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Updated: May 6, 2025
For the importance of panmixia as invalidating Darwin's strongest evidence for use-inheritance namely, that drawn from the effects of disuse in highly-fed domestic animals where there is supposed to be no economy of growth see Professor Romanes on Panmixia, Nature, April 3, 1890. Descent of Man, p. 33. Descent of Man, p. 33. Variation of Animals and Plants under Domestication, i., 453.
Spencer is scarcely borne out by the facts so far as male jaws are concerned. The great reduction in the weight of female jaws and skulls evidently points to sexual selection and to panmixia under male protection.
Panmixia, or the suspension of natural selection, together with altered habits, will account for an increase of short-sight among the population generally. Long-sighted people could not work at watchmaking and engraving so comfortably and advantageously as at other occupations, and hence would be less likely to take to such callings.
Panmixia, however, as Weismann has shown, would probably be the most important factor in causing blindness. Darwin says: "A horse is trained to certain paces, and the colt inherits similar consensual movements." But selection of the constitutional tendency to these paces, and imitation of the mother by the colt, may have been the real causes.
A similar simulation of course occurs under domestication, where natural selection is partly replaced by artificial selection of the best adapted and therefore most flourishing animals, while in disused parts panmixia or the comparative cessation of selection will aid or replace "economy of growth" in causing diminution.
An apparent increase in this liability might arise from greater attention being now paid to it, or from increased use of harder roads; or a real increase might be due to panmixia and some obscure forms of correlation. Of course artificially-caused ill-health or weakness in parents will tend in a general way to injure the offspring.
He failed to adequately notice the effect of panmixia, or the withdrawal of selection, in causing or allowing degeneracy and dwindling under disuse; and he hardly attached sufficient importance to the fact that rudimentary organs and other supposed effects of use or disuse are quite as marked features in neuter insects which cannot transmit the effects of use and disuse as they are in the higher animals.
Romanes, Galton, and Weismann have made great use of this principle in explaining the diminution of disused organs. Weismann has given it the name of Panmixia, all individuals being equally free to survive and commingle their variations, and not merely selected or favoured individuals. Inclusive in each case of fixed strengthening wire weighing about a sixteenth of an ounce or less.
Weismann has emphasized this idea in his doctrine of "panmixia," or the withdrawal of selection, which always results in degeneration. Selection, artificial or natural, may serve to counteract this universal tendency of organic life, but only approximately. As Sir William Dawson says, "All things left to themselves tend to degenerate."
Of the inevitability of selection and of its generally adaptive tendencies "there can be no doubt," and panmixia would tend to reduce disused parts; so that there must always remain grave doubts of the alleged inheritance of the similar effects of use and disuse, unless we can accomplish the extremely difficult feat of excluding both natural and artificial selection as causes of enlargement, and panmixia and selection as causes of dwindling.
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