660 Studies in the Theory of Descent. 



when, on the other hand, we find that all polar 

 animals to which the white coloration is advan- 

 tageous, and indeed none but species of which 

 the nearest allies vary only individually into white, 

 possess this colour, must we not conclude from 

 this alone that single variations can, under 

 favourable conditions, become predominant cha- 

 racters? 



It appears to me that in this question one 

 weighty factor has been too little regarded, even 

 by the supporters of the selection theory, viz., the 

 slowness of most, and especially of climatic 

 changes, which I have already insisted upon. If 

 the transformation of a temperate into an arctic 

 climate occurred so rapidly that the species ex- 

 posed to it had the alternative either of becoming 

 white in ten or twenty generations or of being 

 unable to exist, then the hasty intervention of a 

 directive power could alone save them from ex- 

 termination by causing hundreds of thousands of 

 individuals to become similarly coloured with all 

 speed. But it is quite different if the change of 

 climate takes place only in the course of several 

 thousand generations ; and this, according to the 

 geological evidence, must have been the true state 

 of the case. 



Let us take a definite example the well-known 

 one of the hare. With us this animal remains 

 brown in the winter and but seldom produces 

 white varieties, whilst its ally the Alpine hare is 



