Page:The Effect of External Influences upon Development.djvu/30

26 present only be supposed to occur, another kind of dimorphism is known with certainty, in which also protective colouration is involved. In this case the nature of the colour to be developed is determined by the quality of the light radiating from the surroundings of the animal. Most of you must be familiar with the beautiful experiments of Professor Poulton on caterpillars and pupae, which demonstrate this. The caterpillars of Amphidasis betularia take on a colouration similar to that of the twigs on which they live from the first; and one can cause them to become 'black, brown, or bright green by the presence of similarly-coloured twigs (or paper) in their surroundings, although fed on the same food .' In like manner the pupae of Vanessa urticae become dark blackish-brown when they undergo transformation on a dark ground; but assume a light colour or even a strong golden sheen when they have settled on a light ground. In these cases, again, it cannot be supposed that the primary constituents for polymorphism have possibly arisen directly owing to the light; they must rather have been produced as an adaptation by processes of selection. But each of the possible colours of the skin originating in this way is specially sensitive to certain kinds of light and is roused to activity by these alone.

This instance leads us to those in which the same individual is able to change colour in a short