ITS PLACE IN NATUKE. 3 



in the economy of nature may be most advanta- 

 geously studied by every one who wishes to gain an 

 insight into some of the fundamental principles of 

 biology. In scarcely any other animal has specializa- 

 tion of various parts that is, modification from the 

 general or average type to conform to the require- 

 ments of some special mode of existence been car- 

 ried to such an extreme. In many organs, but espe- 

 cially in the limbs and teeth, we find the strongest 

 evidence of two opposing principles striving against 

 each other for the mastery in fashioning their form 

 and structure. We find heredity , or adherence to a 

 general type derived from ancestors, opposed by spe- 

 cial modifications of or deviations from that type, and 

 the latter generally getting the victory, although in 

 the numerous rudimentary structures that remain 

 there is significant evidence of ancestral conditions 

 long passed away. The various specializations, evi- 

 dently in adaptation to purpose, will be thought by 

 many to be the result of the survival, in the severe 

 struggle for existence, of what is best fitted for the 

 purpose to which it is to be applied. This may or 

 may not be the explanation, but the interest of the 

 study of such an animal as the horse will be in- 

 creased tenfold by the conviction that there is some 

 true and probably discoverable causation for all its 

 modifications of structure, however far we may yet 



