gravely inform his pupils that the 

 horse was a gift of the old world to the new, and was entirely 

 unknown in America until intn duced by the Spaniards. After 

 the lecture I asked him whether no earlier remains of horses had 

 been found on this continent, and was told in reply that the 

 reports to that effect were too unsatisfactory to be presented as 

 facts in science. This remark led me, on my return, to examine 

 the subject m) self, and I have since unearthed, with my own hands, 

 net less than thirty distinct species of the horse tribe, in the ter- 

 tiary deposits of the west alone ; and it is now, I think, generally 

 admitted that America is, after all, the true home of the horse. 



