REVIEWS. 191 
Studies in the Theory of Descent. By Dr. Ava. Wetsman. 
Translated and edited by Raphael Meldola, F.C.S., with a 
prefatory notice by Charles Darwin, F.R.S. Part I., “ On 
the Seasonal Dimorphism of Butterflies.” 
Turis work is a welcome addition to the literature of Darwinism, 
and the thanks of all entomologists are due to Mr. Meldola for 
the excellent translation he has placed before English readers. 
It is well known to British entomologists that the form of 
Pieris napi, which appears in the perfect state in the spring, 
differs very materially from the imagines of the same species 
which appear in the autumn; the difference is so striking that the 
late Mr. J. F. Stephens described them as distinct species, and 
never quite gave up the belief that they were so. 
Dr. Weisman, reflecting on this seasonal dimorphism, con- 
ceived the hypothesis that, during the glacial epoch, but one 
form of the insect existed,—probably the spring, or, as, he terms it, 
“winter form,’ —and that, as the seasons became ameliorated, the 
broods produced from this form had time to pass through all 
their changes from the egg to the imago in a few weeks, and the 
summer form of the insect was the ultimate result. These latter 
laying eggs, which became chrysalids in the autumn, and remained 
in that state until the next spring, when the perfect insect emerged, 
thus completing the seasonal dimorphism. 
Dr. Weisman’s carefully-conducted experiments have placed 
this hypothesis beyond doubt. By the application of cold to 
the chrysalids of the summer form he retarded their development, 
and produced from them the winter form, precisely as is always 
the case in Pieris napi, variety Bryonie, which inhabits the high 
alps, is single-brooded, and appears in one form only, and this 
much darker than our own winter form. It would thus appear 
probable, assuming P. bryonie as representing the original 
coloration, that the ordinary winter form has lost something of its 
colour in acquiring the dimorphic condition. 
Of our British butterflies, Dr. Weisman draws attention to 
seasonal dimorphism in Pieris brassice, P. rape, Plebewus Agestis 
(Alexis, Scop.), Polyommatus Phleas, and Parargia Afgeria, but 
in some of these cases the differentiation is complicated by the 
effects of local climate. Still these species are well worthy 
of investigation, although the variation between the summer and 
winter forms may be very slight. 
