GENERAL STRUCTURE. 1259 



connection with the other families. Similarly, at the same period 

 the Pigs of the Old World appear to be inseparable from the 

 Peccaries of the New ; while the Giraffes were probably as closely 

 connected with the Antelopes on the one hand, and the Deer on 

 the other. Again, in the Perissodactylate suborder Tapir-like forms 

 appear to have passed into Rhinoceroses on the one hand, and into 

 the Horses on the other. 



Many other equally striking instances could be cited of the 

 mutual connection of the Eocene Mammals with one another ; but 

 enough has been stated to show that the present sharply defined 

 distinction of the orders and families into which we find it con- 

 venient to divide the class is, so to speak, but a feature of 

 to-day. 



In the Lower Eocene all the genera of Mammals appear to be 

 extinct ; but in the Upper division of the same period, which is 

 frequently termed the Lower Oligocene, we meet with a few existing 

 genera, such as Didelphys, Rhinoceros, Viverra, Mustela (Weasel), 

 and perhaps Cants. In the succeeding period, or Miocene, existing 

 genera become more common ; thus in the Lower and Middle divi- 

 sions of that period we meet with Otters (Lutra), Rhinoceros, Tapirs 

 (Tapirus), and Gibbons (Hylobates). The middle division of the 

 Miocene is, indeed, noteworthy for the appearance of Anthropoid 

 Apes and the Proboscidea, or Elephants and Mastodons. Deer of 

 extinct genera are abundant ; but these were either devoid of antlers, 

 or, if these appendages were present, they were small and simple. 

 The teeth of the Ruminant Ungulates (Deer and Cattle) were, 

 moreover, low-crowned or brachydont (fig. 12 13), and the Rhino- 

 ceroses had in most cases not yet developed horns. 



With the commencement of the Pliocene period the Mammalian 

 fauna assumes a much more modern appearance. Thus we have 

 Porcupines (Hystrix), Hyaenas, large Tiger-like Cats (Re/is), numer- 

 ous Antelopes, Giraffes, Deer (Cervus), and Horse-like animals 

 (Hipparion) ; while in India true Elephants (Elephas) had made 

 their appearance. There was still, however, a large number of 

 extinct genera. By this time many of the Deer had acquired 

 complicated antlers ; many of the Ruminants had tall-crowned or 

 hypsodont teeth ; the Rhinoceroses had horns ; and the Pigs had 

 developed large tusks in the males. In India during some portion 

 of the Pliocene, not only Elephants, but true Horses (Equus), 

 Hippopotami, Wolves, Bears (Ursus), and Oxen (Bos) had already 

 made their appearance ; but in Europe these genera are unknown 

 before the top of the Pliocene, when we first meet with remains of 

 a few existing species, such as the African Hippopotamus and the 

 Striped Hyaena. 



In the succeeding Pleistocene period, which may in reality be 



