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 Lepidoptera, as well as the perfect insects, acquired these distasteful 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 probably is, that the imitators, being always fewer in number, have not been able to keep pace with the variations of the much more numerous imitated form; another reason may be the ever-increasing acuteness of the enemies, which have again and again detected the imposture and exterminated the feeble race before it has had time to become further modified.

The result of this growing acuteness of enemies has been, that those mimics that now survive exhibit, as Mr.Bates well remarks, "a palpably intentional likeness that is perfectly staggering," and also "that those features of the portrait are most attended to by nature which produce the most effective deception when the insects are seen in nature." No one, in fact, can understand the perfection of the imitation who has not seen these species in their native wilds.

So complete is it in general effect that in almost every box of butterflies, brought from tropical America by amateurs, are to be found some species of the mimicking Pieridae, Erycinidae, or moths, and the mimicked Heliconidae, placed together under the impression that they are the same species.