39 2 Studies in the Theory of Descent. 



another. The larvae and imagines of two species 

 must differ from one another to the same extent, 

 and the same must hold good for the larvae and 

 imagines of two genera, families, and so forth. In 

 brief, a larval system must completely coincide 

 with the system based entirely on imaginal cha- 

 racters, or, what amounts to the same thing, 

 the form-relationships of the larvae must corre- 

 spond exactly with the form-relationships of the 

 imagines. 



On the other hand, the condition of affairs must 

 be quite different if an internal power causing 

 phyletic remodelling does not exist, the transforma- 

 tion of species depending entirely on the action of 

 the environment. In this case dissimilarities in 

 the phyletic development of the different stages of 

 life must be expected, since the temporary, and 

 often widely deviating, conditions of life in the 

 two stages can and must frequently influence the 

 one stage whilst leaving the other unacted upon 

 the former can therefore undergo remodelling 

 while the latter remains unchanged. 1 



1 [Compare this with Darwin's " Origin of Species " (ist. 

 ed. p. 440), where it is stated that when an animal, during any 

 part of its embryonic career, is active, and has to provide for 

 itself, " the period of activity may come on earlier or later in 

 life ; but whenever it comes on, the adaptation of the larva to 

 its conditions of life is just as perfect and beautiful as in the 

 adult animal. From such special adaptations the similarity of 

 the larvae or active embryos of allied animals is sometimes 

 much obscured." R.M.] 



