1 70 Studies in the Theory of Descent. 



the form-relationship expressed the genealogical 

 connection of the species. But it would be quite 

 different if every species presented itself in two or 

 three different forms. If in two species or genera 

 the butterflies as well as the larvae and pupae ex- 

 hibited the same degree of form-relationship, the 

 probability that this expressed also the blood- 

 relationship would then be exceedingly great. 

 Now this agreement certainly does not always 

 occur, and when these different stages are re- 

 lated in form in unequal degrees, the problem 

 then is to decide which of these relationships 

 expresses the genealogy. This decision may be 

 difficult to arrive at in single cases, since the cater- 

 pillar may diverge in form from the next blood- 

 related species to a greater extent than the butter- 

 fly, or, conversely, the butterfly may diverge more 

 widely from its nearest blood-related species than 

 the caterpillar. 



For such cases there remains the develop- 

 mental history of the caterpillar, which will almost 

 always furnish us to a certain extent with infor- 

 mation respecting the true genealogical relation- 

 ship of the forms, because it always reveals a 

 portion of the phyletic (ancestral) development of 

 the species. If we see two species of butterflies 

 quite dissimilar in form of wing and other charac- 

 ters, we should be inclined, in spite of many points 

 of agreement between them, to place them in 

 entirely different genera. But should we then 



