Phylctic Ptiraf/e/ism in Mcfanwrphic Species. 467 



time which is sufficient for the production of 

 species, the one stage only is but seldom and quite 

 exceptionally influenced by transforming impulses, 

 whilst both stages are as a rule affected, although 

 not with the same frequency, it must necessarily 

 follow that on the whole, as the period of time 

 increases, the difference in the number of these 

 impulses which affect the larva and of those 

 which affect the imago must continually decrease, 

 and with this difference the magnitude of the 

 morphological differences resulting from the trans- 

 forming influences must at the same time also 

 diminish. With the number of the successively 

 increasing changes the difference in the magnitude 

 of the change in the two stages would always 

 relatively diminish until it had quite vanished 

 from our perception ; just in the same manner as 

 we can distinguish a group of three grains of corn 

 from one composed of six, but not a heap of 103 

 grains from one containing 106 grains. 



That the small systematic groups must have 

 required a short period and the large groups a 

 long period of time for their formation requires no 

 special proof, but results immediately from the 

 theory of descent. 



All the foregoing considerations would, how- 

 ever, only hold good if the transforming impulses 

 were equal in strength, or, not to speak figura- 

 tively, if the changes only occurred in equivalent 

 portions of the body, i. e. in such portions as those 



H h 2 



