OCR Text |
Show 228 MR. E. B. POULTON ON THE PROTECTIVE [Mar. 1, birds would destroy the same number of individuals of each, before they were educated to avoid them. Then if these insects are thoroughly mixed, and become undistinguishable to the birds, a proportionate advantage accrues to each over its former state of existence. These proportionate advantages are inversely in the duplicate ratio of their respective original numbers, compounded with the ratio of the respective percentages that would have survived without the mimicry." It had been previously argued that in the case of two protected species which had thus come to resemble each other, the proportionate advantage was chiefly on the side of the one which was smaller in numbers, and that when the numerical difference was great the advantage to the other could be neglected. The amended law which is quoted above shows, however, that the proportionate advantage is always the same, and this is also enforced in another part of the same letter:-" It must be remembered, however, that B does no harm to A by mimicking it; on the contrary the act of mimicry is of advantage to A over its former state of existence as well as to B ; but A being more numerous the advantage is less. Still, after the assimilation, neither has an advantage over the other. Proportionally they suffer from the ravages of birds equally ; the percentage of losses is the same ; they are on equal terms. No matter how long they continue the association, neither gains or loses on the other ; though through one being more numerous it loses more individuals, yet equally in proportion with tbe other. So that if one is twice as numerous as the other at the time of assimilation, it must always-other conditions being equal-remain twice as numerous." Dr. Miiller's interpretation was at first criticized in many quarters, the chief objection brought forward being the belief that birds do not learn the meaning of the conspicuous colours by experience, but that they avoid such insects by instinct, the ancestral experience having become hereditary. There is, however, no direct evidence for this view, and I think the account of J. Jenner Weir's observations upon Lizards, and m y own upon Lizards and Frogs (given in the two Appendices to this paper), will go far to furnish an experimental refutation of such a theory, so far as these animals are concerned. In addition to this, I am assured by a very keen observer, Rev. G. J. Burch, that recently hatched chickens certainly do peck at insects which they afterwards learn to avoid without trial, and he believes that the hen assists in their education by indicating that certain insects are not tit for food. His observations were chiefly made upon a common phytophagous Hymenopterous larva which is found upon gooseberry (doubtless Nematus ribesii). Another observation made by Mr. Burch bears upon the same question. He offered his chickens a quantity of chickweed, knowing that this plant was often given as food to Linnets. The chickens ate the plant readily enough, but they were all extremely unwell in consequence, and vomited freely. After this Mr. Burch again offered them chickweed, but they had profited by the experience and would not touch it. The chief attack upon Dr. Miiller's suggestion was made by Mr.W. |