52 The Selection Theory 
These feed by night and betake themselves through the day to the 
trunk of the tree, and hide in the furrows of the bark. We cannot, 
however, conclude from this that they were unable to vary towards 
green, for there are Arctic animals which are white only in winter 
and brown in summer (Alpine hare, and the ptarmigan of the Alps), 
and there are also green leaf-insects which remain green only while 
they are young and difficult to see on the leaf, but which become 
brown again in the last stage of larval life, when they have outgrown 
the leaf. They then conceal themselves by day, sometimes only 
among withered leaves on the ground, sometimes in the earth itself. 
It is interesting that in one genus, Chaerocampa, one species is 
brown in the last stage of larval life, another becomes brown earlier, 
and in many species the last stage is not wholly brown, a part 
remaining green. Whether this is a case of a double adaptation, 
or whether the green is being gradually crowded out by the brown, 
the fact remains that the same species, even the same individual, can 
exhibit both variations. The case is the same with many of the leaf- 
like Orthoptera, as, for instance, the praying mantis (Mantis religiosa) 
which we have already mentioned. 
But the best proofs are furnished by those often-cited cases in 
which the insect bears a deceptive resemblance to another object. 
We now know many such cases, such as the numerous imitations 
of green or withered leaves, which are brought about in the most 
diverse ways, sometimes by mere variations in the form of the insect 
and in its colour, sometimes by an elaborate marking, like that which 
occurs in the Indian leaf-butterflies, Kalluma inachis. In the single 
butterfly-genus Anaea, in the woods of South America, there are 
about a hundred species which are all gaily coloured on the upper 
surface, and on the reverse side exhibit the most delicate imitation 
of the colouring and pattern of a leaf, generally without any indica- 
tion of the leaf-ribs, but extremely deceptive nevertheless. Anyone 
who has seen only one such butterfly may doubt whether many of 
the insignificant details of the marking can really be of advantage 
to the insect. Such details are for instance the apparent holes and. 
splits in the apparently dry or half-rotten leaf, which are usually due to 
the fact that the scales are absent on a circular or oval patch so that 
the colourless wing-membrane lies bare, and one can look through 
the spot as through a window. Whether the bird which is seeking 
or pursuing the butterflies takes these holes for dewdrops, or for the 
work of a devouring insect, does not affect the question; the mirror- 
like spot undoubtedly increases the general deceptiveness, for the 
same thing occurs in many leaf-butterflies, though not in all, and 
in some cases it is replaced in quite a peculiar manner. In one 
species of Anaea (A. divina), the resting butterfly looks exactly like 
