Phyletic Parallelism in Metamorpku Sf>ccies. 465 



organism to some of the influences proceeding 

 from the environment ; every living form would 

 in this case remain constant so long as it was not 

 compelled to change by inciting causes. Such 

 transforming factors can act directly or indirectly, 

 i. e. they can produce new changes immediately, 

 or can bring about a remodelling by the combina- 

 tion, accumulation, or suppression of individual 

 variations already present (adaptation by natural 

 selection). Both forms of this action of external 

 influences have long been shown to be in actual 

 operation, so that no new assumption will be made, 

 but only an attempt to explain the phenomena in 

 question by the sole action of these known factors 

 of species formation. 



If, in the first instance, we fix our attention 

 upon that form of incongruence which manifests 

 itself through unequal divergence of form-rela- 

 tionship, it will appear prominently that this bears 

 precise relations to the different systematic groups. 

 This form of incongruence constitutes the rule in 

 varieties of the order Lepidoptera, it is of very 

 frequent occurrence in species, but disappears 

 almost completely in genera, and entirely in the 

 case of families and the higher groups. On the 

 whole, therefore, as we turn to more and more 

 comprehensive groups, the incongruence diminishes 

 whilst the congruence increases, until finally the 

 latter becomes the rule. 



Now if congruence presupposes an equal 



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