GO 



PSYCHE. 



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lie in better condition to withstand the 

 elTects of the cold. Natural selection 

 would then operate to weed out all 

 pupae having high metabolic processes, 

 for they would be more likely to freeze ; 

 whereas those individuals in which the 

 metabolism was low would be pre- 

 served. Also this inherited tendency 

 in the overwintering pupae to possess 

 low metabolic activity might become so 

 strongly fixed that it would be found 

 difficult to alter it by the mere subjection 

 of the pupae to a high temperature, 

 such as 32°-35° C Moreover it would 

 doubtless be of advantage to the insects 

 if they had the power to resist the in- 

 fluence of such warmth, for there are 

 often hot periods of weather in the 

 autumn through which the over-win- 

 tering pupae must pass; but their 

 development must not be hastened 

 thereby, for if the butterflies emerged 

 they or their progeny would proliably 

 perish of the cold. 



Professor Weismann's former (1S75, 

 '82) idea that in the seasonally-dimor- 

 phic butterflies of the temperate zone 

 the phylogenetically older form issues 

 from the wintered chrysalids, and repre- 

 sents the form which existed during the 

 glacial epoch, is to my mind improb- 

 able. His hypothesis may be true as 

 fir as tlie European Vanessa leva?ia- 

 prorsa is concerned, because, as is well 

 known, the butterflies of Europe are 

 more closely related to those of Siberia, 

 than to those of Africa or India. (See 

 Bath ('95)).* The cause of this lies 



*Bath, W. H. 1S95; Entomologist Vol. 28, p. 247. 



in the well known f;ict that there has 

 been, according to Geikie, no land con- 

 nection between Europe and Africa 

 since the close of tlie glacial epoch. 

 Moreover the deserts of Sahara, and 

 Arabia, and the snow clad peaks of the 

 Himalayas form an insuperable barrier 

 beyond which tropical forms could not 

 pass to enter the northern regions. In 

 America, however, the case is very dif- 

 ferent, for almost 50% of all the known 

 species of Lepidoptera of the world come 

 from South America,* and there can be 

 but little doubt that the ancestors of most 

 of the North American butterflies came 

 from South America. The ancestors 

 of the North American forms have 

 gradually crept in from South America 

 after the glacial epoch, and as their 

 range extended further and further 

 north, they were obliged to become 

 adaptetl to the cold, or perish. This 

 adaptation would mean the acquisition 

 of a low metabolism in the ovcr-ivin- 

 tering pupae. In this connection it is 

 interesting to notice that Merrifield ('93) 

 has shown that in England those pupae 

 of Pyrameis atalonta which form in 

 the autumn all perish of the cold. This 

 insect usuall)' hibernates as an imago, 

 and is not seasonally-dimorphic. In- 

 deed, the seasonally-dimorphic butter- 

 flies, of the temperate zone, according 

 to Scudder (Butterflies of New Eng- 

 land, 1SS9, p. 13S4) probably all TC'////c;- 



* Schatz ('()2) finds that there are in South .America 272 

 genera of butterflies comprising 4560 species, and that 231 

 of these genera are proper to South America alone. In 

 North America, on tlie other hand, there are according to 

 Edwards only 66 genera and 612 species. 



