466 Studies in the Tlieory of Descent. 



number of transforming impulses, we perceive 

 that the number of the impulses which have 

 affected larvae and imagines agree with one 

 another the more closely the larger the systematic 

 groups which are compared together. How can 

 this be otherwise ? The larger the systematic 

 group the longer the period of time which must 

 have been necessary for its formation, and the 

 more numerous the transforming impulses which 

 must have acted upon it before its formation was 

 completed. 



But if the supposition that the impulse to 

 change always comes from the environment in no 

 way favours the idea that such impulses always 

 affect both stages contemporaneously, and are 

 equal in number during the same period of time, 

 there is not, on the other hand, the least ground 

 for assuming that throughout long periods the 

 larvae or the imagines only would have been 

 affected by such transforming influences. This 

 could have been inferred from the fact that 

 varieties frequently depend only upon one stage, 

 whilst specific differences in larvae only also occur 

 occasionally, the imagines remaining alike ; but no 

 dingle genus is known of which all the species 

 passess similar larvae. Within the period of time 

 durifng which genera can be formed the trans- 

 forming impulses therefore never actually affect 

 the onte stage only, but always influence both. 



But ii f this is the case if within the period of 



