COLOEATION AFFECTED BY THE ENVIRONMENT. 79 



the intermediate stages must be useless : a piebald animal 

 would be just as conspicuous as a black one — perhaps even 

 more so, by the unusualuess of its appearance ; even if we 

 postulate a gradual whitening of the fur the difficulties are 

 hardly removed. 



It may be replied, however, to this that albinism, complete 

 or incomplete, occurs in many animals, and it has been known to 

 be hereditary ; for the most part such albinos or partial albinos 

 are weeded out on account of their conspicuousness, but in 

 the Arctic regions the albinos would be just the ones to be 

 preserved, the others dying out. In this way we may possibly 

 account for the origin of the white coloration in the polar 

 bear and in other animals which are white the whole year 

 round. But this line of argument will hardly apply to those 

 creatures which only change in the winter. It is difficult to 

 avoid the conclusion that here the depigmentation of the hair 

 is directly due to cold, or dryness, or whatever the true environ- 

 mental cause may be. Nor, on the other hand, can it be 

 urged that the delicacy of constitution which allows of this 

 yearly recurring change has been produced by natural selec- 

 tion. It must have occurred suddenly, to be of any use ; but 

 when it was once established it may have been perpetuated by 

 natural selection. 



Seasonal Dimorphism.* 



The classical instance of this remarkable phenomenon is of 

 course afforded by the spring and summer broods of the conti- 

 nental Vanessid ( Vanessa prorsa levana). 



* I confess myself unable to understand the necessity for Dr. Weismann's 

 apologies for making use of the above— Mr. Wallace's — term ; nor can I see 

 why Professor Elmer should stigmatise it as " in every respect monstrous." 

 They are true English words— an adjective and substantive ; we might as 

 well object to such expressions as " complete metamorphosis " and " mimetic 

 resemblance." 



