500 Studies in the Theory of Descent. 



equal period of time, this must be decidedly 

 answered in the negative. The unequal develop- 

 ment obviously depends in this case, as in the 

 higher systematic groups of the Lepidoptera, upon 

 the unequal value of the parts affected by the 

 changes. These parts are on the one side of 

 small importance, and on the other side of great 

 importance, to the whole structure of the insect. 

 This is shown in the last-mentioned case of the 

 fleas, where, of the typical parts of the body, only 

 the wings have become rudimentary, whilst the 

 antennae, mouth-parts, and legs, and even the form 

 and mode of segmentation (free thoracic segments), 

 must have suffered most important modifications ; 

 their larvae, on the other hand, can have ex- 

 perienced only unimportant changes, since they 

 still agree in all typical parts with those of the 

 gnat-type. 



Although therefore in this and in similar cases a 

 greater number of transforming impulses may well 

 have occurred on the one side than on the other 

 and it is indeed highly probable that this number 

 has not been absolutely the same nevertheless 

 the chief cause of the striking incongruence is not 

 to be found therein, but rather in the strength of 

 the transforming impulses, if I may be permitted 

 to employ this figure, or, more precisely expressed, 

 in the importance of the parts which become 

 changed and at the same time in the afnount of 

 change. 



