156 THE COLOURS OF ANIMALS 



fact that different individuals will probably meet with 

 different environments. As long ago as 1873, Pro- 

 fessor Meldola argued that this power of adjustment 

 is adaptive and to be explained by the operation of 

 natural selection. 1 



Comparison between the varying effects of green 

 leaves upon the different stages of an insect strongly 

 supports the view that the results are due to adaptation. 

 Thus the caterpillar of the Brimstone Moth remains 

 upon its food-plant for a few weeks in the summer when 

 the leaves are green, and green leaves cause the larva 

 to become green and to lose the dark pigment. But 

 the chrysalis remains among the leaves in winter 

 when they have become brown, and green leaves cause 

 the caterpillar to spin a dark cocoon. Hence precisely 

 opposite effects are produced by the operation of the 

 same force, the nature of the effects having been 

 determined by adaptation. 



Furthermore, there is no positive evidence for any 

 of these effects becoming hereditary. I have carried 

 on some of my experiments for more than one genera- 

 tion, always carefully noting the effects produced in 

 the parents, and have never been able to detect any 

 resulting hereditary tendencies, even when the previous 

 generation had been powerfully influenced. 



When therefore we meet with a dimorphic species 

 which is not influenced by its environment, so as to 

 produce the appropriate form, I do not believe that we 



' Proc. Zool. Soc. 1873, p. 153. 



