It is precisely the case of this last caterpillar, which formerly appeared to me to present insurmountable difficulties to the explanation of the coloured stripes by adaptation, and I believed that this insect would have to be classed with those species which are brightly coloured because they are distasteful, and are avoided by birds. But although we have no experiments on this point, I must reject this view. Unfortunately, we know scarcely anything of the ontogeny of this caterpillar; but we know at least that the young larvæ (stage four) are greener than the more purely yellow ones of the fifth stage (which, however, are also frequently green), and we know further that some adults are of a dark brownish-grey, without any striking colours. From analogy with the dimorphism of the species of _Chærocampa_ and _Sphinx_, fully considered previously, it must therefore be concluded that in this case also, a new process of adaptation has commenced--that the caterpillar is becoming adapted to the soil in and on which it conceals itself by day.[144] An insect which acquires undoubted protective colours cannot, however, be classed with those which possess an immunity from hostile attacks.
That the coloured edges are correctly explained as imitations of the oblique shadows of the leaf-ribs, may also be proved from another point of view. Let us assume, for the sake of argument, that these coloured stripes are not adaptive, and that they have not been produced by natural selection, but by a hypothetical phyletic force. We should then expect to see them appear at some period in the course of the phyletic development--perhaps at first only in solitary individuals, then in several, and finally in all; but we certainly could not expect that at first single, irregular, coloured spots should arise in the neighbourhood of the oblique white stripes--that these spots should then multiply, and fusing together, should adhere to the white stripes, so as to form an irregular spot-like edge, which finally becomes formed into a straight, uniformly broad stripe. The phyletic development of the coloured edges takes place, however, in such a manner, the species of _Smerinthus_, as has already been established, showing this with particular distinctness. In _S. Tiliæ_ the course of development can be followed till the somewhat irregular red border is formed. In the species of _Sphinx_ this border has become completely linear. It is very possible that the ontogeny of _S. Ligustri_ or _Drupiferarum_ would reveal the whole process, although it may also be possible that owing to the contraction of the development, much of the phylogeny is already lost.