For while there has been taking place in the biological world the major differentiation above indicated, there have been taking place certain minor differentiations—there have been arising special views respecting the process of organic evolution. Concerning each of these it is needful to say something.
§ 174b. Among the implied controversies the most conspicuous one has concerned the alleged process called by Prof. Weismann Panmixia—a process which Dr. Romanes had foreshadowed under the name of "the Cessation of Selection." Dr. Romanes says:—"At that time it appeared to me, as it now appears to Weismann, entirely to supersede the necessity of supposing that the effect of disuse is ever inherited in any degree at all."[[55]] The alleged mode of action is exemplified by Prof. Weismann as follows:—
"A goose or a duck must possess strong powers of flight in the natural state, but such powers are no longer necessary for obtaining food when it is brought into the poultry-yard, so that a rigid selection of individuals with well-developed wings, at once ceases among its descendants. Hence in the course of generations, a deterioration of the organs of flight must necessarily ensue, and the other members and organs of the bird will be similarly affected."[[56]]
Here, and throughout the arguments of those who accept the hypothesis of Panmixia, there is an unwarranted assumption—nay, an assumption at variance with the doctrine in support of which it is made. It is contended that in such cases as the one given there will, apart from any effects of disuse, be decrease in the disused organs because, not being kept by Natural Selection up to the level of strength previously needed, they will vary in the direction of decrease; and that variations in the direction of decrease, occurring in some individuals, will, by interbreeding, produce an average decrease throughout the species. But why will the disused organs vary in the direction of decrease more than in the direction of increase? The hypothesis of Natural Selection postulates indeterminate variations—deviations no more in one direction than in the opposite direction: implying that increases and decreases of size will occur to equal extents and with equal frequencies. With any other assumption the hypothesis lapses; for if the variations in one direction exceed those in another the question arises—What makes them do this? And whatever makes them do this becomes the essential cause of the modification: the selection of favourable variations is tacitly admitted to be an insufficient explanation. But if the hypothesis of Natural Selection itself implies the occurrence of equal variations on all sides of the mean, how can Panmixia produce decrease? Plus deviations will cancel minus deviations, and the organ will remain where it was.[[57]]
"But you have forgotten the tendency to economy of growth," will be the reply—"you have forgotten that in Mr. Darwin's words 'natural selection is continually trying to economize in every part of the organization;' and that this is a constant cause favouring minus variations." I have not forgotten it; but have remembered it as showing how, to support the hypothesis of Panmixia, there is invoked the aid of that very hypothesis which it is to replace. For this principle of economy is but another aspect of the principle of functionally-produced modifications. Nearly forty years ago I contended that "the different parts of ... an individual organism compete for nutriment; and severally obtain more or less of it according as they are discharging more or less duty:"[[58]] the implication being that as all other organs are demanding blood, decrease of duty in any one, entailing decreased supply of blood, brings about decreased size. In other words, the alleged economy is nothing else than the abstraction, by active parts, of nutriment from an inactive part; and is merely another name for functionally-produced decrease. So that if the variations are supposed to take place predominantly in the direction of decrease, it can only be by silently assuming the cause which is overtly denied.
But now we come to the strange fact that the particular case in which panmixia is assigned in disproof of alleged inheritance of functionally-produced modifications, is a case in which it would be inapplicable even were its assumption legitimate—the case of disused organs in domestic animals. For since nutrition is here abundant, the principle of economy under the form alleged does not come into play. Contrariwise, there even occurs a partial re-development of rudimentary organs: instances named by Mr. Darwin being the supplementary mammæ in cows, fifth toes on the hind feet of dogs, spurs and comb in hens, and canine teeth in mares. Now clearly, if organs disused for innumerable generations may thus vary in the direction of increase, it must, a fortiori, be so with recently disused organs, and there disappears all plea (even the illegitimate plea) for assuming that in the wing of a wild duck which has become domesticated, the minus variations will exceed the plus variations: the hypothesis of panmixia loses its postulate.
If it be said that Mr. Darwin's argument is based on the changed ratio between the weights of leg-bones and wing-bones, and that this changed ratio may result not from decrease of the wing-bones but from increase of the leg-bones, then there comes a fatal reply. Such, increase cannot be ascribed to selection of varieties, since there is no selective breeding to obtain larger legs, and as it is not pretended that panmixia accounts for increase the case is lost: there remains no cause for such increase save increase of function.
§ 174c. The doctrine of determinate evolution or definitely-directed evolution, which appears to be in one form or other entertained by sundry naturalists, has been set forth by the late Prof. Eimer under the title "Orthogenesis." A distinct statement of his conception is not easily made for the reason that, as I think, the conception itself is indistinct. Here are some extracts from a translation of his paper published at Chicago. Out of these the reader may form a notion of the theory:
"Orthogenesis shows that organisms develop in definite directions without the least regard for utility through purely physiological causes as the result of organic growth, as I term the process."
"I am concerned in this paper with definitely directed evolution as the cause of transmutation, and not with the effects of the use and activity of organs which with Lamarck I adopted as the second main explanatory cause thereof."