Popular Science Monthly/Volume 12/April 1878/Introduction and Succession of Vertebrate Life in America II
By Professor O. C. MARSH.
[concluded from march number.]
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 significant fact that, at essentially the same horizon in each hemisphere, similar low forms of mammals make their appearance. Although only a few incomplete specimens have been discovered, they are characteristic and well preserved, and all are apparently Marsupials, the lowest mammalian group which we know in this country, living or fossil. The American Triassic mammals are known at present only from two small lower jaws, on which is based the genus Dromatherhium, supposed to be related to the insect-eating Myrmecobius, now living in Australia.
Although the Jura of Europe has yielded other similar mammals, we have as yet none of this class from that formation; while, from rocks of Cretaceous age, no mammals are known in any part of the world. This is especially to be regretted, as it is evidently to the Cretaceous that we must look for the first representatives of many of our present groups of mammals, as well as for indications of their more ancient lineage. That some discovery of this nature from the Cretaceous is near at hand, I cannot doubt, when I consider what the last few years have brought to light in the Eocene.
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 constantly occupied by this type of life in the greatest diversity of form. Fortunately, a nearly continuous record of this life, as preserved, is now accessible to us, and insures great additions to our knowledge of the genealogy of mammals, and perhaps the solution of more profound problems. Before proceeding to discuss in detail American fossil Mammalia, it is important to define the divisions of time indicated in our Tertiary and Post-Tertiary deposits, as these in many cases mark successive stages in the development of the mammals.
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 geological bearings of the fossil plants found near this horizon. The accompanying invertebrate 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 favor of the former view.
This brings up an important point in paleontology, one to which my attention was drawn several years since, namely: the comparative value of different groups of fossils in marking geological 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 subdivisons of the latter group, moreover, and in fact all forms of animal life, are of value in this respect, mainly according to the perfection of their organization or zoölogical 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 evidence 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 drawn where the Dinosaurs and other Mesozoic vertebrates disappear, and are replaced by the mammals, henceforth the dominant type.
The Tertiary of Western America comprises the most extensive series of deposits of this age known to geologists, and important breaks in both the rocks and the fossils separate it into three well-marked divisions. These natural divisions are not the exact equivalents of the Eocene, Miocene, and Pliocene of Europe, although usually so considered, and known by the same names; but, in general, the fauna of each appears to be older than that of its corresponding representative in the other hemisphere; an important fact, not hitherto recognized. This partial resemblance of our extinct faunas to others in regions widely separated, where the formations are doubtless somewhat different in geological age, is precisely what we might expect, if, as was probable, the main migrations took place from this continent. It is better at once to recognize this principle, rather than attempt to bring into exact parallelism formations that were not strictly contemporaneous.
The fresh-water Eocene deposits of our Western Territories, which are in the same region at least two miles in vertical thickness, may be separated into three distinct subdivisions. The lowest of these, resting unconformably on the Cretaceous, has been termed the Vermilion Creek, or Wahsatch, group. It contains a well-marked mammalian fauna, the largest and most characteristic genus of which is the ungulate Coryphodon, and hence I have called these deposits the Coryphodon beds. The middle Eocene strata, which have been termed the Green River and Bridger Series, may be designated as the Dinoceras beds, as the gigantic animals of this order are only found here. The uppermost Eocene, or the Uintah group, is especially well characterized by large mammals of the genus Diplacodon, and hence may be termed the Diplacodon beds. The fauna of each of these three subdivisions was essentially distinct, and the fossil remains of each were entombed in different and successive ancient lakes. It is important to remember that these Eocene lake-basins all lie between the Rocky Mountains on the east and the Wahsatch Range on the west, or along the high central plateau of the continent. As these mountain chains were elevated, the inclosed Cretaceous sea, cut off from the ocean, gradually freshened, and formed these extensive lakes, while the surrounding land was covered with a luxuriant tropical vegetation, and with many strange forms of animal life. As the upward movement of this region continued, these lake-basins, which for ages had been tilling up, preserving in their sediments a faithful record of Eocene life-history, were slowly drained by the constant deepening of the outflowing rivers, and they have since remained essentially dry land.
The Miocene lake-basins are on the flanks of this region, where only land had been since the close of the Cretaceous. These basins contain three faunas, nearly or quite distinct. The lowest Miocene, which is only found east of the Rocky Mountains, alone contains the peculiar mammals known as the Brontotheridæ, and these deposits may be called the Brontotherium beds. The strata next above, which represent the middle Miocene, have as their most characteristic fossil the genus Oreodon, and are known as the Oreodon beds. The upper Miocene, which occurs in Oregon, is of great thickness, and from one of its most important fossils, Miohippus, may be designated as the Miohippus Series. The climate here during this period was warm temperate.
Above the Miocene, east of the Rocky Mountains and on the Pacific coast, the Pliocene is well developed, and is rich in vertebrate remains. The strata rest unconformably on the Miocene, and there is a well-marked faunal change at this point, modern types now first making their appearance. For these reasons, we are justified in separating the Miocene from the Pliocene at this break; although in Europe, where no marked break exists, the line seems to have been drawn at a somewhat higher horizon. Our Pliocene forms essentially a continuous series, although the upper beds may be distinguished from the lower by the presence of a true Equus, and some other existing genera. The Pliocene climate was similar to that of the Miocene. The Post-Pliocene beds contain many extinct mammals, and may thus be separated from recent deposits.
Returning now to our subject from this geological digression—which will hardly be deemed unprofitable, since I have given you in few words the results of a great deal of my own hard mountain work—let us consider the Tertiary mammals, as we know them front the remains already discovered, and attempt to trace the history of each order down to the present time. We have seen that a single small Marsupial, from the Trias, is the only mammal found in all the American rocks below the Eocene; and yet in beds of this age, immediately over the Chalk, fossil mammals of many different kinds abound.
The Marsupials, strange to say, are here few in number, and diminutive in size; and have as yet been identified only by fragmentary specimens, and most of them are too imperfect for accurate description. In the higher Eocene deposits, this group is more abundant, but still represented by small animals, most of them insectivorous, or carnivorous in habit, like the existing opossum. From the Miocene and Pliocene, no remains of Marsupials have been described. From the Post-Tertiary, only specimens nearly allied to those now living are known, and most of these were found in the caves of South America.
The Edentate Mammals are evidently an American type, and on this continent attained a great development in numbers and size. No Eocene Edentates have been found here, and, although their discovery in this formation has been announced, the identification proves to have been erroneous. In the Miocene of the Pacific coast, a few fossils have been discovered which belong to animals of this group, and to the genus Moropus. There are two species, one about as large as a tapir, and the other nearly twice that size. This genus is the type of a distinct family, the Moropodidæ. In the lower Pliocene above, well-preserved remains of Edentates of very large size have been found at several widely-separated localities in Idaho and California. These belong to the genus Morotherium, of which two species are known. East of the Rocky Mountains, in the lower Pliocene of Nebraska, a large species apparently of the genus Moropus has been discovered. The horizon of these later fossils corresponds nearly with beds in Europe that have been called Miocene. In the Post-Pliocene of North America, gigantic Edentates were very numerous and widely distributed, but all disappeared with the close of that period. These forms were essentially huge sloths, and the more important genera were Megatherium, Mylodon, and Megalonyx. The genera Megalocnus and Myomorphus have been found only in Cuba.
In South America, during the Pliocene or Post-Pliocene, enormous Edentates were still more abundant, and their remains are usually in such perfect preservation as to suggest a very recent period for their extinction. The Sloth tribe is represented by the huge Mylodon, Megatherium, Megalonyx, Cœlodon, Ochotherium, Gnathopis, Lestodon, Scelidotherium, and Sphœnodon; and among the Armadilloes were Chlamydotherium, Eurydon, Glyptodon, Heterodon, Pachytherium, and Schistopleurum. Glossotherium, another extinct genus, is supposed to be allied to the Ant-Eaters.
It is frequently 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 migration 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 have 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 Cetacea first appear in the Eocene, as in Europe, and are comparatively abundant in deposits of this age on the Atlantic coast. The most interesting remains of this order, yet found, belong to the Zeuglodontidæ, which are carnivorous whales, and the only animals of the order with teeth implanted by two roots. The principal genera of this family are Zeuglodon and Squalodon, the former genus being represented by gigantic forms, some of which were seventy feet in length. The genus Saurocetes, which includes some small animals of this group, has been found in South America. The Dolphin family (Delphinidæ) are well represented in the Miocene, both on the Atlantic and Pacific coast. The best-known genus is Priscodelphinus, of which several species have been described. Several other generic names which have been applied to fragments need not here be enumerated. In none of the Tertiary species of this family were the cervical vertebræ anchylosed. The Sperm Whales (Catodontidæ) were also abundant throughout the Tertiary, and with them, in the earlier beds, various Ziphioid forms have been found. The toothless Balænidæ are only known with certainty as fossils from the later Tertiary and more recent deposits.
The Sirenians, which appear first in the Eocene of the Old World, occur in the Miocene of our eastern coast, and throughout the later Tertiary. The specimens described have all been referred to the genus Manatus, and seem closely related to our living species. In the Tertiary of Jamaica, a skull has been found which indicates a new genus, Prorastomus, also allied to the existing manatee. The genus Rhytina, once abundant on our Northwest coast, has recently become extinct.
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 Perissodactyles of the earlier Tertiary are more or less generalized, they are still quite distinct from the Artiodactyles, even at the base of the Eocene. One family, however, the Coryphodontidæ, 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 equaled 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 proportionally smaller than in any other land mammal. The feet had five toes, and resembled in their general structure those of Coryphydon, 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 Dinocerata. Only three genera are known, Dinoceras, Tinoceras, and Uintatherium, but quite a number of species have been described. During the later part of the middle Eocene these animals were very abundant for a short time, and then became extinct, leaving apparently no successor, unless possibly we have in the Proboscidians their much-modified descendants. Their genetic connection with the Coryphodonts is much more probable, in view of what we now know of the two groups.
Besides these peculiar mammals, which are extinct, and mainly of interest to the biologist, there were others 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 Zoölogy gravely inform his pupils that the horse was a gift of the Old World to the New, and was entirely unknown in America until introduced 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 myself, and I have since unearthed, with my own hands, not less than thirty distinct species of the horse tribe in the Tertiary deposits of the West alone; and it is now, I think, generally admitted that America is, after all, the true home of the horse.
I can offer you no better, illustration than this of the advance vertebrate paleontology has made during the last decade, or of the important contributions to this progress which our Rocky Mountain region has supplied.
The oldest representative of the horse, at present known, is the diminutive Eohippus, from the lower Eocene. Several species have been found, all about the size of a fox. Like most of the early mammals, these Ungulates had forty-four teeth, the molars with short crowns, and quite distinct in form from the premolars. The ulna and the fibula were entire and distinct, and there were four well-developed toes, and a rudiment of another on the fore-feet, and three toes behind. In the structure of the feet, and in the teeth, the Eohippus indicates unmistakably that the direct ancestral line to the modern horse has already separated from the other Perissodactyles. In the next higher division of the Eocene, another genus, Orohippus, makes its appearance, replacing Eohippus, and showing a greater, although still distant, resemblance to the Equine type. The rudimentary first digit of the fore-foot has disappeared, and the last premolar has gone over to the molar series. Orohippus was but little larger than Eohippus, and in most other respects very similar. Several species have been found in the same horizon with Dinoceras, and others lived during the upper Eocene with Diplacodon, but none later.
Near the base of the Miocene, in the Brontotherium beds, we find a third closely-allied genus, Mesohippus, which is about as large as a sheep, and one stage nearer the horse. Three are only three toes and a rudimentary splint hone in the fore feet, and three toes behind. Two of the premolar teeth are quite like the molars. The ulna is no longer distinct, or the fibula entire, and other characters show clearly that the transition is advancing. In the upper Miocene,Mesohippus is not found, but in its place a fourth form, Miohippus, continues the line. This genus is near the Anchitherium of Europe, but presents several important differences. The three toes in each foot are more nearly of a size, and a rudiment of the fifth metacarpal bone is retained. All the known species of this genus are larger than those of Mesohippus, and none pass above the Miocene.
The genus Protohippus, of the lower Pliocene, is yet more equine, and some of its species equaled the ass in size. There are still three toes on each foot, but only the middle one, corresponding to the single toe of the horse, comes to the ground. This genus resembles most nearly the Hipparion of Europe. In the Pliocene, we have the last stage of the series before reaching the horse, in the genus Pliohippus, which has lost the small hooflets, and in other respects is very equine. Only in the upper Pliocene does the true Equus appear, and complete the genealogy of the horse, which in the Post-Tertiary roamed over the whole of North and South America, and soon after became extinct. This occurred long before the discovery of the continent by Europeans, and no satisfactory reason for the extinction has yet been given. Besides the characters I have mentioned, there are many others, in the skeleton, skull, teeth, and brain, of the forty or more intermediate species, which show that the transition from the Eocene Eohippus to the modern Equus has taken place in the order indicated; and I believe the specimens now at New Haven will demonstrate the fact to any anatomist. They certainly carried prompt conviction to the first of anatomists, who was the honored guest of the Association a year ago, whose genius had already indicated the later genealogy of the horse in Europe, and whose own researches so well qualified him to appreciate the evidence here laid before him. Did time permit, I might give you at least a probable explanation of this marvelous transition, but justice to the comrades of the horse in his long struggle for existence demands that some notice of their efforts should be placed on record.
Besides the horse and his congeners, the only existing Perissodactyles are the rhinoceros and tapir. The last is the oldest type, but the rhinoceros had near allies throughout the Tertiary; and, in view of the continuity of the equine line, it is well worth while to attempt to trace his pedigree. At the bottom of the Eocene, in our Western lake-basins, the tapiroid genus Helaletes is found, represented by numerous small mammals hardly larger than the diminutive horses of that day. In the following epoch of the Eocene, the closely-allied Hyrachyus was one of the most abundant animals. This genus was nearly related to the Lophiodon of Europe, and in its teeth and skeleton strongly resembled the living tapir, whose ancestry, to this point, seems to coincide with that of the rhinoceros we are considering. Strangely enough, the rhinoceros line, before it becomes distinct, separates into two branches. In the upper part of the Dinoceras beds we have the genus Colonoceras, which is really a Hyrachyus with a transverse pair of very rudimentary horn-cores on the nasal bones. In the lower Miocene west of the Rocky Mountains this line seems to pass on through the genus Diceratherium, and in the higher Miocene this genus is well represented. Some of the species nearly equaled in size the existing rhinoceros, which Diceratherium strongly resembled. The main difference between them is a most interesting one. The rudimentary horn-cores on the nasals, seen in Colonoceras, are in Diceratherium developed into strong bony supports for horns, which were placed transversely, as in the ruminants, and not on the median line, as in all existing forms of rhinoceros. In the Pliocene of the Pacific coast, a large rhinoceros has been discovered, which may be a descendant of Diceratherium; but, as the nasal bones have not been found, we must wait for further evidence on this point. Returning now to the other branch of the rhinoceros group, which left their remains mainly east of the Rocky Mountains, we find that all the known forms are hornless. The upper Eocene genus, Amynodon, is the oldest known rhinoceros, and by far the most generalized of the family. The premolars are all unlike the molars; the four canines are of large size, but the inner incisor in each jaw is lost in the fully adult animal. The nasals were without horns. There were four toes in front, and three behind. The genus Hyracodon, of the Miocene, which is essentially a rhinoceros, has a full set of incisor and canine teeth; and the molars are so nearly like those of its predecessor, Hyrachyus, that no one will question the transformation of the older into the newer type. Hyracodon, however, appears to be off the true line, for it has but three toes in front. In the higher Miocene beds, and possibly with Hyracodon, occurs a larger rhinoceros, which has been referred to the genus Aceratherium. This form has lost the canine and one incisor above, and two incisors below. In the Pliocene are several species closely related, and of large size. Above the Pliocene in America, no vestiges of the rhinoceros have been found; and our American forms, doubtless, became extinct at the close of this period.
The tapir is clearly an old American type; and we have seen that, in the Eocene, the genera Helaletes and Hyrachyus were so strongly tapiroid in their principal characters that the main line of descent probably passed through them. It is remarkable that the Miocene of the West, so greatly developed as it is on both sides of the Rocky Mountains, should have yielded but a few fragments of tapiroid mammals; and the same is true of the Pliocene of that region. In the Miocene of the Atlantic coast, too, only a few imperfect specimens have been found. These forms all apparently belong to the genus Tapiravus, although most of them have been referred to Lophiodon—a lower Eocene type. In the Post-Tertiary, a true Tapirus was abundant; and its remains have been found in various parts of North America. The line of descent, although indistinct through the middle and upper Tertiary, was doubtless continuous in America; and several species exist at present from Mexico southward. It is worthy of notice that the species north of the Isthmus of Panama appear all to be generically distinct from those of South America.
In addition to these three Perissodactyle types, which, as the fittest, have alone survived, and whose lineage I have endeavored to trace, there were many others in early Tertiary times. Some of these disappeared with the close of the Eocene, while others continued, and assumed strange specialized shapes in the Miocene, before their decline and extinction. One series of the latter deserves especial mention, as it includes one of the most interesting families of our extinct animals. Among the large mammals in the lower Eocene is Limnohyus, a true Perissodactyle, but only known here from fragments of the skeleton. In the next higher beds, this genus is well represented, and with it is found a nearly allied form, Palæosyops. In the upper Eocene, both have left the field, and the genus Diplacodon, a very near relative, holds the supremacy. The line seems clear through these three genera, but on crossing the break into the Miocene, we have apparently, as next of kin, the huge Brontotheridæ. These strange beasts show in their dentition and some other characters the same transition steps beyond the Diplacodon, which that genus had, made beyond Palæosyops. The Brontotheridæ were nearly as large as the elephant, but had much shorter limbs. The skull was elongated, and had a transverse pair of large horn-cores on the maxillaries, in front of the orbits, like the middle pair in Dinoceras, There were four toes in front and three behind, and the feet were similar to those of the rhinoceros. There are four genera in this group, Brontotherium; Diconodon; Menodus (Titanotherium); and Megacerops, which have been found only in the lowest Miocene, east of the Rocky Mountains.
In the higher Miocene beds of Oregon, an allied genus, Chalicotherium, makes its appearance. It is one stage further on in the transition, and perhaps a descendant of the Brontotheridæ; but here, so far as now known, the line disappears. It is a suggestive fact that this genus has now been found in Western America, China, India, Greece, Germany, and France, indicating thus, as I believe, the path by which many of our ancient mammals helped to people the so-called Old World.
The Artiodactyles, or even-toed Ungulates, are the most abundant of the larger mammals now living; and the group dates back at least to the lowest Eocene. Of the two well-marked divisions of this order, the Bunodonts and the Selenodonts, as happily defined by Kowalevsky, the former is the older type, which must have separated from the Perissodactyle line after the latter had become differentiated from the primitive Ungulate. In the Coryphodon beds of New Mexico occurs the oldest Artiodactyle yet found, but it is at present known only from fragmentary specimens. These remains are clearly Suilline in character, and belong to the genus Eohyus. In the beds above, and possibly even in the same horizon, the genus Helohyus is not uncommon, and several species are known. The molar teeth of this genus are very similar to those of the Eocene Hyracotherium, of Europe, which is supposed to be a Perissodactyle, while Helohyus certainly is not, but apparently a true lineal ancestor of the existing pigs. In every vigorous primitive type which was destined to survive many geological changes, there seems to have been a tendency to throw off lateral branches, which became highly specialized and soon died out, because they are unable to adapt themselves to new conditions. The narrow path of the persistent Suilline type, throughout the whole Tertiary, is strewed with the remains of such ambitious offshoots; while the typical pig, with an obstinacy never lost, has held on in spite of catastrophes and evolution, and still lives in America to-day. In the lower Eocene, we have in the genus Parahyus apparently one of these short-lived, specialized branches. It attained a much larger size than the true lineal forms, and the number of its teeth was reduced. In the Dinoceras beds, or middle Eocene, we have still, on or near the true line, Helohyus, which is the last of the series known from the American Eocene. All these early Suillines, with the possible exception of Parahyus, appear to have had at least four toes of usable size.
In the lower Miocene, we find the genus Perchœrus, seemingly a true Suilline, and with it remains of a larger form, Elotherium, are abundant. The latter genus occurs in Europe in nearly the same horizon, and the specimens known from each continent agree closely in general characters. The name Pelonax has been applied erroneously to some of the American forms; but the specimens on which it was based clearly belong to Elotherium. This genus affords another example of the aberrant Suilline offshoots, already mentioned. Some of the species were nearly as large as a rhinoceros, and in all there were but two serviceable toes; the outer digits, seen in living animals of this group, being represented only by small rudiments concealed beneath the skin. In the upper Miocene of Oregon, Suillines are abundant, and almost all belong to the genus Thinohyus, a near ally of the modern peccary (Dicotyles), but having a greater number of teeth, and a few other distinguishing features. In the Pliocene, Suillines are still numerous, and all the American forms yet discovered are closely related to Dicotyles. The genus Platygonus is represented by several species, one of which was very abundant in the Post-Tertiary of North America, and is apparently the last example of a side branch, before the American Suillines culminate in existing peccaries. The feet in this species are more specialized than in the living forms, and approach some of the peculiar features of the ruminants; as, for example, a strong tendency to coalescence in the metapodial bones. The genus Platygonus became extinct in the Post-Tertiary, and the later and existing species are all true peccaries.
No authenticated remains of the genera Sus, Porcus, Phacochœrus, or the allied Hippopotamus, the Old World Suillines, have been found in America, although several announcements to that effect have been made.
In the series of generic forms between the lower Eocene Eohyus and the existing Picotyles, which I have very briefly discussed, we have apparently the ancestral line ending in the typical American Suillines. Although the demonstration is not yet as complete as in the lineage of the horse, this is not owing to want of material, but rather to the fact that the actual changes which transformed the early Tertiary pig into the modern peccary were comparatively slight, so far as they are indicated in the skeletons preserved, while the lateral branches were so numerous as to confuse the line. It is clear, however, that from the close of the Cretaceous to the Post-Tertiary the Bunodont Artiodactyles were especially abundant on this continent, and only recently have approached extinction.
The Selenodont division of the Artiodactyles is a more interesting group and, so far as we now know, makes its first appearance in the upper Eocene of the West, although forms, apparently transitional, between it and the Bunodonts occur in the Dinoceras beds, or middle Eocene. These belong to the genus Homacodon, which is very nearly allied to Helohyus, and but a single step away from this genus toward the Selenodonts. By a fortunate discovery, a nearly complete skeleton of this rare intermediate form has been brought to light, and we are thus enabled to define its characters. Several species of Homacodon are known, all of small size. This primitive Selenodont had forty-four teeth, which formed a nearly continuous series.
The molar teeth are very similar to those of Helohyus, but the cones on the crowns have become partially triangular in outline, so that, when worn, the Selenodont pattern is clearly recognizable. The first and second upper molars, moreover, have three distinct posterior cusps, and two in front; a peculiar feature, which is seen also in the European genera Dichobune and Cainotherium. There were four toes on each foot, and the metapodial bones were distinct. The type species of this genus was. about as large as a cat. With Helohyus, this genus forms a well-marked family, the Helohyidæ.
In the Diplacodon horizon of the upper Eocene, the Selenodont dentition is no longer doubtful, as it is seen in most of the Artiodactyla yet found in these beds. These animals are all small, and belong to at least three distinct genera. One of these, Eomeryx, closely resembles Homacodon in most of its skeleton, and has four toes, but its teeth show well-marked crescents, and a partial transition to the teeth of Hyopotamus, from the Eocene of Europe. With this genus is another, Parameryx, also closely allied to Homacodon, but apparently a straggler from the true line, as it has but three toes behind. The most pronounced Selenodont in the upper Eocene is the Oromeryx, which genus appears to be allied to the existing Deer family, or Cervidæ, and if so is the oldest known representative of the group. These facts are important, as it has been supposed, until very recently, that our Eocene contained no even-hoofed mammals.
In the lowest Miocene of the West, no true crescent-toothed Artiodactyla have as yet been identified, with the exception of a single species of Hyopotamus; but, in the overlying beds of the middle Miocene, remains of the Oreodontidæ occur in such vast numbers as to indicate that these animals must have lived in large herds around the borders of the lake-basins in which their remains have been entombed. These basins are now the denuded deserts so well termed Mauvaises Terres by the early French trappers. The least specialized, and apparently the oldest, genus of this group is Agriochœrus, which so nearly resembles the older Hyopotamus, and the still more ancient Eomeryx, that we can hardly doubt that they all belonged to the same ancestral line. The typical Oreodonts are the genera Oreodon and Eporeodon, which have been aptly termed by Leidy ruminating hogs. They had forty-four teeth, and four well-developed toes on each foot. The true Oreodons, which were most numerous east of the Rocky Mountains, were about as large as the existing peccary, while Eporeodon, which was nearly twice this size, was very abundant in the Miocene of the Pacific slope.
In the succeeding Pliocene formation, on each side of the Rocky Mountains, the genus Merychyus is one of theforms, and continues the line on from the Miocene, where the true Oreodons became extinct. Beyond this, we have the genus Merychochœrus, which is so nearly allied to the last that they would be united by many naturalists. With the close of the Pliocene, this series of peculiar ruminants abruptly terminates, no member surviving until the Post-Tertiary, so far as known.
A most interesting line, that leading to the camels and llamas, separates from the primitive Selenodont branch in the Eocene, probably through the genus Parameryx. In the Miocene, we find in Pœbrotherium and some nearly allied forms unmistakable indications that the Cameloid type of ruminant had already become partially specialized, although there is a complete series of incisor teeth, and the metapodial bones are distinct. In the Pliocene, the camel tribe was, next to the horses, the most abundant of the larger mammals. The line is continued through the genus Procamelus, and perhaps others, and in this formation the incisors first begin to diminish, and the metapodials to unite. In the Post-Tertiary we have a true Auchenia, represented by several species, and others in South America, where the alpacas and llamas still survive. From the Eocene almost to the present time, North America has been the home of vast numbers of the Camelidæ, and there can be little doubt that they originated here, and migrated to the Old World.
Returning once more to the upper Eocene, we find another line of descent starting from Oromeryx, which, as we have seen, had apparently then just become differentiated from the older Bunodont type. Throughout the middle and upper Miocene, this line is carried forward by the genus Leptomeryx and its near allies, which resemble so strongly the Pliocene Cervidæ that they may fairly be regarded as their probable progenitors. Possibly some of these forms may be related to the Tragulidæ, but at present the evidence is against it.
The deer family has representatives in the upper Miocene of Europe, which contains fossils strongly resembling the fauna of our lower Pliocene, a fact always to be borne in mind in comparing the horizon of any group in the two continents. Several species of Cervidæ, belonging to the genus Cosoryx, are known from the lower Pliocene of the West, and all have very small antlers, divided into a single pair of tynes. The statement recently published, that most of these antlers had been broken during the life of the animals, is unsupported by any evidence, and is erroneous. These primitive deer do not have the orbit closed behind, and they have all the four metapodial bones entire, although the second and fifth are very slender. In the upper Pliocene, a true Cervus of large size has been discovered. In the Post-Tertiary, Cervus, Alces, and Tarandus, have been met with, the latter far south of its present range. In the caves of South America, remains of Cervus have been found, and also two species of antelopes, one referred to a new genus, Leptotherium.
The hollow-horned ruminants, in this country, appear to date back no further than to the lower Pliocene, and here only two species of Bison have as yet been discovered. In the Post-Tertiary this genus was represented by numerous individuals and several species, some of large size. The musk-ox (Ovibos) was not uncommon during some parts of this epoch, and its remains are widely distributed.
No authentic fossil remains of true sheep, goats, or giraffes, have as yet been found on this continent.
The Proboscidians, which are now separated from the typical Ungulates as a distinct order, make their first appearance in North America in the lower Pliocene, where several species of Mastodon have been found. This genus occurs, also, in the upper Pliocene, and in the Post-Tertiary; although some of the remains attributed to the latter are undoubtedly older. The Pliocene species all have a band of enamel on the tusks, and some other peculiarities observed in the oldest mastodons of Europe, which are from essentially the same horizon. Two species of this genus have been found in South America, in connection with the remains of extinct llamas and horses. The genus Elephas is a later form, and has not yet been identified in this country below the upper Pliocene, where one gigantic species was abundant. In the Post-Pliocene, remains of this genus are numerous. The hairy mammoth of the Old World (Elephas primigenius) was once abundant in Alaska, and great numbers of its bones are now preserved in the frozen cliffs of that region. This species does not appear to have extended east of the Rocky Mountains, or south of the Columbia River, but was replaced there by the American elephant, which preferred a milder climate. Remains of the latter have been met with in Canada, throughout the United States, and in Mexico. The last of the American mastodons and elephants became extinct in the Post-Tertiary.
The order Toxodontia includes two very peculiar genera, Toxodon and Nesodon, which have been found in the Post-Tertiary deposits of South America. These animals were of huge size, and possessed such mixed characters that their affinities are a matter of considerable doubt. They are thought to be related to the Ungulates, Rodents, and Edentates; but, as the feet are unknown, this cannot at present be decided.
Macrauchenia and Homalodontotherium are two other peculiar genera from South America, now extinct, the exact affinities of which are uncertain. Anoplotherium and Palæotherium, so abundant in Europe, have not been found in our North American Tertiary deposits, although reported from South America.
Perhaps the most remarkable mammals yet found in America are the Tillodontia, which are comparatively abundant in the lower and middle Eocene. These animals seem to combine the characters of several different groups, viz., the Carnivores, Ungulates, and Rodents. In the genus Tillotherium, the type of the order, and of the family Tillotheridæ, the skull resembles that of the bears; the molar teeth are of the ungulate type, while the large incisors are very similar to those of Rodents. The skeleton resembles that of the Carnivores, but the scaphoid and lunar bones are distinct, and there is a third trochanter on the femur. The feet are plantigrade, and each had five digits, all with long, pointed claws. In the allied genus Stylinodon, which belongs to a distinct family, the Stylinodontidæ, all the teeth were rootless. Some of these animals were as large as a tapir. The genus Dryptodon has been found only in the Coryphodon beds of New Mexico, while Tillotherium and Stylinodon occur in the middle Eocene of Wyoming. Anchippodus probably belongs to this group, which may perhaps include some other forms that have been named from fragmentary specimens.
The Rodents are an ancient type, and their remains are not unfrequently disinterred in the strata of our lowest fresh-water Eocene. The earliest known forms are, apparently, all related to the squirrels; and the most common genus is Sciuravus, which continued throughout the Eocene. A nearly allied form, which may prove to be the same, is Paramys, the species of which are larger than those of the older type. In the Dinoceras beds, the genus Colonomys is found, and the specimens preserved point to the Muridæ as the nearest living allies. A peculiar genus, Apatemys, which also occurs in the middle Eocene, has gliriform incisors; but the molars resemble those of Insectivores. All the Eocene Rodents known are of small size, the largest being about as large as a rabbit.
In the middle and upper Miocene lake-basins of the West, Rodents abound, but all are of moderate size. The hares first appear in the Oreodon beds, and continue in considerable numbers through the rest of the Tertiary and Post-Tertiary, to the present day. In these beds, the most common forms belong to the Leporidæ, and mainly to the genus Palæolagus. The Squirrel family is represented by Ischyromys, the Muridæ by the genus Eumys, and the beavers by Palæocastor. In the upper Miocene of Oregon, most of the same genera are found; and with them some peculiar forms, very unlike anything now living. One of these is the genus Allomys, possibly related to the flying-squirrels, but having molar teeth somewhat like those of the Ungulates. In the Pliocene, east and west of the Rocky Mountains, Rodents continue abundant; but most of them belong to existing genera. Among these are Castor, Hystrix, Cynomys, Geomys, Lepus, and Hesperomys. In the Post-Tertiary, the gigantic beaver, Castoroides, was abundant throughout most of North America. Hydrochœrus has been found in South Carolina. In the caves of the island of Anguilla, in the West Indies, remains of large extinct Rodents, belonging to the Chinchillidæ, have been discovered.
The early Tertiary Rodents known from South America are the genera Megamys, Theridromys, and a large species referred to Arvicola. In Brazil, the Pliocene Rodents found are referred to the existing genera, Cavia, Kerodon, Lagostomus, Ctenomys, Hesperomys, Oxymycterus, Arvicola, and Lepus. A new genus, Cardiodus, described from this horizon, is a true rodent; but the peculiar Typotherium, which has been referred to this order by some authorities, has perhaps other affinities. In the Post-Tertiary, the Rodents were very abundant in South America, as they are at present. The species are, in most instances, distinct from those now living, but the genera are nearly the same. The Caviidæ were especially numerous. Cercolabes, Myopotamus, and Lagostomus, are also found; and two extinct genera, Phyllomys and Lonchophorus.
The Cheiroptera, or bats, have not been found in this country below the middle Eocene, where two extinct genera, Nyctilestes and Nyctitherium, are each represented by numerous remains. These fossils all belong to small animals, and, so far as they have been investigated, show no characters of more than generic importance to distinguish them from the bats of to-day. No other members of this group are known from our Tertiary. In the Post-Tertiary, no extinct species of bats have been found in North America, but from the caves of Brazil quite a number have been reported. These all belong to genera still living in South America, and most of them to the family Phyllostomidæ.
The Insectivores date back, in this country, at least to the middle Eocene. Here numerous remains occur, which have been described as belonging to this order, although it is possible that some of them were insect-eating Marsupials. The best-known genera are—Hemiacodon, Centetodon, Talpavus, and Entomacodon; all represented by animals of small size. In the Miocene, the bones of Insectivores are comparatively abundant, and the genera best determined are Ictops and Leptictis. A few specimens only have been found in the Pliocene and Post-Pliocene, most of them related to the moles. No extinct Insectivores are known from South America, and no member of the group exists there at present.
The Carnivora, or true flesh-eating animals, are an old type, well represented in the Eocene, and, as might be expected, these early forms are much less specialized than the living species. In the Coryphodon beds, the genus Limnocyon, allied to the Pterodon of the European Eocene, is abundant. Another genus, apparently distinct, is Prototomus, and several others have been named from fragmentary fossils. In the middle Eocene, Carnivores were still more numerous, and many genera have been discovered. One of these, Limnofelis, was nearly as large as a lion, and apparently allied to the cats, although the typical Felidæ seem not yet to have been differentiated. Another Carnivore, of nearly equal size, was Orocyon, which had short, massive jaws and broad teeth. Dromocyon and Mesonyx were large animals, allied to Hyœnodon. The teeth were narrow, and the jaws long and slender. Among the smaller Carnivores were—Vulpavus, Viverravus, Sinopa, Thinocyon, and Ziphacodon.
In our Western Miocene, Carnivores are abundant, and make an approach to modern types. The Felidæ are well represented, the most interesting genus being Machairodus, which is not uncommon in the Oreodon beds on both sides of the Rocky Mountains. An allied genus is Dinictis, and several smaller cats are known from about the same horizon. The Canidæ are represented by Amphicyon, a European genus, and by several species of Canis, or a very nearly allied form. The peculiar genus Hyænodon, found also in Europe, and the type of a distinct family, is abundant in the Miocene east of the Rocky Mountains, but has not yet been found on the Pacific coast. In the Pliocene of both regions the Canidæ are numerous, and all apparently belong to the existing genus Canis. The genus Machairodus is still the dominant form of the cats, which are abundant, and for the most part belong to the genus Felis. The extinct Leptarctus is supposed to belong to the Ursidæ, and, if so, is the oldest American representative of this family. In the Post-Pliocene, the extinct Felidæ include species nearly as large as a lion, and smaller forms very similar to those still living. Bears, raccoons, and weasels, have also been found.
In the Pliocene of South America, Machairodus represents the Felidæ, while the genera Arctotherium and Hyænarctus belong to the Bear family. Species of. Mustela and Canis have also been found. In the caves of Brazil, the fauna of which is regarded as Post-Pliocene, one species of Machairodus is known, and one of Synælurus. Canis and Icticyon, still living in Brazil, and the extinct genus Speothos, represent the Canidæ. Mephitis and Galictis, among the weasels, were also present, and with them species of Nasua and Arctotherium.
We come now to the highest group of Mammals, the Primates, which includes the Lemurs, the Apes, and Man. This order has a great antiquity, and even at the base of the Eocene we find it represented by several genera belonging to the lower forms of the group. In considering these interesting fossils, it is important to have in mind that the Lemurs, which are usually regarded as Primates, although at the bottom of the scale, are found at the present day only in Madagascar and the adjacent regions of the globe. All the American monkeys, moreover, belong to one group, much above the Lemurs, while the Old World apes are higher still, and most nearly approach man.
In the lower Eocene of New Mexico we find a few representatives of the earliest known Primates, and among them are the genera Lemuravus and Limnotherium, each the type of a distinct family. These genera became very abundant in the middle Eocene of the West, and with them are found many others—all, however, included in the two families Lemuravidæ and Limnotheridæ. Lemuravus appears to have been most nearly allied to the Lemurs, and is the most generalized form of the Primates yet discovered. It had forty-four teeth, forming a continuous series above and below. The brain was nearly smooth, and of moderate size. The skeleton most resembles that of the Lemurs. A nearly allied genus, belonging to the same family, is Hyopsodus. Limnotherium (Tomitherium) also is nearly related to the Lemurs, but shows some affinities with the South American marmosets. This genus had forty teeth. The brain was nearly smooth, and the cerebellum large, and placed mainly behind the cerebrum. The orbits are open behind, and the lachrymal foramen is outside the orbit. Other genera belonging to the Limnotheridæ are—Notharctos, Hipposyus, Microsyops, Palæacodon, Thinolestes, and Telmatolestes. Besides these, Antiacodon (Anaptomorphus), Bathrodon, and Mesacodon, should probably be placed in the same group. In the Diplacodon beds, or upper Eocene, no remains of Primates have yet been detected, although they will doubtless be found there. All the Eocene Primates known from American strata are low generalized forms, with characters in the teeth, skeleton, and feet, that suggest relationships with the Carnivores, and even with the Ungulates. These resemblances have led paleontologists to refer some imperfect specimens to both these orders.
In the Miocene lake-basins of the West, only a single species of the Primates has been identified with certainty. This was found in the Oreodon beds of Nebraska, and belongs to the genus Laopithecus, apparently related both to the Limnotheridæ and to some existing South American monkeys. In the Pliocene and Post-Pliocene of North America no remains of Primates have yet been found.
In the Post-Pliocene deposits of the Brazilian caves, remains of monkeys are numerous, and mainly belong to extinct species of Callithrix, Cebus, and Jacchus, all living South American genera. Only one extinct genus, Protopithecus, which embraced animals of large size, has been found in this peculiar fauna.
It is a noteworthy fact, that no traces of any Anthropoid apes, or indeed of any Old World monkeys, have yet been detected in America. Man, however, the highest of the Primates, has left his bones and his works from the arctic circle to Patagonia. Most of these specimens are clearly Post-Tertiary, although there is considerable evidence pointing to the existence of man in our Pliocene. All the remains yet discovered belong to the well-marked genus Homo, and apparently to a single species, at present represented by the American Indian.
In this rapid review of mammalian life in America, from its first known appearance in the Trias down to the present time, I have endeavored to state briefly the introduction and succession of the principal forms in each natural group. If time permitted, I might attempt the more difficult task of trying to indicate what relations these various groups may possibly bear to each other; what connection the ancient mammals of this continent have with the corresponding forms of the Old World; and, most important of all, what real progress mammalian life has here made since the beginning of the Eocene. As it is, I can only say, in summing up, that the Marsupials are clearly the remnants of a very ancient fauna, which occupied this continent millions of years ago, and from which the other mammals were doubtless all derived, although the direct evidence of the transformation is wanting.
Although the Marsupials are nearly related to the still lower Monotremes, now living in the Australian region, we have as yet no hint of the path by which these two groups became separated from the inferior vertebrates. Neither have we to-day much light as to the genetic connection existing between Marsupials and the placental Mammalia, although it is possible that the different orders of the latter had their origin each from a separate group of the Marsupials.
The presence, however, of undoubted Marsupials in our lower and middle Eocene, some of them related to the genus Didelphys, although remotely, is important evidence as to the introduction of these animals into America. Against this, their supposed absence in our Miocene and Pliocene can have but limited weight, when taken in connection with the fact that they flourished in the Post-Tertiary, and are still abundant. The evidence we now have is quite as strongly in favor of a migration of Marsupials from America to the Old World, as the reverse, which has been supposed by some naturalists. Possibly, as Huxley has suggested, both countries were peopled with these low mammals from a continent now submerged.
The Edentate mammals have long been a puzzle to zoölogists, and up to the present time no clew to their affinities with other groups seems to have been detected. A comparison of the peculiar Eocene mammals which I have called the Tillodontia, with the least specialized Edentates, brings to light many curious resemblances in the skull, teeth, skeleton, and feet. These suggest relationship, at least, and possibly we may yet find here the key to the Edentate genealogy. At present, the Tillodonts are all from the lower and middle Eocene, while Moropus, the oldest Edentate genus, is found in the middle Miocene, and one species in the lower Pliocene.
The Edentates have been usually regarded as an American type, but the few living forms in Africa, and the Tertiary species in Europe, the oldest known, have made the land of their nativity uncertain. I have already given you some reasons for believing that the Edentates had their first home in North America, and migrated thence to the southern portion of the continent. This movement could not have taken place in the Miocene period, as the Isthmus of Darien was then submerged; but, near the close of the Tertiary, the elevation of this region left a much broader strip of land than now exists there, and Over this the Edentates and other mammals made their way, perhaps urged on by the increasing cold of the glacial winters. The evidence to-day is strongly in favor of such a southern migration. This, however, leaves the Old World Edentates, fossil and recent, unaccounted for; but I believe the solution of this problem is essentially the same, namely, a migration from North America. The Miocene representatives of this group, which I have recently obtained in Oregon, are older than any known in Europe, and, strangely enough, are more like the latter and the existing African types than like any of our living species. If, now, we bear in mind that an elevation of only 180 feet would close Behring's Straits, and give a road thirty miles wide from America to Asia, we can easily see how this migration might have taken place. That such a Tertiary bridge did exist, we have much independent testimony, and the known facts all point to extensive migrations of animals over it.
The Cetacea are connected with the marine Carnivores through the genus Zeuglodon, as Huxley has shown, and the points of resemblance are so marked that the affinity cannot be doubted. That the connection was a direct one, however, is hardly probable, since the diminutive brain, large number of simple teeth, and reduced limbs in the whales, all indicate them to be an old type, which doubtless branched off from the more primitive stock leading to the Carnivores. Our American extinct Cetaceans, when carefully investigated, promise to throw much light upon the pedigree of these strange mammals. As most of the known forms were probably marine, their distribution is of little service in determining their origin.
That the Sirenians are allied to the Ungulates is now generally admitted by anatomists, and the separation of the existing species in distant localities suggests that they are the remnants of an extensive group, once widely distributed. The large number of teeth in some forms, the reduced limbs, and other characters, point back to an ancestry near that of the earliest Ungulates. The gradual loss of teeth in the specialized members of this group, and in the Cetaceans, is quite parallel with the same change in Edentates, as well as in Pterodactyles and Birds.
The Ungulates are so distinct from other groups that they must be one of the oldest natural divisions of mammals, and they probably originated from some herbivorous marsupial. Their large size, and great numbers, during Tertiary and Post-Tertiary time, render them most valuable in tracing migrations induced by climate, as well as in showing the changes of structure which such a contest for existence may produce.
In the review of the extinct Ungulates, I have endeavored to show that quite a number of genera, usually supposed to belong originally to the Old World, are in reality true American types. Among these were the horse, rhinoceros, and tapir, all the existing odd-toed Ungulates, and, besides these, the camel, pig, and deer. All these I believe, and many others, went to Asia from our Northwest coast. It must, for the present, remain an open question whether we may not fairly claim the Bovidæ, and even the Proboscidea, since both occur in our strata at about the same horizon as on the other continent. On this point there is some confusion, at least in names. The Himalayan deposits called upper Miocene, and so rich in Proboscidians, indicate in their entire fauna that they are more recent than our Niobrara River beds, which, for apparently good reasons, we regard as lower Pliocene. The latter appear to be about the same horizon as the Pikermi deposits in Greece, also regarded as Miocene. Believing, however, that we have here a more complete Tertiary series, and a better standard for comparison of faunas, I have preferred to retain the names already applied to our divisions, until the strata of the two continents are more satisfactorily coördinated.
The extinct Rodents, Bats, and Insectivores of America, although offering many suggestive hints as to their relationship with other groups, and their various migrations, cannot now be fully discussed. There is little doubt, however, that the Rodents are a New World type, and, according to present evidence, they probably had their origin in North America. The resemblance in so many respects of this order to the Proboscidians is a striking fact, not yet explained by the imperfectly known genealogy of either group.
The Carnivores, too, I must pass by, except to call attention to a few special forms which accompanied the migrations of other groups. One of these is Machairodus, the sabre-toothed tiger, which flourished in our Miocene and Pliocene, and apparently followed the huge Edentates to South America, and the Ungulates across Asia to Europe. With this genus went Hyæodon, and some typical wolves and cats, but the bears probably came the other way with the antelopes. That the gazelle, giraffe, hippopotamus, hyena, and other African types, once abundant in Asia, did not come, is doubtless because the Miocene bridge was submerged before they reached it.
The Edentates, in their southern migration, were probably accompanied by the horse, tapir, and rhinoceros, although no remains of the last have yet been found south of Mexico. The mastodon, elephant, llama, deer, peccary, and other mammals, followed the same path. Why the mastodon, elephant, rhinoceros, and especially the horse, should have been selected with the huge Edentates for extinction, and the other Ungulates left, is at present a mystery, which their somewhat larger size hardly explains.
The relations of the American Primates, extinct and recent, to those of the other hemisphere, offer an inviting topic, but it is not within my present province to discuss them in their most suggestive phases. As we have here the oldest and most generalized members of the group, so far as now known, we may justly claim America for the birthplace of the order. That the development did not continue here until it culminated in man, was due to causes which at present we can only surmise, although the genealogy of other surviving groups gives some data toward a solution. Why the Old World apes, when differentiated, did not come to the land of their earlier ancestry, is readily explained by the then intervening oceans, which likewise were a barrier to the return of the horse and rhinoceros.
Man, however, came—doubtless first across Behring's Straits; and at his advent became part of our fauna, as a mammal and primate. In these relations alone it is my purpose here to treat him. The evidence, as it stands to-day, although not conclusive, seems to place the first appearance of man in this country in the Pliocene, and the best proof of this has been found on the Pacific coast. During several visits to that region, many facts were brought to my knowledge which render this more than probable. Man at this time was a savage, and was doubtless forced by the great volcanic outbreaks to continue his migration. This was at first to the south, since mountain-chains were barriers on the east. As the native horses of America were now all extinct, and as the early man did not bring the Old World animal with him, his migrations were slow. I believe, moreover, that his slow progress toward civilization was in no small degree due to this same cause, the absence of the horse.
It is far from my intention to add to the many theories extant in regard to the early civilizations in this country, and their connections with the primitive inhabitants, or the later Indians; but two or three facts have recently come to my knowledge which I think worth mentioning in this connection. On the Columbia River, I have found evidence of the former existence of inhabitants much superior to the Indians at present there, and of which no tradition remains. Among many stone carvings which I saw there, were a number of heads which so strongly resemble those of apes that the likeness at once suggests itself. Whence came these sculptures, and by whom were they made? Another fact that has interested me very much is the strong resemblance between the skulls of the typical mound-builders of the Mississippi Valley and those of the Pueblo Indians. I had long been familiar with the former; and, when I recently saw the latter, it required the positive assurance of a friend, who had himself collected them in New Mexico, to convince me that they were not from the mounds. A third fact, and I leave man to the archaeologists, on whose province I am even now trenching. In a large collection of mound-builders' pottery, over a thousand specimens, which I have recently examined with some care, I found many pieces of elaborate workmanship so nearly like the ancient water-jars from Peru, that no one could fairly doubt that some intercourse had taken place between the widely-separated people that made them.
The oldest known remains of man on this continent differ in no important characters from the bones of the typical Indian, although in some minor details they indicate a much more primitive race. These early remains, some of which are true fossils, resemble much more closely the corresponding parts of the highest Old World apes, than do the latter our Tertiary Primates, or even the recent American monkeys. Various living and fossil forms of Old World Primates fill up essentially the latter gap. The lesser gap between the primitive man of America and the Anthropoid apes is partially closed by still lower forms of men, and doubtless also by higher apes, now extinct. Analogy, and many facts as well, indicate that this gap was smaller in the past. It certainly is becoming wider now with every generation, for the lowest races of men will soon become extinct, like the Tasmanians, and the highest apes cannot long survive. Hence the intermediate forms of the past, if any there were, become of still greater importance. For such missing links, we must look to the caves and later Tertiary of Africa, which I regard as now the most promising field for exploration in the Old World. America, even in the tropics, can promise no such inducements to ambitious explorers. We have, however, an equally important field, if less attractive, in the Cretaceous mammals, which must have left their remains somewhere on this continent. In these two directions, as I believe, lie the most important future discoveries in paleontology.
As a cause for many changes of structure in mammals during the Tertiary and Post-Tertiary, I regard as the most potent, natural selection, in the broad sense in which that term is now used by American evolutionists. Under this head I include not merely a Malthusian struggle for life among the animals themselves, but the equally important contest with the elements and all surrounding Nature. By changes in the environment, migrations are enforced, slowly in some cases, rapidly in others, and with change of locality must come adaptation to new conditions, or extinction. The life-history of Tertiary mammals illustrates this principle at every stage, and no other explanation meets the facts.
The real progress of mammalian life in America, from the beginning of the Tertiary to the present, is well illustrated by the brain growth, in which we have the key to many other changes. The earliest known Tertiary mammals all had very small brains, and in some forms this organ was proportionally less than in certain reptiles. There was a gradual increase in the size of the brain during this period, and it is interesting to find that this growth was mainly confined to the cerebral hemispheres, or higher portion of the brain. In most groups of mammals, the brain has gradually become more convoluted, and thus increased in quality as well as quantity. In some, also, the cerebellum and olfactory lobes, the lower parts of the brain, have even diminished in size. In the long struggle for existence during Tertiary time, the big brains won, then as now; and the increasing power thus gained rendered useless many structures inherited from primitive ancestors, but no longer adapted to new conditions.
Another of the interesting changes in mammals during Tertiary time was in the teeth, which were gradually modified with other parts of the structure. The primitive form of tooth was clearly a cone, and all others are derived from this. All classes of Vertebrates below mammals, namely, fishes, amphibians, reptiles, and birds, have conical teeth, if any, or some simple modification of this form. The Edentates and Cetaceans with teeth retain this type, except the zeuglodonts, which approach the dentition of aquatic Carnivores. In the higher mammals, the incisors and canines retain the conical shape, and the premolars have only in part been transformed. The latter gradually change to the more complicated molar pattern, and hence are not reduced molars, but transition forms from the cone to more complex types. Most of the early Tertiary mammals had forty-four teeth, and in the oldest forms the premolars were all unlike the molars, while the crowns were short, covered with enamel, and without cement. Each stage of progress in the differentiation of the animal was, as a rule, marked by a change in the teeth; one of the most common being the transfer, in form at least, of a premolar to the molar series, and a gradual lengthening of the crown. Hence, it is often easy to decide from a fragment of a jaw to what horizon of the Tertiary it belongs. The fossil horses of this period, for example, gained a grinding-tooth for each toe they lost, one in each epoch. In the single-toed existing horses, all the premolars are like the molars, and the process is at an end. Other dental transformations are of equal interest, but this illustration must suffice.
The changes in the limbs and feet of mammals during the same period were quite as marked. The foot of the primitive mammal was, doubtless, plantigrade, and certainly five-toed. Many of the early Tertiary forms show this feature, which is still seen in some existing forms. This generalized foot became modified by a gradual loss of the outer toes, and increase in size of the central ones, the reduction proceeding according to systematic methods, differing in each group. Corresponding changes took place in the limb-bones. One result was a great increase in speed, as the power was applied so as to act only in the plane of motion. The best effect of this specialization is seen to-day in the horse and antelope, each representing a distinct group of Ungulates, with five-toed ancestors.
If the history of American mammals, as I have briefly sketched it, seems, as a whole, incomplete and unsatisfactory, we must remember that the genealogical tree of this class has its trunk and larger limbs concealed beneath the débris of Mesozoic time, while its roots doubtless strike so deeply into the Paleozoic that for the present they are lost. A decade or two hence we shall probably know something of the mammalian fauna of the Cretaceous, and the earlier lineage of our existing mammals can then be traced with more certainty.
The results I have presented to you are mainly derived from personal observation; and, since a large part of the higher vertebrate remains hitherto found in this country have passed through my hands, I am willing to assume full responsibility for my presentation of the subject.
For our present knowledge of the extinct Mammals, Birds, and Reptiles of North America, science is especially indebted to Leidy, whose careful, conscientious work has laid a secure foundation for our vertebrate paleontology. The energy of Cope has brought to notice many strange forms, and greatly enlarged our literature. Agassiz, Owen, Wyman, Baird, Hitchcock, Deane, Emmons, Lea, Allen, Gibbes, Jefferson, DeKay, and Harlan, deserve honorable mention in the history of this branch of science. The South American extinct Vertebrates have been described by Lund, Owen, Burmeister, Gervais, Huxley, Flower, Desmarest, Aymard, Pictet, and Nodot. Darwin and Wallace have likewise contributed valuable information on this subject, as they have on nearly all forms of life.
In this long history of ancient life I have said nothing of what Life itself really is—and for the best of reasons—because I know nothing. Here, at present, our ignorance is dense, and yet we need not despair. Light, Heat, Electricity, and Magnetism, Chemical Affinity, and Motion, are now considered different forms of the same force; and the opinion is rapidly gaining ground that Life, or vital force, is only another phase of the same power. Possibly the great mystery of Life may thus be solved, but, whether it be or not, a true faith in science knows no limit to its search for Truth.