INTRODUCTION. 



i7 



old forms, resemble hoofed mammals, especially 

 Artiodactyla, in dentition. Even the placenta 

 bespeaks a close affinity to the latter. Very 

 probably, therefore, our present mode of division 

 unites in a single order animals which are derived 

 from quite different ancestors, but which have 

 approached one another in respect of a number of 

 secondary characters. 



The same holds good also of the apes and 

 monkeys, which cann*^ be derived from a single 

 ancestor. On their first appearance in the lower 

 Miocene the SimiiE of the New World are as 

 different from those of the Old as they are at the 

 present day. Moreover, there are Simise which are 

 rather insectivorous in their habits, while others 

 again approach more nearly the carnivorous and 

 omnivorous forms. 



We should thus have a group of orders, the Chir- 

 optera, Prosimii, and Simise, which might, at least 

 in part, be placed very near the old Insectivora. 



Take now the Carnivora. We know even in 

 Jurassic strata marsupials whose dentition ap- 

 proaches the carnivorous type, and from more 

 recent strata, as well as the fauna of the present 

 era, we are acquainted with many, which, if we may 

 so e.xprcss ourselves, are more carnivorous than 

 the placental carnivores. The Hyaenodons are an 

 unmistakable connecting link. The Carnivora are 

 accbrdingly descended without doubt from mar- 

 supials. The seals, as we have already mentioned, 

 are only a branch of this group adapted to an 

 aquatic mode of life. 



The Perissodactyla and Artiodactyla belong to 

 the Eocene nobility. May we perhaps connect 

 them with the Jurassic genus Stereognathus.' 

 However that may be, these two orders form, so 

 to speak, trees, whose original trunks, the tapirs 

 and pigs, have continued on to our own day. 

 Both stems have put forth many branches now 

 extinct, which we cannot here enumerate, but the 

 Solidungula and rhinoceroses form series which 

 have developed from the ancient Perissodactyla, 

 the former from those of Eocene, the latter 

 those of Miocene times. Very probably the Pro- 

 boscidea and Sirenia are also to be reckoned to this 

 group. Further, there is probably no doubt as to 

 the fact that the ruminants and hippopotamuses are 



descended from the first Artiodactyla, the pigs. In 

 these two great orders, also, the marked distinction 

 between the primitive stocks of the Old and the 

 New World is plainly manifest. 



The rodents of the Eocene nobility cannot be 

 traced with certainty from any marsupial form. 

 Only the genus Plagiaulax, from Jurassic times, ex- 

 hibits certain distant resemblances. The members 

 of this order have remained what they were at first; 

 they have not essentially altered throughout the 

 whole geological period in which they are known. 



The same is the ca.se with the edentates. This 

 group is manifestly made up of the descendants of 

 several ancestors; but we can only trace them 

 back to the Miocene, and in doing so we cannot 

 discover any unequivocal relations to older types. 



From all this two main conclusions in our opinion 

 may be drawn. First, that it is a perfectly ground- 

 less hypothesis, that the mammals can all be traced 

 back to a single stock ; and, second, that the various 

 original stocks, which we feel compelled to assume, 

 have developed, in the regions to which they are 

 confined, independently of each other, and often in 

 such a manner that the ultimate forms which they 

 have attained have more resemblance to each other 

 than the types from which they have proceeded. 



In the body of the present work we have sub- 

 divided the mammals in accordance with the points 

 of view which we have just explained. We follow 

 the series from the most perfect forms, those stand- 

 ing nearest to man, the Simian, down to the types 

 with the lowest organization. The aplacental 

 forms, the marsupials and monotremes, were 

 obliged to take the last place; and among the 

 placental forms the edentates and rodents un- 

 doubtedly form the lowest steps in the ladder. 

 After the Simla; the orders standing next to the 

 stem-group of the Insectivora, namely the Prosimii 

 and Chiroptera, naturally range themselves in im- 

 mediate succession, and then follow the Carnivora, 

 among which the Pinnipedia (seals) form the 

 transition to the whales, with which again are con- 

 nected the Sirenia and Proboscidea as transitions to 

 the Even-toed Ungulates (Artiodactyla). 



The table on page i8 exhibits these subdivisions, 

 with their general characters, in the order in which 

 they are treated in the body of the work. 



