io6 Studies in the Theory of Descent. 



species must have gone on from this sole cause 

 of alternation of climate. When we consider 

 that many species elsewhere extinct have become 

 locally preserved, and when, further, to these we 

 add those local forms which have arisen by the 

 prevention of crossing (amixia), and finally take 

 into consideration the important effects of sexual 

 selection, we can no longer be astonished at the 

 vast numbers of species of butterflies which we 

 now meet with on the earth. 



Should any one be inclined to conclude, from my 

 reversion experiments with seasonally dimorphic 

 butterflies, that the secondary species when ex- 

 posed to the same climate as that which produced 

 it must revert to the primary, he forgets that this 

 reversion to the winter form is nothing but a 

 reversion i.e., a sudden return to a primary form 

 through peculiar laws of inheritance and by no 

 means a gradual re-acquisition of this primary 

 form under the gradual influence of the primary 

 climate. Reversion to the winter form occurs also 

 through other influences, as, for instance, by high 

 temperature. Reversions of this kind, depending 

 on laws of heredity, certainly happen with those 

 cases of transmutation which do not alternate with 

 the primary form, as in seasonal dimorphism, but 

 which occur continuously. They would, how- 

 ever probably be more quickly suppressed in such 

 cases than in seasonal dimorphism, where the 

 constant alternation of the primary and secondary 



