48 John Henry Cotnstock 



generalized condition of other characters ; while in another 

 line of descent very highly specialized forms may still possess 

 the organ in question. 



A good illustration of this is presented by the condition of 

 the mouth in the Macrolepidoptera. In many moths the 

 mouth parts are wanting, while in other moths and in butter- 

 flies the maxillae are ver}' highly specialized. It cannot be 

 concluded from this fact that the mouthless forms are farther 

 removed from the primitive type than are the sphinges and 

 butterflies for example. A study of other structures would 

 not support such a conclusion. We have to do in a case of 

 this kind either with verj^ distinct lines of descent or with a 

 sidewise development. 



In the case of the organ selected, the mouth, there comes 

 into play, I believe, a very peculiar principle. For a long 

 time I was greatly puzzled by the many instances in which 

 absence of mouth parts is correlated with a very generalized 

 condition of other structures. The explanation of this phe- 

 nomenon I now believe to be as follows : Under certain condi- 

 tions natural selection may tend to change the length of the 

 adult stage. In some cases those individuals that most quickly 

 provide for the perpetuation of the species are the ones that are 

 most likely to have offspring. Under such conditions there 

 would be a shortening of the duration of the adult stage until a 

 point was reached at which it would not be necessary for the in- 

 sect to take food during the adult stage, and the mouth parts 

 would be lost in this stage. 



But this shortening of the duration of the adult stage would 

 also tend to a great degree to remove the species that had ac- 

 quired it from the struggle for existence in this stage. A 

 species that found it necessary to fly only a few hours or even 

 days in order to provide for the perpetuation of its kind would 

 not offer such an opportunity for the action of natural selec- 

 tion upon the .structure of its wings and other organs peculiar 

 to the adult, as would surely occur in a species having a 

 longer period of flight. 



