The gradual dying out of the less favourably conditioned forms of a species is a law which follows as a logical necessity from the essence of the process of selection, but its reality may be inferred from the phenomena themselves. On it depends, as far at least as adaptations are concerned, the transformation of species. A beautiful example of the crowding out of a less favoured form of a species by a more favoured one is afforded by a butterfly of North America, of which the two female forms have long been known, although the reason for their dimorphism was not understood. A yellow butterfly, _Papilio turnus_, not unlike our swallow-tail, has yellow females in the north and east of the United States, but black ones in the south and west. There was much guessing as to what the cause of this striking phenomenon might be, and it was for a time thought that this difference was directly due to the influence of climate, and, later, the black form of female was regarded as protectively coloured, because of the supposed greater persecution by birds in the south, since the female would be less easily recognized if of a dark colour, and would thus be better protected. This last explanation could hardly be looked upon as satisfactory, for a black butterfly in flight would be very easily seen by sharp-sighted birds; indeed, against a light background, it would be even more readily seen than a light one. Since we have acquired a more exact knowledge of the immune species of _Papilio_ this case has become clear to us. For on those stretches of country on which the black female of _Papilio turnus_ lives there occurs another _Papilio_ which is black in both sexes, _Papilio philenor_, and this is one of those species which are protected by their unpleasant taste and odour. Here, therefore, we have a case of mimicry, the female of _Papilio turnus_ imitates the immune _Papilio philenor_, and thereby secures protection for itself; but as the immune model only occurs in the southern half of the distribution of _Papilio turnus_ a somewhat sharp separation of the two forms of female has been evolved; the black, mimetic form, being the most fit, has completely crowded the primitive yellow form out of the area inhabited by _Papilio philenor_, while beyond this area, to the north and west, the yellow form alone prevails. The extensive and careful studies of Edwards have shown that the two forms occur together only in a very narrow transition region.