ITS ANCESTORS AND RELATIONS 



13 



organisation common to all members of a group which 

 underlies the various external or superficial modifications 

 by which it becomes adapted to the different surround- 

 ing 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 

 rhinocero s are formed on the other — the Perissodactyle 

 type. 



To understand one of the principal distinctions 

 between these two forms, it must be premised by way of 

 explanation that the number of digits (a convenient 

 common term to express either fingers or toes, whether 

 of the fore or hind foot) in mammals never exceeds five 

 on each limb. For convenience of description, they are 

 designated numerically from the inner side of the limb — 

 I., II., III., IV. and V. (fig. 1, p. 14)— the pollex (thumb) 

 and hallux (great toe) being the first of the fore and 

 hind limbs respectively, and the third is the middle of 

 the complete series. When the number falls short of five, 

 it is always easy to determine, by their relations to the 

 bones of the wrist or ankle, which of the typical series 

 are present and which are missing. 



In the Artiodactyles the third and fourth digits of 



