groups. The fashion is in each case set by the most abundant form, generally an Ithomiine of the genus _Melinaea_. The rest are mimics of this dominant species, either in the {136} Batesian or Müllerian sense. Batesian mimics are such genera as _Dismorphia_ and _Protogonius_, to which there are no reasons for attributing disagreeable properties. Of the nature of Müllerian mimics on the other hand are the various Heliconines and Ithomiines which enter into the combination. In each case the whole assemblage is a great "mimicry ring," of which the pattern is dictated by the Ithomiine that predominates in point of numbers. It is, however, very doubtful whether this can be accepted as a satisfactory explanation. The four groups which we have considered are all characterised by a peculiar and distinctive coloration, and in each case the pattern must on the theory of mimicry be regarded as a highly efficient warning pattern. One or other of these patterns must doubtless be looked upon as the most primitive. If so the question at once arises as to why a distasteful genus should change from one efficient warning pattern to another quite distinct one. If the newer pattern affords better protection we should expect it to have spread and eventually to have ousted the older one. That it has not done so must probably be attributed to the old pattern being as efficient as the new one. But if this is so we are left without grounds for assuming the change to have been brought about by natural selection through the agency of enemies to whom warning colours appeal. For natural selection can only bring about a change that is beneficial to the species. Hence we must suppose the change on the part of the dominant model to have been independent of natural selection by {137} enemies, and due to some condition or set of conditions of which we are ignorant. It is not inconceivable that the new colour scheme was associated with some physiological peculiarity which was advantageous to the species in its altered surroundings. If so natural selection may have favoured the new variety, not because of its colour scheme, but owing to the underlying physiological differences of which the pattern is but an outward sign. And if this could happen in one species there seems to be no reason why it should not happen in others. The weak point of the explanation on the mimicry hypothesis is that it offers no explanation of the change in the so-called dominant Ithomiine pattern as we pass from one region to another. Whatever the cause of this change may be there would appear to be nothing against it having also operated to produce similar changes in other unrelated species, in which case the mimicry hypothesis becomes superfluous. It is not unlikely that the establishing of these new forms was due to natural selection. If they were associated with physiological peculiarities better adapted for their environment it is reasonable to