456 Studies in t/te Theory of Descent. 



Hydroid Polypes and inferior Medusae. This 

 case of Porcellus, however, shows that those are 

 correct who maintain that systemy claims to 

 express, although incompletely, the blood-relation- 

 ship, and that systematists have always uncon- 

 sciously formed their groups as though they 

 intended to express the genetic connection of the 

 forms. Only on this supposition can it appear 

 incorrect to us to thus separate two species of 

 which the larvae agree so completely. 



I cannot conclude this review of the various 

 systematic groups without taking a glance at the 

 groups comprised within species, viz. varieties. 

 Whilst in species incongruence is of frequent 

 occurrence, in varieties this is the rule, for which 

 reason it admits in this case of being more 

 sharply defined, since we are not concerned with 

 a double difference but only with the question 

 whether in the one stage a difference or an 

 absolute similarity is observable. By far the 

 majority of varieties are either simply imaginal or 

 merely larval varieties only the one stage 

 diverges, the other is quite constant. 



Thus, as has already been shown, in all the 

 seasonally dimorphic butterflies known to me the 

 caterpillars of the two generations of imagines, 

 which are often so widely different, are exactly 

 alike ; and the same obtains for the majority of 

 purely climatic varieties of butterflies. Unfor- 

 tunately there are as yet no connected observa- 



