SPECIALISATION. 



the primitive number, five : its limbs are specially fitted for 

 swift movement over bard ground. To take another instance, 

 the teeth of the tiger are said to be highly " specialised/' 

 because there are only thirty of the original forty-four, and 

 these have become specially adapted for seizing living prey and 

 cutting and tearing its flesh. It must be added that all the 

 characters of a group of animals do not necessarily become 

 specialised, but that some may remain in the primitive condi- 

 tion. Thus man, in some ways the most highly specialised of 

 mammals, still retains the primitive number of five digits on 

 both limbs. 



Fig. 3. 



ii 



Diagram showing the gradual loss of toes on the fore foot (a) and increase 

 of complexity in the grinding teeth (b) of successive horse-like Ungulata 

 from Europe, namely Hyracotherinm (Eocene) 4, Anchitherium (Mio- 

 cene) :3, HippaHon (Pliocene) 2, Equus (Pleistocene and Recent) 1. 



In order to trace back a modern specialised group of mammals 

 to its early primitive ancestors, a long series of fossil remains 

 from the successive geological periods is necessary. Unfor- 

 tunately, in many cases these fossils have yet to be found, but 

 every year further discoveries are made and gaps of more or 

 less importance are filled up. The series of changes undergone 

 by a group of mammals is perhaps best known in the horse 



