88 Shi dies in the Theory of Descent. 



But even if seasonal dimorphism is to be ascribed 

 to heterogenesis, it must by no means be asserted 

 that those cases of cyclical propagation hitherto 

 designated as heterogenesis are completely iden- 

 tical with seasonal dimorphism. Their identity 

 extends only to their origin and manner of de- 

 velopment, but not to the mode of operation of 

 the causes which bring about their transformation. 

 Both phenomena have a common mode of origina- 

 tion, arising from similar (monomorphic) sexual 

 generations and course of development, a cycle of 

 generations with gradually diverging characters 

 coming into existence by the action of alternating 

 influences. On the other hand, the nature of the 

 changes by which the secondary differs from the 

 primary generation may be referred to another 

 mode of action of the exciting causes. In seasonal 

 dimorphism the differences between the two gene- 

 rations are much less than in other cases of hetero- 

 genesis. These differences are both quantitatively 

 less, and are likewise qualitative, affecting only 

 characters of biological insignificance. 5 The va- 

 riations in question are mostly restricted to the 

 marking and colouring of the wings and body, 

 occasionally affecting also the form of the wing, 

 and in a few cases the size of the body (Plebeius 

 Amyntas), whilst the bodily structure so far at 



6 See my essay " Uber den Einfluss der Isolirung auf die 

 Artbildung." Leipzig, 1872. 



