Phyictic Para i It /ism in Metamorphic Species. 503 



frequently connected with the first kind, and is 

 mostly the direct consequence of the latter. 



The cause of the incongruences is* to be found 

 in unequal phyletic development, either the one 

 stage within the same period of time having been 

 influenced by a greater number of transforming- 

 impulses than the other, or else these impulses 

 have been different in strength, i.e. have affected 

 parts of greater or less physiological value, or 

 have influenced parts of equal value with unequal 

 strength. 



In all these cases in which there are deep- 

 rooted form-differences, it can be shown that these 

 correspond exactly with inequalities in the con- 

 ditions of life, this correspondence being in two 

 directions, viz. in strength and in extent : the 

 former determines the degree of form -difference, 

 the latter its extent throughout a larger or smaller 

 group of species. 



The different forms of incongrtience are mani- 

 fested in the following manner : 



(i.) Different amount of form-divergence 

 between the larvae on the one side and the 

 imagines on the other. Among the Lepidoptera 

 this is found most frequently in varieties and 

 species, and there is evidence to show that in this 

 case the one stage has been affected by trans- 

 forming influences, either alone (varieties), or at 

 any rate to a greater extent (species). In the 

 last case it can be shown in many ways that one 



