90 HEREDITY AND SELECTION IN SOCIOLOGY 



of these beetles in regard to egg-laying is so regulated that it 

 co-operates with the instinctive appetite of the larvse for the 

 Atrofa belladonna ; and the eggs are invariably deposited in such 

 a manner that the larva can be sure of immediately obtaining 

 food when it breaks through the egg. It must obviously be a 

 great advantage to this species that its food substance is not 

 shared by any rivals, for thereby the severity of its struggle for 

 existence is considerably — ^indeed, very greatly — diminished. 

 Several species of caterpillar are likewise monophagous — that 

 is to say, they subsist on a single species of plant. It may be 

 asked how such an instinct could be developed, unless con- 

 sciousness be at its origin ? The reply is that the species in 

 question are too low down on the scale to be able to correlate 

 cause and effect ; that they are unable to judge of what is good 

 or not good to eat ; and that, as a matter of fact, they are un- 

 doubtedly able to eat other plants than the specified one in 

 each case. Such an instinct as the monophagous instinct can 

 only have arisen as a result of selection. Utility for the species 

 being the raison d'etre of monophagy, selection must gradually 

 have eliminated those individuals which did not possess this 

 instinct ; with the result that the determinants of that instinct 

 have become ever more and more fixed in the germ-plasm of 

 the species. 



In many cases we find a correlation of colour and instinct 

 which cannot be explained otherwise than as the result of selec- 

 tion. Thus, for instance, the butterfly Xylina resembles, in its 

 colouring, a broken-off and decayed piece of wood ; and this 

 extraordinary colouring would not avail the animal unless it 

 were accompanied by the instinct to lie perfectly still, with 

 feelers and legs drawn in against the body, until danger is past. 

 There are hundreds of other instances of this correlation of 

 instinct and colouring which could be cited, and which show us 

 that these self-preservative instincts — for such, in the case of 

 Xylina, is the instinct to be still — are the results of natural 



