Does the habit prevail only among the species of the one genus _Deilephila_, and in all the species of this genus? This is by no means the case, since, on the one hand, many species of _Deilephila_, such as _D. Euphorbiæ_, _Galii_, _Nicæa_, and _Dahlii_, do not possess the habit, and, on the other hand, it occurs in species of other genera, such as _Macroglossa Stellatarum_, _Sphinx Convolvuli_, and _Acherontia Atropos_. The habit in question must therefore be the result of certain external conditions of life common to all those species which rest by day. The mode of life common to them all is that they do not live on trees with large leaves or with thick foliage, but on low plants or small-leaved shrubs, such as the Sea Buckthorn.[136] I believe I do not err when I attribute the habit possessed by the adult larvæ, of concealing themselves by day, to the fact that the green colour is protective only so long as they are small--or, more precisely speaking, as long as their size does not considerably exceed that of a leaf or twig of their food-plant. When they become considerably larger, they must become conspicuous in spite of their adaptive colour, so that it would then be advantageous for them to conceal themselves by day, and to feed only by night. This habit they have acquired, and still observe, even when the secondary adaptation to the colour of the soil, &c., has not been brought about. We learn this from _D. Hippophaës_, which remains green throughout its whole larval existence; and no less from the green forms of the adult larvæ of _Sphinx Convolvuli_, _Chærocampa Elpenor_, and _Porcellus_, all of which conceal themselves by day in the same manner as their brown allies. It may be objected that there are Sphinx-larvæ--instances of which I have myself adduced--which live on low small-leaved plants, and which nevertheless do not hide themselves by day. This is the case with the spurge-feeding _D. Euphorbiæ_, so common in many parts of Germany. This caterpillar must, however, be classed with those which, on account of their distastefulness, or for other reasons to be subsequently considered, are rejected by birds and other larger foes, and which, as Wallace has shown, derive advantage from being coloured as vividly as possible. I shall return to this subject later, when treating of the biological value of special markings. On the other hand, it is readily conceivable that, from the conditions of life of caterpillars living on trees or shrubs with dense foliage, the habit of resting by day and descending from the tree for concealment would not have been acquired. Such larvæ are sufficiently protected by their green colour among the large and numerous leaves; and I shall have occasion to show subsequently that their markings increase this protective resemblance.