The extraordinary perfection of these mimetic resemblances is most wonderful. I have heard this urged as a reason for believing that they could not have been produced by natural selection, because a much less degree of resemblance would have protected the mimetic species. To this it may be answered, that natural selection not only tends to pick out and preserve the forms that have protective resemblances, but to increase the perceptions of the predatory species of insects and birds, so that there is a continual progression towards a perfectly mimetic form. This progressive improvement in means of defence and of attack may be illustrated in this way. Suppose a number of not very swift hares and a number of slow-running dogs were placed on an island where there was plenty of food for the hares but none for the dogs, except the hares they could catch; the slowest of the hares would be first killed, and the swifter preserved. Then the slowest-running dogs would suffer, and having less food than the fleeter ones, would have least chance of living, and the swiftest dogs would be preserved; thus the fleetness of both dogs and hares would be gradually but surely perfected by natural selection, until the greatest speed was reached that it was possible for them to attain. I have in this supposed example confined myself to the question of speed alone, but in reality other means of pursuit and of escape would come into play and be improved. The dogs might increase in cunning, or combine together to work in couples or in packs by the same selective process; and the hares on their part might acquire means of concealment or stratagem to elude their enemies; but, on both sides, the improvement would be progressive until the highest form of excellence was reached. Viewed in this light, the wonderful perfection of mimetic forms is a natural consequence of the selection of the individuals that, on the one side, were more and more mimetic, and on the other (that of their enemies) more and more able to penetrate through the assumed disguises. It has doubtless happened in some cases that species, having many foes, have entirely thrown off some of them through the disguises they have been brought to assume, but others they still cannot elude.