48 THE COLOURS OF ANIMALS 



the ordinary operation of natural selection, leading to 

 the extermination of the less protected variety. I 

 have experimented with all the dimorphic larvae men- 

 tioned above, and could not find any trace of suscep- 

 tibility to the influence of surroundings, so as to lead 

 to the production of the appropriate form. When 

 such susceptibility is present, of course the dimorphism 

 has a far higher protective value. The description of 

 such cases is reserved for a future chapter. 



Occasionally the two forms of a dimorphic species 

 appear at different times and correspond to the tints 

 which successively predominate in the surroundings. 

 At one time I thought the brown form of the Large 

 Emerald caterpillar might appear rather later than 

 the other, when the green catkins had been replaced 

 by brown ; but further examination did not confirm 

 the observations which pointed in this direction. Dr. 

 Alexander Wallace, of Colchester, has, however, found 

 that the moths of Bombyx cynthia which are the first 

 to emerge from the pupae possess, as a rule, an olive- 

 green ground colour, while those which emerge in 

 September are generally of a yellow tint. These 

 colours harmonise with the appearance of the Ailanthus 

 leaflets at corresponding times of the year. 



Dimorphism in the Perfect Insect 



Dimorphism is also met with in perfect insects, 

 and it is especially frequent in the females (see page 



