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Show 260 MR. E. B. POULTON ON THE PROTECTIVE [Mar. 1, most complete opposite of that in which tbe larvse render themselves conspicuous in various ways. The experimental evidence shows, however, that the larva has a most disagreeable taste and (almost certainly) smell, so that the most ravenous of all m y Lizards would not eat it. It is perfectly clear that these two methods of protection are antagonistic if present in the degree and kind possessed by this larva. One of them must be useless and merely incidental, and as it is quite certain that the highly specialized protective colouring and habits of concealment are of value to the organism, the unpleasant taste must be the useless character. And this was seen in its treatment by the Lizard, for the larva was recognized at once as something which was expected to be palatable, and was at first seized with great vigour, and it was only when the larva was injured beyond hope of recovery that its enemy recognized the unpleasant attributes and relinquished it. I witnessed the whole process ; it afforded the most instructive comparison with the reluctant and hesitating way in which a very hungry Lizard would approach a highly coloured larva which it knew to be distasteful. It was quite obvious that the Lizard fully expected a palatable insect, and was greatly surprised at the unwelcome result. After the larva had bled freely, another Lizard approached, but did not taste the insect, evidently repelled by the unpleasant smell of tbe freshly escaped fluids. It is obvious that a larva of this kind, being unpalatable, and yet giving off no strong smell from its surface, by which to warn its enemies, belonging, moreover, to an immense group of similarly protected insects of which the vast majority are highly relished,-it is certain that such a larva can gain nothing by an unpleasant taste which can only be appreciated after fatal injury, and which is not associated with any colour, marking, or habit by which the disagreeable experience could be remembered. W e are therefore driven to the conclusion that tbe unpleasant quality is in this case a merely useless character, probably some incidental result of the physiological processes of digestion or metabolism. But such a condition is most important on theoretical grounds, for it at once supplies the necessary steps by which a species can change from one protective method to another. The most constant objection or difficulty which is raised against the explanation of the rise of anv well-marked structure or function as due to the action of natural selection, deals especially with the initial stages. It is asked how natural selection can accumulate the earliest variations, which are (the objectors assume) of insufficient importance to act as criteria by which life and death can be settled. Darwin set the great example of giving a satisfactory answer to such objections by carefully working out one by one those cases in which especial difficulty was assumed. And here, by the instance of the larva of M. typica, we see at once how the difficulty of tbe origin of nauseous forms may be overcome; lor this larva possesses a useless attribute ready-made as the incidental result of some physiological process, and at so high a stage of efficiency that there is no difficulty whatever in imagining that it might readily Decome an important criterion of existence, |
