188 DEVELOPMENT AND HEREDITY. 



the extreme summer and winter forms. Each form, 

 therefore, must be regarded as the result of the in- 

 fluence of a particular environment during develop- 

 ment. Of a number of such possible forms, we may 

 suppose the average alternations of seasons to pro- 

 duce alternately only the two extreme forms ; or, 

 on the other hand, a change in the climate toward 

 greater heat might cause the omission of one of the 

 extreme forms, and instead, the production alter- 

 nately of one of the intermediate forms instead of 

 the extreme cold form. The following illustration 

 of this refers to Lyccsna agestis : " This butterfly 

 occurs in three forms : A and B alternate in Ger- 

 many as winter and summer form ; B and C are the 

 winter and summer form in Italy. Thus the form 

 B occurs in both cHmates, but appears in Germany 

 as the summer, in Italy as the winter, form. The 

 German winter form A is completely wanting in 

 Italy, while the Italian summer form (van Allous) 

 does not occur in Germany." 



It is evident, from the facts and principles which 

 we have discussed in previous chapters, that a per- 

 manent new form is not the result of the brief 

 action of a new set of forces upon the individual 

 during the short period of a single lifetime. A new 

 form is produced by the long-continued action of 

 the. changed environment, which causes the gradual 



