172 [August, 



which they rested, naturally fell a pi'ey to their enemies more readily 

 than the dark-coloured individuals; the darker ones were left to 

 j)ropaf;ate the species, and, in time, a dark race was formed. I am 

 well aware that I am now trespassing on ground which is far better 

 understood by my English brethren, but I trust that I may be 

 permitted to call attention to this matter for the purpose of consider- 

 ing 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 developed from some 

 progenitor in which the sexes were of the same colour, 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 colour, 

 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-coloured 

 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 

 colour would not be apparent to the birds, and no discrimination would 

 be made in favour of such variations. For this reason the males have 

 probably retained more of their primitive colour 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 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 

 Eigs. 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 became too heavy for fli-:h<^ and at last 



