Lhe Influence of External Conditions 23 
probable that the original form of the Vanessas was darker 
than the present forms, to judge by related groups. Uncertain 
as arguments, pro and con, based on these phylogenetic specu- 
lations necessarily are, there is fortunately experimental evi- 
dence that shows how little ground there is for Fischer’s argu- 
ment. Grafin Marie von Linden has examined the developing 
pigment in the wing itself. She finds that the lighter ‘colors 
develop first and that in the younger stages the red and yellow 
tones occupy a greater area than they do later. This is the 
reverse of Fischer’s primary assumption of sequence. In pupe 
exposed to freezing and to heat it seems that disturbances in the 
development of the color occur. Standfuss observed in these cir- 
cumstances that the color develops later than in the normal; and 
von Linden finds that the black color appears relatively earlier 
than the others, and at times even before the red and yellow. 
Grafin von Linden has also shown that changes in certain 
colors similar to these shown by the pupe can be produced in 
vitro. Extracts of the red color, when heated, become fiery red 
or more red-brown in color; while on ice the red and the yellow- 
red tone remain constant. It is likely, therefore, that some of 
the effects of high and low temperature can be explained en- 
tirely as due to the direct influence of the temperature on the 
chemical composition of the pigment. 
Not only have the summer and winter forms been changed 
by changing the temperature, but in two cases, in which sexual 
dimorphism exists, it has been possible to change the female 
coloration into that of the male. For example, the colors of 
the female of Parnassius apollo can be changed into those of the 
male. In the female of Rhodocera rhammi the white colora- 
tion of the wings can be changed by warmth into the intensely 
yellow color of the male. It has been suggested that while 
ordinary temperatures suffice to cause the development of the 
deeper color in the male, it requires.a higher temperature than 
that ordinarily met with to cause the same change in the tissues 
of the female. Standfuss has pointed out that while in many 
cases a lower temperature may cause a darker color and a higher 
