272 THE GYPSY MOTH. 



the purpose of considering the bearing of environment and 

 climate on the gypsy moth. 



If we suppose that the native home of this insect was in 

 central Europe or south-western Asia, and that it was de- 

 veloped from some progenitor in which the sexes were of the 

 same color, possibly not very unlike the males of the present 

 time, we may well suppose that, as the females developed 

 a larger number of eggs, rendering them so heavy that they 

 flew but rarely, and, in time, not at all, their wings, not being 

 used, would grow weaker and less useful as organs of flight, 

 as we now find them. Under these circumstances, suppose 

 them to be inhabiting trees and shrubs, the bark of which is 

 of light color, as birch, their enemies, the birds, etc., could 

 readily distinguish these strongly contrasting females on the 

 trunks of the trees and destroy the darker forms, leaving 

 the lighter examples to propagate the species ; and, as this 

 went on, in time permanently light-colored females would be 

 produced. 



The males fly actively during the day, and are captured, 

 while on the wing, by insectivorous birds ; but in this case 

 slight variations in color would not be apparent to the birds, 

 and no discrimination would be made in favor of such vari- 

 ations. For this reason the males have probably retained 

 more of their primitive color and appearance, while the 

 females have made a most remarkable change; and, as a 

 result, this insect furnishes a most striking illustration of 

 what is called sexual dimorphism. 



Another reason for thinking that the males have changed 

 far less than the females, and that the changes made were 

 perhaps in a different direction, is the fact that they more 

 nearly resemble the males of allied species than they do the 

 females of their own. Compare Figs. 3 and 4, on Plate I., 

 with Figs. 11, 14 and 17, on Plate 39, and also with Figs. 1 

 and 2, on Plate I. The three species of Orgyia just referred 

 to have wingless females (one of which, O. leucostigma, is 

 represented on Plate 39, Fig. 18), which are so heavy that 

 it would be impossible for them to fly with wings of ordinary 

 size. It is probable, however, that the remote ancestors of 

 these insects were winged in both sexes, and that, as the female 

 developed a larger reproductive system and more eggs, she 



