[Illustration: FIG. 7. Caterpillar of a North American _Darapsa_ in its "terrifying attitude" (after Abbot and Smith).]
I should like to discuss one other phenomenon, which is well calculated to give us a deeper insight into the transformation processes of organisms--I refer to the remarkable dimorphism of colour which occurs in many of the species of caterpillar just described.
The caterpillar of the convolvulus hawk-moth (_Sphinx convolvuli_) is in its full-grown stage green, like the wild convolvulus on which it lives, or brown like the ground on which its food-plant grows. It thus shows a double adaptation, each of which is capable of protecting it to a certain extent, and we might think to the _same_ extent. But that is not so, the brown colouring is a more effective protection than the green, as we may learn from two facts. In the first place, the four young stages of the caterpillar are green, and it only becomes brown in the last stage, though sometimes even then it remains green. This shows that the brown is a relatively modern adaptation, and it could not have arisen had it not been better than the original green. In the second place, the green-coloured caterpillars of the convolvulus hawk-moth are nowadays much less numerous than the brown ones, and this implies that the latter survive oftener in the struggle for existence. We have here an interesting case of an easily recognizable process of selection still going on between the old green and the newer brown variety.
It is hardly necessary to ask why the brown colour should in this case be a better protection than the green, for it is obvious that such a large green body as that of the full-grown convolvulus-caterpillar would be but badly concealed among the little leaves of the convolvulus plant in spite of its green colour; while the brown caterpillar, on the brown soil, with its pebbles, hollows, and irregular shadows, is excellently protected, especially if it passes the day concealed in the ground, as is actually the case.
Our view is materially strengthened by the fact that the same phenomenon of double colouring occurs in several allied species of Sphingidæ, but in a manner which shows us that we have to do with a similar process of transformation, only at a more advanced stage. The caterpillar of _Chærocampa elpenor_ (Fig. 4) shows the same state of things as that of the convolvulus hawk-moth; it is brown or green, and the green form is the less common. But in the two other European species of _Chærocampa_ the full-grown caterpillar is always brown, and indeed it becomes brown in the fourth stage, instead of, like _Chærocampa elpenor_, only in the fifth and last. Another indigenous sphingid species, _Deilephila vespertilio_, only remains green during the first two stages, and assumes in the third stage the grey-brown colour which it afterwards retains. The dark colour has obviously prevailed among the full-grown caterpillars for a considerable length of time, for it is in this, the largest and most conspicuous stage, that the change of colour must have been most necessary, and consequently the process of selection must have begun in it, and only after the more protective brown became general would it have extended to the next stage below, if it were of use there too, and, later on, to still earlier stages in the life-history.