470 Mr. Voulion's further experiments upon 



environment does not necessarily destroy individual 

 variability, but the most powerful forms of environment, 

 when applied to highly sensitive species, very nearly do 

 away with it. 



If the environment be mixed, there does not appear to 

 be any instinctive knowledge leading the larvffi to rest 

 only on appropriate objects. Thus, if they have become 

 green, and are beyond the power of change, they will 

 nevertheless rest on brown twigs in preference to leaves, 

 if offered to them. 



The instinct of these Geometrce is to rest upon twigs 

 under any circumstances, and this is probably the reason 

 why so small a proportion of twigs produces so great an 

 effect {A. hetalaria, 1889). Contact, or at all events the 

 closest proximity, is required to effect the change. Al- 

 though they are so much more susceptible to brown sur- 

 roundings when these are mixed with green, there were 

 no exceptions among 105 larvae which in 1889 became 

 green among leaves and shoots. 



The effects produced on the larvae do not influence the 

 colours of the moths {A. hetalaria). 



Darkness does not produce so great an effect as black 

 surroundings in strong light {A. hetalaria, R. cratcegata, 

 C. elingaaria). Overcrowding tends to produce dark 

 larvae {A. hetalaria, li. cratcegata). 



In the case of R. cratcegata and A. hetalaria, there is 

 direct evidence of the power being efficient in concealing 

 the wild larvae. 



The larvae are probably chiefly sensitive at the time 

 when they quit the leaves and first begin to rest on the 

 twigs. 



The protective significance of the colour changes. — 

 Looking at the results here recorded, as a whole, there 

 can be no doubt about these changes being such as to 

 promote concealment. In the majority of the larvae 

 the only possible change appears to be from dark to 

 light brown or greenish brown. But the latter are far 

 less conspicuous on the leaves than the dark varieties 

 would have been, although they are not nearly so well 

 concealed as the latter upon the dark twigs. When 

 the larvae of any one of these species hatch upon a 

 tree, or part of a tree, with a great abundance of 

 young green shoots, their susceptibility would certainly 



