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, Kallima 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 aftect 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 



