42 Studies in the Theory of Descent. 



in their habitats, they have, to a certain extent, 

 acquired and preserved their present form. 



The var. Bryonicz is, however, of quite special 

 interest, since it makes clear the relation which 

 exists between climatic variation and seasonal 

 dimorphism, as will be proved in the next section. 

 The correctness of the present theory must first 

 here be submitted to further proof. 



It has been shown that the secondary forms of 

 seasonally dimorphic butterflies do not all possess 

 the tendency to revert in the same degree, but that 

 this tendency rather varies with each individual. 

 As the return to the primary form is synonymous 

 with the relinquishing of the secondary, the greater 

 tendency to revert is thus synonymous with the 

 greater tendency to relinquish the secondary form, 

 but this again is equivalent to a lesser stability of 

 the latter ; it must consequently be concluded that 

 the individuals of a species are very differently 

 influenced by climatic change, so that with some 

 the new form must become sooner established than 

 with others. From this a variability of the gene- 

 ration concerned must necessarily ensue, i. e., the 

 individuals of the summer generation must differ 

 more in colour and marking than is the case with 

 those of the winter generation. If the theory is 

 correct, the summer generations should be more 

 variable than the winter generations at least, so 

 long as the greatest possible equalization of indi- 

 vidual variations has not occurred through the 

 continued action of warmth, combined with the 



