Essays upon Heredity, vol. i, p. 90.
In a letter published by Dr. Romanes in Nature, for April 26, 1894, he alleges three reasons why "as soon as selection is withdrawn from an organ the minus variations of that organ outnumber the plus variations." The first is that "the survival-mean must descend to the birth-mean." The interpretation of this is that if the members of a species are on the average born with an organ of the required size, and if they are exposed to natural selection, then those in which the organ is relatively small will some of them die, and consequently the mean size of the organ at adult age will be greater than at birth. Contrariwise, if the organ becomes useless and natural selection does not operate on it, this difference between the birth-mean and the survival-mean disappears. Now here, again, the plus variations and their effects are ignored. Supposing the organ to be useful, it is tacitly assumed that while minus variations are injurious, plus variations are not injurious. This is untrue. Superfluous size of an organ implies several evils:—Its original cost is greater than requisite, and other organs suffer; the continuous cost of its nutrition is unduly great, involving further injury; it adds needlessly to the weight carried and so again is detrimental; and there is in some cases yet a further mischief—it is in the way. Clearly, then, those in which plus variations of the organ have occurred are likely to be killed off as well as those in which minus variations have occurred; and hence there is no proof that the survival-mean will exceed the birth-mean. Moreover the assumption has a fatal implication. To say that the survival-mean of an organ is greater than the birth-mean is to say that the organ is greater in proportion to other organs than it was at birth. What happens if instead of one organ we consider all the organs? If the survival-mean of a particular organ is greater than its birth-mean, the survival mean of each other organ must also be greater. Thus the proposition is that every organ has become larger in relation to every other organ!—a marvellous proposition. I need only add that Dr. Romanes' inferences with respect to the two other causes—atavism and failing heredity—are similarly vitiated by ignoring the plus variations and their effects.
Westminster Review, January, 1860. See also Essays, &c., vol. i, p. 290.
"On Orthogenesis and the Impotence of Natural Selection in Species-Formation," pp. 2, 19, 22, 24.
Address to Plymouth Institution, at opening of Session 1895-6.