[2] _Contributions to an Insect Fauna of the Amazon Valley_, Trans. Linn. Soc., Vol. XXIII, 1862.
Bates's hypotheses have been subsequently confirmed in the most striking way. The Heliconiidæ do possess a disagreeable taste and odour, and are utterly rejected by birds, lizards, and other animals. It has been directly observed that puff-birds, species of _Trogon_, and other insectivorous birds, looking down from the tops of trees in search of food, allowed to pass unheeded the swarms of gaily coloured Heliconiidæ which were fluttering among the leaves, and experiments with various insectivorous animals yielded the same result: _the Heliconiidæ are immune_. We can, therefore, not only understand that it must be advantageous to resemble them, we can also appreciate many of their peculiar characters, such as their gay coloration, which must serve as a sign of their disagreeable taste, and their slow, fluttering flight, as well as their habit of flocking together, which must make it easier for the birds to recognize them as uneatable. Everything which marks out these unpalatable morsels, and makes them more readily recognizable, must be to their advantage, and therefore must have been favoured by natural selection (Pl. II, Fig. 13).
In the same way, every increase of resemblance on the part of the mimics would increase their chances of escaping notice, and any one who is accustomed to observe butterflies in nature can well understand that even very slight resemblances may have formed the beginning of the selection process; perhaps even a mere variation in the manner of flight, combined with the habit of associating with the swarms of Heliconiidæ. I myself have many times been momentarily deceived in our own woods by a White of unusually majestic flight, so that I took it for an _Apatura_ or a _Limenitis_. If, therefore, individual Whites occurred here and there in the Amazon valley, which flew somewhat after the manner of the Heliconiidæ, and associated with them, they might possibly have attained a certain degree of security through that alone, and it would be greatly increased if at the same time they varied somewhat in colour in the direction of their companions.
In any case there can be no doubt whatever that in these cases a real transformation of the species in colour and marking, and perhaps often, too, in form of wing, has taken place, and that within comparatively modern times--let us say during the distribution of a species which required protection over a large continent, or since the last breaking up of an immune species into local species. Various facts prove this; above all, the circumstance that it is often only the females which exhibit this protective mimicry; and that one and the same species may mimic a different immune species in different areas, but always the one occurring abundantly in that area, and so on.