34 THE INFLUENCE OF ENVIRONMENT ON 



and species of Crustacea ; while the ultimate cri- 

 terion of distinct species fertile breeding bears 

 the matter out also, for no one of the three forms 

 is capable of living in the medium dwelt in by 

 either of the others. 



We also know of several cases among butterflies 

 in which, out of a very complex environment, we 

 are able to put our finger on the one element which 

 is the actual determining cause of the structural 

 changes that we observe. One of the most im- 

 portant of these examples is furnished by two 

 butterflies of widely different appearance, Vanessa 

 levana and Vanessa prorsa. 



Vanessa levana has a red ground colour, with 

 black spots and dashes and a row of blue spots 

 round the margin of the hind wings ; Vanessa prorsa 

 is deep black, with a broad yellowish-white band 

 across both wings and with no blue spots. These 

 were formerly called distinct species, but have 

 recently been shown to be varieties of one and the 

 same species. The relation between them is a 

 very definite one, and they are what is called 

 seasonally dimorphic : that is, they are double 

 brooded, and have two generations in the course of 

 the year a winter brood and a summer brood. In 

 the summer or autumn Prorsa is alone met with, 

 and the pupae developed from the autumn brood of 

 eggs of Prorsa hybernate during the winter and 

 hatch in April or May of the following spring, not 

 as Prorsa, but as Levana. This in turn produces 

 a brood of eggs which develop into Prorsa again. 

 This phenomenon was investigated by Weismann, 



