524 



manual of elementary zoology 



enable us to infer past history from present facts. Un- 

 fortunately, owing to what is known as the imperfection of 

 the geological record, such complete proof is impossible. 

 The unsuitability of the bodies of many animals for pre- 

 servation owing to the absence of hard parts, the destruc- 

 tion of immense layers of rocks, and the small proportion 

 of those which remain that can be examined, bring this 

 about. But it is established that throughout geological 



Fig. 390. — The bones of the foot of a horse compared with those 

 of earlier members of its family. — After Neumayr. 



1, Palaeotherium, which appeared in the "Eocene" period of geology; 



2, Anchitherium, which appeared^ in the following " Miocene " period ; 



3, Hippotherium, which appeared in the " Pliocene " period ; 4, the modern 

 horse. Note the continuous reduction of the side toes. 



history there is a continual change in the types of animal 

 life, leading up to those that exist at the present day, and 

 in a few rare cases, particularly among Mollusca (Fig. 389), 

 it is possible to trace fully the evolution of species, while 

 in others, such as those of horses and of elephants, the 

 origin of higher groups can be followed in the appearance 

 of successive genera (Fig. 390). 



As to how evolution has been brought about, there is 

 a conflict of opinion. One theory supposes that the 

 modifications which arise in each individual in the course 



