48 



PSYCHE. 



[April iSqy. 



This change is not accomplished how- 

 ever without resistance, for the chrysa- 

 lids show a strong tendency to produce 

 prorsas, as is seen by the production ot 

 many porimas among the butterflies 

 whose chrysaHds have been subjected to 

 the cold. On the other hand the third, 

 or autumnal, generation of chrysalids 

 shows a strong tendency to over-winter 

 and produce butterflies of the levana 

 type next spring. Heat of _37°-30°C., 

 can, however, counteract this tendency 

 and cause some of these chrysalids to 

 give rise to porimas, or even to prorsas. 

 In (i875-'82) VVeismann was lead 

 into the conclusion that levana represents 

 the more primitive or ancestral form of 

 the butterfly which existed in Europe 

 at the time of the glacial epoch. As 

 the mean temperature at that time was 

 much lower than at present, and the 

 suminer was short, the butterfly was 

 probably single brooded, and consisted 

 of only the form levana. The form 

 prorsa, however, gradually made its 

 appearance after the glacial epocli when 

 the climate became milder, and the but- 

 terfly began to produce svunmer genera- 

 tions. The form prorsa, according to 

 this hypothesis, is phylogenetically 

 newer than levana, and the application 

 of cold simply causes it to revert to its 

 ancestral type. The levana form, on the 

 other hand, could not possibly be made 

 to revert into prorsa because prorsa is 

 phylogenetically younger than levana. 

 In 1S95, however, VVeismann finds that 

 he is mistaken in this for, it will be re- 

 membered, he succeeded in forcing chrys- 

 alids which weie naturally destined to 



produce levana to give rise to prorsa by 

 subjecting them to a high temperature. 

 He is therefore obliged to modif\- his 

 former (iS75-'S2) conclusions, and 

 finally decides that there are two kinds 

 of seasonal-dimorphism ; one of which 

 he calls " direct seasonal-dimorphism," 

 and the other " adaptive seasonal- 

 dimorphism." By " direct " seasonal- 

 dimorphism Weismann means the direct 

 effect of the temperature stimulus upon 

 the pupae at the time when the colors 

 are produced. This direct influence 

 may induce chemical changes, etc., 

 which determine the coloration of the 

 wings. An excellent example of direct 

 seasonal-dimorphism is aflbrded by 

 Chiysophanus fhlceas where heat 

 causes the pupae of any brood to give 

 rise to dark colored butterflies; while 

 cold induces them to give light golden 

 red forms. 



In " adaptive " seasonal-dimorphism, 

 on the other hand, we have the addi- 

 tional factor that one or both of the 

 dimorphic forms possesses a peculiar 

 advantage correlated with the season in 

 which it occurs. Under these circum- 

 stances there has arisen, through the 

 agency of natural selection, a tendency 

 to produce different forms in the difler- 

 ent seasons. For example we find in- 

 herent in the pupae of Vauessa levana- 

 prorsa, two separate, and distinct, ten- 

 dencies; the one to produce levana, and 

 the other prorsa. The tendency to 

 produce levana is strong in the over- 

 wintering pupae, while the tendency to 

 produce prorsa is strong in the summer 

 pupae. These tendencies can hovve^•|;r 



