36 THE THEORY OF EVOLUTION 



tends to become more and more convoluted, more and 

 more decorated with lines and excrescences, or some 

 definite organ is added and thereby indirectly the 

 entire organism becomes more and more specialized 

 for a definite function (mode of progression, transfor- 

 mation of the limbs for swimming, flying, running, 

 digging, etc.), and as this proceeds always in the same 

 direction it becomes more and more adapted to the 

 particular function concerned. With these phenomena 

 is associated the well-known example of the so-called 

 horse-foot series, which demonstrates how, from a 

 normal five-toed foot, the one-toed foot of the present 

 horse has been quite gradually evolved, as a constantly 

 one-sided, and therefore, in a certain sense, a more and 

 more perfect, adaptation for speedy running. With 

 it is furthermore associated the evolution of the paddle 

 hand of the Sirens (sea-cows), the evolution of defensive 

 and gripping weapons for instance, the horns of the 

 stag which from small beginnings arrived at colossal 

 dimensions in the extinct gigantic deer ; also the tusks 

 of the Proboscidse, etc. 1 



' Specialization ' signifies, therefore, the development 

 in one direction of an organ or of the entire organism. 

 What causes are effective are not always demonstrable ; 

 probably the necessity of purposeful adaptation to 

 changed environments was the reason. 



' Differentiation ' is the development of numerous 

 variations of one and the same fundamental type by 

 specialization of separate individuals in different direc- 



1 Deperet-Wcgner : Die Umbildung der Tierwelt, chap. xx. 



