But, during the early stages of this process, some of the Pieridae, inhabiting the same district, happened to be sufficiently like some of the Heliconidae to be occasionally mistaken for them. These, of course, survived while their companions were devoured. Those among their descendants that were still more like Heliconidae again survived, and at length the imitation would become tolerably perfect. Thereafter, as the protected group diverged into distinct species of many different colours, the imitative group would occasionally be able to follow it with similar variations,--a process that is going on now, for Mr. Bates informs us that in each fresh district he visited he found closely allied representative species or varieties of Heliconidae, and along with them species of Leptalis (Pieridae), which had varied in the same way so as still to be exact imitations. But this process of imitation would be subject to check by the increasing acuteness of birds and other animals which, whenever the eatable Leptalis became numerous, would surely find them out, and would then probably attack both these and their friends the Heliconidae in order to devour the former and reject the latter. The Pieridae would, however, usually be less numerous, because their larvae are often protectively 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 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.