16 Experimental Zoology 
required the experimental work of Dorfmeister and of Weis- 
mann to show that this supposition was correct. 
Weismann showed for Vanessa levana-prorsa that when a 
pupa, destined to give rise to the summer form, is kept at a low 
temperature, it may produce the winter form, V. levana, or a 
type transitional between the summer and the winter forms. 
He also succeeded, by raising the temperature, in changing the 
winter pupa so that it gave rise to the summer type of butterfly. 
More recently Merrifield, Standfuss, Fischer, Grafin von 
Linden, and others have carried out extensive experiments on 
the effects of temperature. The butterflies and moths used 
for this work are usually those having summer and winter broods, 
that differ in color and often in size, and even in the shape of 
the wings. In other cases, however, similar changes have been 
brought about in forms that do not show seasonal dimor- 
phism. It has been found that not only the summer form can 
be changed into the winter form, and vice versa, but in certain 
cases the type may be changed by cold so that it resembles 
northern varieties of the same species, and by heat to resem- 
ble southern varieties. Temperatures that are only somewhat 
higher or lower than normal produce the southern and northern 
types respectively, while much higher or lower temperatures 
produce effects that are rarely or never found in nature. These 
latter changes are sometimes called aberrations. We may first 
examine a few examples of these effects given by Standfuss. 
The effect of heat on Vanessa cardui is shown in Fig. 5. The 
colors are much lighter above than those of the normal butterfly. 
The black bands are much reduced. Similar changes are ob- 
servable on the under side of the wing. The pupa had been 
kept for 60 hours at 36-37° C., and then at normal temperatures 
for six to seven days, when the butterfly emerged. The effect of 
cold on Vanessa cardui is shown in Fig. 6. The color is darker 
and the white spots are reddish in tint as seen especially on the 
under side. The pupe had been kept for 33 days in an ice 
chest, then for five days in a cellar (+13°C.), and lastly for nine 
days at room temperature. 
