ANCESTRY AND CLASSIFICATION. 235 



single-brooded, and the variety Marcellus would 

 become extinct ; suppose again both features to 

 hold with different sets of individuals, gradually 

 communicating this tendency in greater and 

 greater force to their offspring, and we should 

 behold the spring and summer varieties changed 

 to separate species. This is one example of a mode 

 in which seasonal dimorphism may become an 

 originator of new forms. It is plain that entirely 

 similar results may follow from unequal lethargy 

 in one brood of caterpillars, such as have been 

 referred to in some of the fritillaries. In one in- 

 stance we saw reason to believe that there were 

 two series of individuals in a single species, more 

 or less independent, and with a distinct cycle of 

 changes ; very slight differences have been found 

 between the individuals belonging to these two 

 series ; but with complete independence we may 

 be sure that these differences would intensify, and 

 distinct species be formed. 



Ordinary dimorphism again, or the appearance 

 of different varieties in each brood, running 

 through both sexes, must surely be a precursor of 

 a division in the species ; no doubt the change is 

 gradual, so that centuries of direct experimenta- 

 tion would throw no light upon the change ; but 

 we have only to suppose each form breeding true 

 to itself and the separation will be accomplished. 

 In the case of the Yiolet-tip, we have two strik- 



