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Show 160 MR. R. MELDOLA ON VARIABLE PROTECTIVE [Feb. 4, duct of natural selection, which works in this case, as in all other cases, upon the variations of colour presented to it by nature, quite regardless of the manner in which the colour is primarily produced -regardless whether it is hypodermal or epidermal, whether it is a colour due to interference, or a colour due to pigment-cells in the skin. A similar mode of reasoning applied to such cases of variable protective colouring as have been considered in this paper will, I imagine, serve to establish the truth of the proposition, that such cases are " attributable, in great part at least, to the action of the ' survival of the fittest.' " Let us take the several groups of cases included in Class V. in the order already dealt with, and apply this reasoning to them. We have first to deal with larvae which feed on several plants of different colours, and which are capable of adapting themselves to the colour of the particular plant on which they are feeding. Now, granting that this power of changing colour is beneficial to the insects by affording them concealment, a truth which no entomologist who has witnessed any of these cases will deny, it follows that natural selection would eliminate any variations tending to depart from this useful power of adaptability to the colour of the food-plant. Here, then, the function of natural selection, as in the illustration first brought forward, is simply to maintain a power possessed by the larva, regardless whether this power resulted in the first instance from the direct action of external conditions-regardless whether it is under the control of the creature's will or not. Assuming, in these cases, that the change of colour is due to the presence of the colouring-matter of the food-plant in the tissues of the caterpillar (as it most probably is), we might say in more concrete language that natural selection is and has been at work weeding out those individuals whose skins were not sufficiently transparent to allow the colouring-matter to appear through them. Passing on to those pupae which appear to be photographically sensitive, we shall find a similar mode of reasoning to obtain. Larvae in selecting a suitable place to assume the pupal condition are liable to be exposed on surfaces of different colours. It will be admitted that pupee which harmonize with the colour of their rest-ing- surface would be more likely to escape detection than individuals not thus coloured. I need only allude, en passant, to the perfect manner in which the pupa of Synchloe brassicce matches the speckled wall on which we so often find it. N o w a pupa is liable to be exposed on a surface of any colour ; h o w can such a state of affairs be met? obviously only by giving to each individual a power of changing colour in correspondence with the colour of its resting-surface. Observe, now, that we are not here in any way concerned with the primary cause of such a faculty: natural selection only takes advantage of the property, no matter how it has originated. W e may finally proceed to the examples of this class furnished by insects in the perfect state. An insect adapted to the colour of one district, but forced to roam into other districts in search of food |