The inadequacy of "natural selection" / by Herbert Spencer ; reprinted from the Contemporary review.

  • Spencer, Herbert, 1820-1903.
Date:
[1893]
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Licence: Public Domain Mark

Credit: The inadequacy of "natural selection" / by Herbert Spencer ; reprinted from the Contemporary review. Source: Wellcome Collection.

    19/79 (page 165)
    having the smaller will continually survive and propagate, instead of those having the larger. Properly to appreciate this supposition, we must use figures. To give it every advantage we will assume that there have been only two thousand generations, and we will assume that, instead of being reduced to a rudiment, the eye has disappeared altogether. What amounts of variation shall we suppose ? If the idea is that the process has operated uniformly on each generation, the implication is that some advantage has been gained by the individuals having the eyes Wooth less in weight; and this will hardly be contended. Not to put the hypothesis at this disadvantage^ let us then imagine that there take place, at long intervals, decreasing variations considerable in amount—say xVth, once in a hundred generations. This is an interval almost too long to be- assumed ; but yet if we assume the successive decrements to occur more frequently, and therefore to be smaller, the amount of each becomes too insignificant. If, seeing the small head, we assume that the eyes of the Proteus originally weighed some ten grains each, this would give us, as the amount of the decrement of Joth, occurring once in a hundred generations, one grain. Suppose that this eel- shaped amphibian, about a foot long and more than half an inch in diameter, weighs three ounces—a very moderate estimate. In such case the decrement would amount to tttoth of the creature’s weight; or, for convenience, let us say that it amounted to xoVoth, which would allow of the eyes being taken at some fourteen grains each/ To this extent, then, each occasional decrement would profit the organism. The economy in weight to a creature having nearly the same specific gravity as its medium, would be infinitesimal. The economy in nutrition of a rudimentary organ, consisting of passive tissues, would also be but nominal. The only appreciable economy would be in the original building up of the creature’s structures • and the hypothesis of Weismann implies that the economy of this thousandth part of its weight, by decrease of the eyes, would so- benefit the rest of the creature’s organisation as to give it an * I find that the eye of a small smelt (the only appropriate small fish obtainable here, St. Leonards) is about T|Tth of its weight; and since in young fish the eyes are disproportionately large, in the full-grown smelt the eye would be probably not more than ^-th of the creature’s weight. On turning to highly-finished plates, published by the Bibliographisches Institut of Leipzig, of this perenni branchiate Proteus, and other amphibians, I find that in the nearest ally there represented, the caducibran- chiate axolotl, the diameter of the eye, less than half that of the smelt, bears a much smaller ratio to the length of the body: the proportion in the smelt being ^th of the length, and in the axolotl about (the body being also more bulky than that of the smelt). If, then, we take the linear ratio of the eye to body in this amphibian as one-half the ratio which the fish presents, it results that the ratio of the mass of the eye to the mass of the body will be but one-eighth. So that the weight of the eye of the amphibian will be but i^V/h of that of the body. It is a liberal estimate, there- fore, to suppose that its original weight in the Proteus was 1000th of that of the body. I may add that any one who glances at the representation of the axolotl, will see that, were the eye to disappear entirely by a single variation, the economy achieved could not have any appreciable physiological effect on the organism.