50 THE COLQUES OF ANIMALS 



different in appearance, and often in size, from the 

 later ones. Professor Weismann has investigated this 

 question, and he finds that while the later broods can 

 be readily made, by the application of ice in the pupal 

 stage, to assume the form of the first or winter gene- 

 ration, the latter cannot be made to assume the form 

 of the summer brood by the application of warmth. 

 He infers that such species were single-brooded in the 

 short summers which succeeded the Glacial Period, 

 and that the appearance was that of the present winter 

 form. As the summers became longer, other newer 

 generations with a different appearance were added 

 (summer broods), but the species always tends easily 

 to revert to the more ancient form. An important 

 part of the evidence consists in the proof that such 

 species are now single -brooded in the northern part of 

 their range, and that the one form is that of the 

 winter brood of more southern localities. ^ 



I have given a very brief sketch of dimorphism, 

 hardly alluding to polymorphism, which is only an 

 extension of the same principle. Although the subject 

 is only touched upon, enough has been said to show 

 that there are many distinct kinds of dimorphism, 

 some of which are very obscure. By far the most 

 important kind of di- or polymorphism remains to be 

 described below (see Chapters VIII. and IX.), in which 



' See Studies in the Theory of Descent, by August Weismann. 

 English translation by Professor Meldola. 



