From all these facts, which I have summarized as briefly as possible, we see that the older phyletic characters are gradually crowded by the newer into ever-younger stages in the ontogeny, until ultimately they disappear altogether. We have now to ask to what this phenomenon is due; is it a simple crowding out of the old and less advantageous by the new and better characters as a result of natural selection, or is there some other factor at work? It is clear in regard to these forms of marking that they can have been developed at first only in the almost full-grown larva by natural selection, because they are of use only there, and that, at the same time, the old marking must have been set aside through the influence of the same factor, in as far as it prejudiced the effect of the new adaptation. This seems to be indicated by the persistence of the sub-dorsal line on those segments which are drawn in when _Chærocampa_ assumes a terrifying attitude, or which do not bear oblique stripes in the leaf-like caterpillars, e.g. the three anterior segments in the species of _Sphinx_ and _Smerinthus_. When newly acquired schemes of marking like the eye-spots of _Chærocampa_ are transmitted from the last stage to the stage before, this can be explained by following the same train of thought, for the caterpillar is already of sufficient size to be able to inspire terror with its eyes; but in still younger stages the spots would not be likely to have that effect, and yet they occur in quite small animals (20 mm.). More obvious still is the uselessness of the oblique striping in the young stages of the _Sphinx_ and _Smerinthus_ caterpillars, for in the earliest stages of life the caterpillars are much too small to look like a leaf, and the oblique stripes stand much closer together than the lateral ribs of any leaf. Moreover, the little green caterpillars require no further protection when they sit on the under side of a leaf; they might then very easily be mistaken _in toto_ for a leaf-rib. _Thus it is certainly not natural selection which effects the shunting back of the new characters._ Nor can this be caused by the fact that the new character can only be developed gradually and in several stages, for the oblique striping at any rate arises in the ontogeny all at once. There must therefore be some mechanical factor in development to which is due the fact that characters acquired in the later stages are gradually transferred to the younger stages. But this shifting backwards can be checked by the agency of natural selection as soon as it becomes disadvantageous for the stage concerned.