ORIGIN OF THE SPECIFIC TYPE 345 



of cold brings them rapidly to a climax. Here, therefore, the necessary 

 variations of the relevant germinal parts must have continually 

 presented themselves for selection, which is intelligible enough, since 

 it is merely a question of plus- or minus-variations. The fact that the 

 six-months' dress can be transformed into an eight-months' dress 

 must have its cause in some minute biological units of the germ- 

 plasm : the determinants of the fur must be able to vary in such a way 

 that a longer or shorter duration of the winter's coat is the result. 

 The possibility of the whole variation depends upon the continual 

 fluctuations of all determinants, now towards plus, now towards 

 minus, and the necessity and inevitableness of each adaptation to the 

 duration of the winter lies in the unceasing personal selection — the 

 inexorable preferring of the better adapted. 



