On the Seasonal Dimorphism of Butterflies. 89 



least as my investigations extend appears to be 

 the same in both generations. 6 



The state of affairs is quite different in the re- 

 maining cases of heterogenesis ; here the entire 

 structure of the body appears to be more or less 

 changed, and its size is often very different, 

 nearly all the internal organs differing in the two 

 generations. According to Claus, 7 " we can 

 scarcely find any other explanation of the mode 

 of origination of heterogenesis than the gradual 

 and slow advantageous adaptation of the organiza- 

 tion to important varying conditions of life " a 

 judgment in which this author is certainly correct. 

 In all such cases the change does not affect unim- 

 portant characters, as it does in butterflies, but 

 parts of biological or physiological value ; and we 

 cannot, therefore, consider such changes to have 

 originated through the direct action of altered 

 conditions of life, but indirectly through natural 

 selection or adaptation. 



Thus, the difference between seasonal dimor- 

 phism and the other known cases of heterogenesis 



[In the case of monogoneutic species which, by artificial 

 ' forcing,' have been made to give two generations in the year, 

 it has generally been found that the reproductive system has 

 been imperfectly developed in the second brood. A minute 

 anatomical investigation of the sexual organs in the two broods 

 of seasonally-dimorphic insects would be of great interest, and 

 might lead to important results. R.M.] 



T " Grundziige der Zoologie." 2nd ed. Leipzig, 1872. In- 

 troduction. 



