Darwin, and After Darwin, Volume 2 of 3 / Post-Darwinian Questions: Heredity and Utility

by George John Romanes

Here, then, we have three estimates of the amount of degeneration which can be produced by panmixia alone, where mere size or bulk of an organ is concerned—say, 3 to 5 per cent., 10 to 20 per cent., and 95 per cent. to 0. At first sight, these differences appear simply ludicrous; but on seeking for the reasons of them, we find that they are due to different views touching the manner in which panmixia operates. The oversights which have led to Weismann's extremely high estimate have already been stated. The reason of the difference between the extremely low estimate of Professor Lloyd Morgan, as compared with my own intermediate one, is, that he supposes the power of panmixia to become exhausted as soon as the level of mediocrity of the first generations has become the general level in succeeding generations. In my view, however, the level of mediocrity is itself a sinking level in successive generations, with the result that there is no reason why the reducing power of panmixia should ever become exhausted, save that the more reduction it effects the greater is the force of heredity which remains to be overcome, as previously explained. Thus the only question between Professor Lloyd Morgan and myself is—Does the level of mediocrity fall in successive generations under the cessation of selection, or does it remain permanently where it used to be under the presence of selection? Does the "birth-mean" remain constant throughout any number of generations, notwithstanding that the sustaining influence of selection has been withdrawn; or does it progressively sink as a consequence of such withdrawal?

In order to answer this question we had better begin by considering now the case of organization of structure, as distinguished from mere size of structure. Take any case where a complex organ—such as a compound eye—has been slowly elaborated by natural selection, and is it not self-evident that, when natural selection is withdrawn, the complex structure will deteriorate? In other words, the level of mediocrity, say in the hundred thousandth generation after the sustaining influence of natural selection has been withdrawn, will not be so high as it was in the first generations. For, by hypothesis, there is now no longer any elimination of unfavourable variations, which may therefore perpetuate themselves as regards any of the parts of this highly complex mechanism; so that it is only a matter of time when the mechanism must become disintegrated. I can scarcely suppose that any one who considers the subject will question this statement, and therefore I will not say anything that might be said in the way of substantiating it. But, if the statement be assented to, it follows that there is no need to look for any cause of deterioration, further than the withdrawal of selection—or cessation of the principle which (as we are supposing) had hitherto been the sole means of maintaining efficient harmony among all the independently variable parts of the highly complex structure.

Now, I hold that the same thing is true, though in a lesser degree, as regards degeneration of size. That there is no difference in kind between the two cases, Professor Lloyd Morgan implicitly allows; for what he says is—

"In any long-established character, such as wing-power in birds, brain-development, the eyes of crustacea, &c., no shortcomer in these respects would have been permitted by natural selection to transmit his shortcomings for hundreds of generations. All tendency to such shortcomings would, one would suppose, have been bred out of the race. If after this long process of selection there still remains a strong tendency to deterioration, this tendency demands an explanation[144]."

Here, then, deterioration as to size of structure (wings of birds), and deterioration as to complexity of structure (brain and eyes) are expressly put upon the same footing. Therefore, if in the latter case the "tendency to deterioration" does not "demand an explanation," beyond the fact that the hitherto maintaining influence has been withdrawn, neither is any such further explanation demanded in the former case. Which is exactly my own view of the matter. It is also Mr. Galton's view. For although, in the passage formerly quoted, Professor Lloyd Morgan appears to think that by the phrase "Regression to Mediocrity" Mr. Galton means to indicate that panmixia can cause degeneration only as far as the mediocrity level of the first generations, this, in point of fact, is not what Galton means, nor is it what he says. The phrase in question occurs "in another connexion," and, indeed, in a different publication. But where he expressly alludes to the cessation of selection, this is what he says. The italics are mine.

"A special cause may be assigned for the effects of use in causing hereditary atrophy of disused parts. It has already been shown that all exceptionally developed organs tend to deteriorate: consequently, those that are not protected by selection will dwindle. The level of muscular efficiency in the wing of a strongly flying bird [curiously enough, the same case that is chosen by Professor Lloyd Morgan to illustrate his opposite view], is like the level of water in the leaky vessel of a Danaid, only secured to the race by constant effort, so to speak. Let the effort be relaxed ever so little, and the level immediately falls[145]."

I take it, then, that the burden of proof lies with Professor Lloyd Morgan to show why the withdrawal of selection is not sufficient to account for degeneration any further than the mediocrity-level in the former presence of selection. Why does "the strong tendency[146] to deterioration demand an explanation," further than the fact that when all variations below the average in every generation are allowed to survive, they must gradually lower the average itself through a series of generations? To answer that any such tendency "would have been bred out of the race" by the previous action of selection, is to suppose that the function of selection is at an end when once it has built up a structure to the highest point of working efficiency,—that the presence of selection is no longer required to maintain the structure at that point. But it is enough to ask in reply—Why, under the cessation of selection, does complexity of structure degenerate so much more rapidly than size of structure? Why is it, for instance, that "the eyes of crustacea" in dark caves have entirely disappeared, while their foot-stalks (when originally present) still remain? Can it be maintained that "for hundreds of generations" natural selection was more intent on developing the foot-stalks than the eyes which were mounted upon them—so that while the latter were left by selection with "a strong tendency to deterioration," the former have had this tendency "bred out in the race"[147]?

To sum up. There is now no question in any quarter touching the fact that panmixia, or the cessation of selection, is a true cause of degeneration. The only question is as to the amount of degeneration which it is able to effect when not assisted by the reversal of selection, or any other cause of degeneration. Moreover, even with regard to this question of amount, there is no doubt on any side that panmixia alone causes degeneration more rapidly where it has to do with complexity of organization, than it does where it is concerned with a mere reduction of mass.

The question as to the amount of degeneration that is caused by the cessation of selection alone is without any practical importance where species in a state of nature are concerned, because here the cessation of selection is probably always associated more or less with the reversal of it; and it is as impossible as it is immaterial to determine the relative shares which these two co-operating principles take in bringing about the observed results. But where organisms in a state of domestication are concerned, the importance of the question before us is very great. For if the cessation of selection alone is capable of reducing an organ through 10 or 12 per cent. of its original size, nearly all the direct evidence on which Darwin relied in favour of use-inheritance is destroyed. On the other hand, if reduction through 5 per cent. be deemed a "very liberal estimate" of what this principle can accomplish, the whole body of Darwin's direct evidence remains as he left it. I have now given my reasons for rejecting this lower estimate on the one band, and what seems to me the extravagant estimate of Weismann on the other. But my own intermediate estimate is enough to destroy the apparent proof of use-inheritance that was given by Darwin. Therefore it remains for those who deny Lamarckian principles, either to accept some such estimate, or else to acknowledge the incompatibility of any lower one with the opinion that there is no evidence in favour of these principles.