EVOLUTION OF THE M.MIi1ALIA. During the age of reptiles the mammals had been slowly evolving, but very little is known of them. The remains that have been found are exceedingly rare and very fragmentary, and indicate very primitive tyres, of minute size and apparently arboreal habit. lint at the beginning of the Tertiary period they appear in force, and rapidly inerease in size and variety. taking the plaee of the reptiles as the type, and like them brariehing, out into terrestrial, arboreal, anri a I, marine, and some amphibious races. The :twos t ry of many modern animals has been traced back through successive stages nearly to the beginning of the Tertiary period, always eonverging to ward a common ancestor, The most complete and instructive of these lines of descent is that of the horse (see house, Fossil.) ; that of the camel stands next in enninletelleSs ; and less complete series represent the ancestry of Many Of the higher rm»inants, of the elephants, rhinoceroses. tapirs, (lugs, and many other animals. Indeed. it may be said that there are comparatively few lainl mammals of which some ancestral types are not known; and these ancestral forms in variably present primitive characters linking the modern representative to the common ancestor of the mammals, although they often combine tbesr primitive characters with peculiar which show that they are not in the direct line of descent. And these primitive characters are more pronounced in the more ancient forms.
We have, hot•ever, very little positive knowl edge of the ancestry of the marine ma»inials, the seals, cetaceans (whales, dolphins, and porpoises, etc.), and si renians (dugongs and inanat(ps). Setting aside these, with a few rare and peculiar land mammals whose history is unknown, and the Australian mammals (marsupials and null titubcrculates) which probably diverged from the common stock at an earlier date, we can infer with a high degree of probability from these known lines of descent a common ancestor to the modern mammals, which lived during •the Cretaceous period and whose descendants at the beginning of the Tertiary period hail diverged comparatively little in characters. This common
ancestor was of small size, of a low order of in telligence for a mammal, the legs short, five toes on each foot and claws on the toes, and a long heavy tail. Its teeth were 44 in number, 1 1 on each side of each jaw, consisting of three incisors, a canine, four premolars, and three molars. The molars had each three sharp cusps arranged in the form of a triangle; the remaining teeth had but one cusp, but the canine was larger than the rest. The animal walked on the sole of the foot, and there are some reasons for thinking that it was at least partly arboreal. Its food was prob ably chiefly insects or other small animals. Only a few fragments of jaws and teeth of Cretaceous mammals have been found: but some of them cor respond, as far as they go, to the characters of the hypothetical type, while others represent more ancient offshoots from the central stock.
The oldest Tertiary mammals preserve nearly all the above characters of their supposed ances tor, but show the early steps of the divergence which led into the various types of the present day. Some were becoming herbivorous, develop ing hoofs instead of claws. lengthening the limbs and losing one or more of the side toes on each foot to increase the speed, and changing the character of the teeth to fit them better for a vegetarian diet. Others were becoming more strictly carnivorous, adapting the teeth to cut and tear flesh; while others again were arboreal and frugivorous as judged from the characters of their limbs and teeth. In the succeeding stages of the Tertiary the characters of each line become more marked, some lines leading tip into modern animals, others into side-branehes which have died out. Several of these side-branches ended in clumsy monstrous races of huge size and peculiar form. occupying the same place in the various Tertiary faunas that the elephants, rhinoceroses. and hippopotami do in that of to-flay. Such were the C'oryphodon and Uinta therium of• the Eocene, the Titanotherium and Elotherium of the Oligocene, the Chalicotherimn of the Aliocene, and various peculiar extinct rhinoceroses in America and Europe.