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Undoubtedly "such" or similar modifications have often been inherited, but how can Darwin possibly tell that they are not due to the simulation of use-inheritance by natural or artificial selection acting upon general variability?
If individual disuse diminished the weight of the duck's wing-bones by 9 per cent. there would be nothing left to account for. I suspect that investigation would reveal anomalies inconsistent with the theory of use-inheritance.
But how plasticity is related to race we do not yet know. It may be that use-inheritance somehow incorporates its effects in the offspring of the plastic parents. Or it may be simply that plasticity increases with inter-breeding on a wider basis. These problems have still to be solved.
Such cumulative tendencies of use-inheritance, if they exist, are obviously well kept under by natural selection. Although the ultimate effect of use is generally growth or enlargement through increased flow of blood, the first effect usually is a loss of substance, and a consequent diminution of size and strength.
But use-inheritance is no more proven by one of these necessary coincidences than special design is by the other. The inevitable simulation of use-inheritance may be entirely deceptive. Darwin thinks that "there can be no doubt that use in our domestic animals has strengthened and enlarged certain parts, and disuse diminished them; and that such modifications are inherited."
It must not be forgotten that it would be easy to cite an enormous number of cases which are in direct conflict with the supposed law of use-inheritance. Natural selection evolves blind races whenever eyes are useless or disadvantageous, as with parasites.
Use-inheritance is normally so weak that it appears to be quite helpless when opposed to any other factor of evolution. Natural selection evolves and maintains the instincts of ants and termites in spite of use-inheritance to a more wonderful degree than it evolves the instincts of almost any other animal with the fullest help of use-inheritance.
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