468 Studies in the TJteory of Descent. 



in which the changes are of the same physiological 

 and morphological importance to the whole 

 organism. 



Now in the lower systematic groups this is 

 always the case. Varieties, species, and genera 

 are always distinguished by only relatively small 

 differences ; deep-seated distinctions do not here 

 occur, as is implied in the conception of these 

 categories. The true cause of this is, I believe, 

 to be found in the circumstance that all changes 

 take place only by the smallest steps, so that 

 greater differences can only arise in the course of 

 longer periods of time, within which a great 

 number of types (species) can, however, come 

 into existence, and these would be related by 

 blood and in form in different degrees, and would 

 therefore form a systematic group of a higher 

 rank. 



The short periods necessary for the production 

 of inferior groups, such as genera, would not 

 result in incongruences if only untypical parts of 

 the larvae, such as marking or spines, underwent 

 change, whilst in the imagines typical parts 

 wings and legs became transformed. The 

 changes which could have occurred in the wings, 

 &c., during this period of time would have been 

 much too small to produce any considerable influ- 

 ence on the other parts of the body by correlation ; 

 and two species of which the larvae and imagines, 

 had changed with the same frequency, would show 



