ITS ANCESTORS AND RELATIONS. 13 



throughout the whole of the Tertiary period sepa- 

 rated into two perfectly distinct sections, differing 

 from each other not only in the obvious characters 

 of the structure of their limbs, but in numerous im- 

 portant points in other portions of their organi- 

 zation, such as their skull, vertebral column, teeth, 

 digestive organs, etc. The characters of these two 

 groups, first indicated by Cuvier, were thoroughly 

 established by Owen, by whom the names by which 

 they are now generally known were proposed. These 

 are Artiodactyla, or even-toed, and Perissodactyla, or 

 odd-toed* 



It is only by studying the fundamental type of 

 organization common to all members of a group 

 which underlies the various external or superficial 

 modifications by which it becomes adapted to the 

 different surrounding conditions under which it has 

 to carry on its existence, that the true relationship of 

 animals can be determined. In this way it can be 

 clearly demonstrated that the pig, the deer, the ox, 

 sheep, goat, antelope, and camel, including even such 

 extreme forms as the giraffe and the hippopotamus, 

 are formed on one plan — the Artiodactyle ; while the 

 horse, the tapir, and the rhinoceros are formed on 

 the other — the Perissodactyle type. 



* From the Greek artios, even in number, and perissos, un- 

 even ; combined with daktylos, finger or toe. 



