PS re HE. 



over as pupae. Dr. Scudder has also 

 observed, that in a few cases of seasonal- 

 dimorphism we find differences between 

 the earlier and later appearing members 

 of the spring brood, the later members 

 showing an approach toivard the sum- 

 mer form. 



It is highly probable that the vast 

 majority of Lepidoptera which existed 

 in North America before the glacial 

 epoch, simply retreated southward upon 

 the approach of the ice. For to have 

 them remain we are obliged to assume 

 that not only they, but their food plants, 

 also, became acclimated to the cold. 

 Many of those which did remain, and 

 succeeded in defying the cold would 

 probably for the most part become so 

 thoroughly acclimated to it that they 

 would finally prefer a cold climate, 

 and when the ice retreated they would 

 probably follow it northward leaving 

 a few representatives stranded, as it 

 were, upon the tojjs of the highest 

 mountains ; as has been shown by Grote 

 and, also, Scudder ('89 p. 5SS) to 

 have been the case with Oeneis semidea 

 of Mount Washington. 



In the case of Papilio ajax, which 

 was experimented upon by Edwards, 

 I have reason to believe that it could 

 not have existed in North America dur- 

 ing the ice period. For its nearest allies 

 are all in Mexico, and South America, 

 none of them being found in the colder 

 parts of North America. Moreover its 

 food plant, the papaw {Ashnina triloba, 

 Dunal.) belongs to the characteristically 

 tropical family Anonaceae, and cannot 

 live in a cold climate. This leads me 



to suggest that \valshii or telemonides 

 are not primitive forms as would be the 

 case were Weismann's 1875 hypothesis 

 true. I also predict that if the over- 

 wintering pupae of Papilio ajax be 

 subjected to a constant heat of from 

 30°-35° C. they will be transformed into 

 the summer form marcellus. 



Conclusions : In lepidoptera of the 

 temperate regions it is an advantage for 

 the summer pupae to possess high meta- 

 bolic processes, for under these circum- 

 stances their development is rapid. On 

 the other hand it is an advantage for the 

 overwintering pupae to posses /ow meta- 

 bolic process, for under these circum- 

 stances they would be the better able to 

 withstand the influence of warm periods 

 of weather in the autumn ; for if the 

 butterflies emerged at this time they or 

 their progeny would probably perish of 

 the cold. Moreover in order that the 

 pupae mav withstand the influence of 

 the winter's cold it is essential that they 

 possess a low metabolism. Natural selec- 

 tion would then operate to cause all 

 summer pupae to inherit a high meta- 

 bolism, while all overwintering pupae 

 would be forced to iiiherit a low meta- 

 bolism. Pupae which possess a constitu- 

 tional tendency toward high metabolism 

 give rise to the summer form of imago, 

 while those pupae which possess a con- 

 stitutional tendencv toward low meta- 

 liolism give rise to the overwintered 

 form of butterfly. The summer and 

 winter forms of imago are only expres- 

 sions of this difference in constitution of 

 the summer and winter pupae. 



It is well known through the re- 



