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 facts concerning seasonal dimorphism; and experimenting 
on chrysalids of Pieris Napi, which presents itself under both a winter and sum¬ 
mer form, and upon Vanessa Levana, which is the winter form, and Prorsa, the 
summer form of one and the same species, he found that by application 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, w r hich 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 winter, could now, during the same summer in which they 
as larvrn 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 
by the changing of the summer form. When the summer became longer, a third 
generation could be interpolated, so that two summer generations alternated with 
one winter. 
Dr. Weismann further states that individuals of the Prorsa (summer) form 
sometimes emerge very late in the year (like those of the fourth brood of 
