PHYCIODES I., II. 



and rest nearly immovable for thirty-six hours. The body contracts, and as the 

 time for the moult approaches, the skin becomes glassy, as it separates from the 

 newly formed skin beneath. The new spines lie folded down and back, and as 

 the old skin, after splitting behind the head, is shuffled past the successive seg- 

 ments, the spines and pencils of bristles suddenly spring up, and the latter in- 

 stantly become divergent. For some moments the old mask adheres to the new 

 face, but the larva presently proceeds to rub it off with its feet. When the 

 larva prepares for chrysalis, it spins a button of white silk, and hangs suspended 

 for about twenty-four hours, its position being nearly circular. 



Dr. Aug. Weismann, in his essay, " Ueber den Saison-Dimorphismus der Schmet- 

 terlinge," Leipsic, 1875, relates the history of experiments made by him with the 

 view of determining the fxcts concerning seasonal dimorphism ; and experimenting 

 on chrysalids of Pieris JVapi, which presents itself under both a winter and sum- 

 mer form, and upon Vanessa Leuana, which is the winter form, and Prorsa, the 

 summer form of one and the same species, he found that by aj^plication of cold 

 of the temperature 33° Far., to the chrysalids of the summer brood, the result- 

 ing butterflies could be changed more or less completely from the summer to the 

 winter form, and yet emerge the same season, but that it was not possible to 

 constrain the winter into the summer form by the application of heat. And he 

 concludes that this artificial change is based upon a reversion to the original form 

 of the species, or atavism, which is most readily called out by cold ; that is, by 

 means of the same outside influence to which the original form was exposed 

 through a long period of time, and the continuance of which has preserved in 

 the winter form, to this day, the primitive markings and color. The arising of 

 the summer form he believes to have occurred thus : During the so-called ice 

 period, the summer was short and cool, and the existing butterflies could pro- 

 duce only one generation in a year. As the climate gradually became warmer, 

 a period must have come on in which the summer lasted so long that a second 

 generation could be interpolated. The chrysalids of Levana, which had hitherto 

 slept through the long wiuter, could now, during the same summer in which they 

 as larvae had hatched, fly as butterflies. There had come to be a state of things 

 in which one generation grew up under very different climatic influences from 

 the other, and gradually the difference which now exists between the two arose 

 bv the changing; of the summer form. When the summer became longer, a third 

 generation coifld be interpolated, so that two summer generations alternated with 

 one winter. 



Dr. Weismann further states that individuals of the Pr-orsa (summer) form 

 sometimes emerge very late in the year (like those of the fourth brood of 



