darwin and modern science-第50章
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lainly derived from those of Eohippus； have made a long stride toward the horse pattern； in that the separate cusps have united to form a continuous outer wall and two transverse crests。
In the lower Miocene the interesting genus Desmatippus shows a further advance in the development of the teeth； which are beginning to assume the long…crowned shape； delaying the formation of roots； a thin layer of cement covers the crowns； and the transverse crests of the upper grinding teeth display an incipient degree of their modern complexity。 This tooth…pattern is strictly intermediate between the recent type and the ancient type seen in Mesohippus and its predecessors。 The upper Miocene genera； Protohippus and Hipparion are； to all intents and purposes； modern in character； but their smaller size； tridactyl feet and somewhat shorter…crowned teeth are reminiscences of their ancestry。
From time to time； when a land…connection between North America and Eurasia was established； some of the successive equine genera migrated to the Old World； but they do not seem to have gained a permanent footing there until the end of the Miocene or beginning of the Pliocene； eventually diversifying into the horses； asses； and zebras of Africa； Asia and Europe。 At about the same period； the family extended its range to South America and there gave rise to a number of species and genera； some of them extremely peculiar。 For some unknown reason； all the horse tribe had become extinct in the western hemisphere before the European discovery； but not until after the native race of man had peopled the continents。
In addition to the main stem of equine descent； briefly considered in the foregoing paragraphs； several side…branches were given off at successive levels of the stem。 Most of these branches were short…lived； but some of them flourished for a considerable period and ramified into many species。
Apparently related to the horses and derived from the same root…stock is the family of the Palaeotheres； confined to the Eocene and Oligocene of Europe； dying out without descendants。 In the earlier attempts to work out the history of the horses； as in the famous essay of Kowalevsky （〃Sur l'Anchitherium aurelianense Cuv。 et sur l'histoire paleontologique des Chevaux〃； 〃Mem。 de l'Acad。 Imp。 des Sc。 de St Petersbourg〃； XX。 no。 5； 1873。）； the Palaeotheres were placed in the direct line； because the number of adequately known Eocene mammals was then so small； that Cuvier's types were forced into various incongruous positions； to serve as ancestors for unrelated series。
The American family of the Titanotheres may also be distantly related to the horses； but passed through an entirely different course of development。 From the lower Eocene to the lower sub…stage of the middle Oligocene the series is complete； beginning with small and rather lightly built animals。 Gradually the stature and massiveness increase； a transverse pair of nasal horns make their appearance and； as these increase in size； the canine tusks and incisors diminish correspondingly。 Already in the oldest known genus the number of digits had been reduced to four in the fore…foot and three in the hind； but there the reduction stops； for the increasing body… weight made necessary the development of broad and heavy feet。 The final members of the series comprise only large； almost elephantine animals； with immensely developed and very various nasal horns； huge and massive heads； and altogether a grotesque appearance。 The growth of the brain did not at all keep pace with the increase of the head and body； and the ludicrously small brain may will have been one of the factors which determined the startlingly sudden disappearance and extinction of the group。
Less completely known； but of unusual interest； is the genealogy of the rhinoceros family； which probably； though not certainly； was likewise of American origin。 The group in North America at least； comprised three divisions； or sub…families； of very different proportions； appearance and habits； representing three divergent lines from the same stem。 Though the relationship between the three lines seems hardly open to question； yet the form ancestral to all of them has not yet been identified。 This is because of our still very incomplete knowledge of several perissodactyl genera of the Eocene； any one of which may eventually prove to be the ancestor sought for。
The first sub…family is the entirely extinct group of Hyracodonts； which may be traced in successive modifications through the upper Eocene； lower and middle Oligocene； then disappearing altogether。 As yet； the hyracodonts have been found only in North America； and the last genus of the series； Hyracodon； was a cursorial animal。 Very briefly stated； the modifications consist in a gradual increase in size； with greater slenderness of proportions； accompanied by elongation of the neck； limbs； and feet； which become tridactyl and very narrow。 The grinding teeth have assumed the rhinoceros…like pattern and the premolars resemble the molars in form； on the other hand； the front teeth； incisors and canines； have become very small and are useless as weapons。 As the animal had no horns； it was quite defenceless and must have found its safety in its swift running； for Hyracodon displays many superficial resemblances to the contemporary Oligocene horses； and was evidently adapted for speed。 It may well have been the competition of the horses which led to the extinction of these cursorial rhinoceroses。
The second sub…family； that of the Amynodonts； followed a totally different course of development； becoming short…legged and short…footed； massive animals； the proportions of which suggest aquatic habits； they retained four digits in the front foot。 The animal was well provided with weapons in the large canine tusks； but was without horns。 Some members of this group extended their range to the Old World； but they all died out in the middle Oligocene； leaving no successors。
The sub…family of the true rhinoceroses cannot yet be certainly traced farther back than to the base of the middle Oligocene； though some fragmentary remains found in the lower Oligocene are probably also referable to it。 The most ancient and most primitive member of this series yet discovered； the genus Trigonias； is unmistakably a rhinoceros； yet much less massive； having more the proportions of a tapir； it had four toes in the front foot； three in the hind； and had a full complement of teeth； except for the lower canines； though the upper canines are about to disappear； and the peculiar modification of the incisors； characteristic of the true rhinoceroses； is already apparent； the skull is hornless。 Representatives of this sub…family continue through the Oligocene and Miocene of North America； becoming rare and localised in the Pliocene and then disappearing altogether。 In the Old World； on the other hand； where the line appeared almost as early as it did in America； this group underwent a great expansion and ramification； giving rise not only to the Asiatic and African forms； but also to several extinct series。
Turning now to the Artiodactyla； we find still another group of mammals； that of the camels and llamas； which has long vanished from North America； yet took its rise and ran the greater part of its course in that continent。 From the lower Eocene onward the history of this series is substantially complete； though much remains to be learned concerning the earlier members of the family。 The story is very like that of the horses； to which in many respects it runs curiously parallel。 Beginning with very small； five…toed animals； we observe in the successive genera a gradual transformation in all parts of the skeleton； an elongation of the neck； limbs and feet； a reduction of the digits from five to two； and eventually the coalescence of the remaining two digits into a 〃cannon…bone。〃 The grinding teeth； by equally gradual steps； take on the ruminant pattern。 In the upper Miocene the line divides into the two branches of the camels and llamas； the former migrating to Eurasia and the latter to South America； though representatives of both lines persisted in North America until a very late period。 Interesting side…branches of this line have also been found； one of which ended in the upper Miocene in animals which had almost the proportions of the giraffes and must have resembled them in appearance。
The American Tertiary has yielded several other groups of ruminant…like animals； some of which form beautifully complete evolutionary series； but space forbids more than this passing mention of them。
It was in Europe that the Artiodactyla had their principal development； and the upper Eocene； Oligocene and Miocene are crowded with such an overwhelming number and variety of forms that it is hardly possible to marshal them in orderly array and determine their mutual relationships。 Yet in this chaotic exuberance of life； certain important facts stand out clearly； among these none is of greater interest and importance than the genealogy of the true Ruminants； or Pecora； which may be traced from the upper Eocene onward。 The steps of modification and change are very similar to those through which the camel phylum passed in North America； but it is instructive to note that； despite their many resemblances； the two series can be connected only in their far distant beginnings。 The pecoran stock became vastly more expanded and diversified than did the camel line and was evidently more plastic and adaptable； spreading eventually over all the continents except Australia； and forming to…day one of the dominant types of mammals； while the camels are on the decline and not far from extinction。 The Pecora successively ramified into the deer； antelopes； sheep； goats and oxen； and did not reach North America till the Miocene； when they were already far advanced in specialisation。 To this invasion of the Pecora； or true ruminants； it seems probable that the decline and eventual disappearance of the camels is to be ascribed。
Recent discoveries in Egypt have thrown much light upon a problem which long baffled the palaeontologist； namely； the origin of the elephants。 （C。W。 Andrews； 〃On the Evolution of the Proboscidea〃； 〃Phil。 Trans。 Roy。 Soc。〃 London； Vol。 196； 1904； page 99。） Early representatives of this order； Mastodons； had appeared almost simultaneously （in the geological sense of that word） in the upper Miocene of Europe and North America； but in neither continent was any more ancient type known which could plausibly be regarded as ancestral to them。 Evidently； these problematical animals had reached the northern continents by migrating from some other region； but no one could say where that region lay。 The Eocene and Oligocene beds of the Fayoum show us that the region sought for is Africa； and that the elephants form just such a series of gradual modifications as we have found among other hoofed animals。 The later steps of the transformation； by which the mastodons lost their lower tusks； and their relatively small and simple grinding teeth acquired the great size and highly complex structure of the true elephants； may be followed in the uppermost Miocene and Pliocene fossils of India and southern Europe。
