244 DARWINISM 



the former and reject the latter. The Pieridae would, however, 

 usually be less numerous, because their larvae are often pro- 

 tectively coloured and therefore edible, while the larvae of the 

 Heliconidae are adorned with warning colours, spines, or 

 tubercles, and are uneatable. It seems probable that the 

 larvae and pupae of the Heliconidae were the first to acquire 

 the protective distastefulness, both because in this stage they 

 are more defenceless and more liable to fatal injury, and also 

 because we now find many instances in which the larvae are 

 distasteful while the perfect insects are eatable, but I believe 

 none in which the reverse is the case. The larvae of the 

 Pieridae are now beginning to acquire offensive juices, but 

 have not yet obtained the corresponding conspicuous colours ; 

 while the perfect insects remain eatable, except perhaps in 

 some Eastern groups, the under sides of whose wings are 

 brilliantly coloured although this is the part which is exposed 

 when at rest. 



It is clear that if a large majority of the larvae of Lepido- 

 ptera, as well as the perfect insects, acquired these distaste- 

 ful properties, so as seriously to diminish the food supply of 

 insectivorous and nestling birds, these latter would be forced 

 by necessity to acquire corresponding tastes, and to eat with 

 pleasure what some of them now eat only under pressure of 

 hunger ; and variation and natural selection would soon bring 

 about this change. 



Many writers have denied the possibility of such wonderful 

 resemblances being produced by the accumulation of fortuitous 

 variations, but if the reader will call to mind the large amount 

 of variability that has been shown to exist in all organisms, 

 the exceptional power of rapid increase possessed by insects, and 

 the tremendous struggle for existence always going on, the 

 difficulty will vanish, especially when we remember that 

 nature has the same fundamental groundwork to act upon in 

 the two groups, general similarity of forms, wings of similar 

 texture and outline, and probably some original similarity of 

 colour and marking. Yet there is evidently considerable 

 difficulty in the process, or with these great resources at her 

 command nature would have produced more of these mimicking 

 forms than she has done. One reason of this deficiency prob- 

 ably is, that the imitators, being always fewer in number, have 



