On the Seasonal Dimorphism of Butterflies. 1 1 1 



in kind. But we are not yet able to extend our 

 researches to such fine distinctions. 



As a final, and not unimportant result of these 

 investigations, I may once more insist that dis- 

 similar influences, when they alternatingly affect 

 a long series of originally similar generations in 

 regularly recurring change, only modify the gene- 

 rations concerned, and not intermediate ones. Or, 

 more briefly, cyclically acting causes of change 

 produce cyclically recurring changes : under their 

 influence series of monomorphic generations be- 

 come formed into a cycle of di- or polymorphic 

 generations. 



There is no occasion to return here to the im- 

 mediate evidence and proof of the foregoing law. 

 In the latter, however, is comprised the question 

 is not the cycle of generations produced by cycli- 

 cal heredity ultimately equivalent to Darwin and 

 Haeckel's homochronic heredity which forms the 

 ontogenetic stages into a cycle ? It is possible 

 that from this point, in the future, the nature of the 

 processes of heredity, which are still so obscure, 

 may be penetrated into, and both phenomena 

 traced to the same cause, as can now be only sur- 

 mised but not clearly perceived. 



Finally, the most general, and in so far chief 

 result of these investigations, appears to me to lie 

 in the conclusion, which may be thus formulated : 

 A species is only caused to change through the 

 influence of changing external conditions of life, 



