But now, as all these butterflies were alike or nearly alike, how could Mr. Bates tell—or how had anybody been able to tell before him—that they were really not all the same species—that a _Heliconea_ was not a _Papilio_, or that a _Papilio_, _Pieris_, _Euterpe_, etc., were not all of them _Heliconeas_? This, at first sight, seems a difficult question to answer, but really it is not, because, in all these families of butterflies, the various species composing them bear a kind of generalised resemblance to one another: there is a family likeness, in fact, and this is not only the case in regard to their outward appearance—the shape and colour of their wings, etc.—but it applies, in a still greater degree, to their structure and internal economy. Thus, however strongly a _Pieris_, or one of those others, might resemble a _Heliconea_, the trained eye of an entomologist could easily see that it really was a member of another family, and since, in resembling the _Heliconea_, it departed from the general type of pattern and colouring exhibited by the family to which it belonged, whilst this one species of _Heliconea_ it resembled was like the others, it might be inferred that the latter was the imitated and not the imitating form. Again—and this is still more decisive evidence in the cases where it applies—the resemblance is often confined to one sex of the copying species, viz. the female, so that whilst she is hardly to be distinguished from the model on which she has founded herself, the male retains the appearance, together with all the other characteristics, of the race to which both he and she belong. But now came a further question, the most puzzling or, at any rate, the most important one of all, viz. Why should the one butterfly imitate, or rather resemble the other, in such an extraordinary degree—a degree seeming to preclude the possibility of mere chance having brought it about? This question Mr. Bates is supposed to have been the first to answer, though I cannot help thinking, myself, that he has only extended an explanation, which, in some cases, was so obvious that no one had thought of pointing it out, to these other cases where it was not nearly so easy to see. For what can be plainer, as Mr. Bates himself remarks, than that a moth, for instance, by closely resembling a hornet, would escape the attacks of birds that might otherwise have devoured it? I cannot think but that so patent an explanation had been in the minds of many long before 1862, and though no one previous to that date may have applied the principle of natural selection to such cases, it must be remembered that natural selection had been established by Darwin some ten or twelve years before.