The 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 pupae 

 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 pupae 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 



