POLYMORPHISM IN SPECIES. 189 



acquisition of new methods of growth responsive to 

 new stimuli. Since each organism in its develop- 

 ment passes through the various stages of growth 

 which have been attained successively by its ances- 

 tors, it follows that when any organism fails to 

 attain its full development, it must remain at some 

 point of development which, while incomplete for 

 the present generation, was complete for some past 

 generation. A careful study of the dimorphic but- 

 terflies has led the authorities on those points to 

 the conviction that some of the forms — the winter 

 forms — are the incomplete or ancestral stages of 

 development. So that while the species in the course 

 of its existence has gone on developing new char- 

 acteristics, it has been forced by the alternation of 

 seasons to reproduce an ancestral form every second 

 or third generation. Thus we see that neither the 

 extreme summer nor winter forms are forms which 

 are newly and suddenly produced ; and the same is 

 true of the intermediate forms, which represent an 

 intermediate stage of development. 



The main point to be deduced from all this is that 

 as a result of peculiar conditions of environment, 

 ancestral or imperfectly developed forms may exist 

 alternately or simultaneously in the same species 

 with more advanced and more highly developed 

 forms. This fundamental principle explains, I think, 



