NA TURE 



[March 13, 1890 



There is, however, one respect -in which Prof. Weismann's 

 statement of the principle of panmixia differs from tliat which 

 was considered by Mr. Darwin ; and it is this difference of state- 

 ment — which amounts to an important difference of theory — that 

 I now wish to discuss. 



The difference in question is, that while Prof. Weismann 

 believes the cessation of selection to be capable of inducing de- 

 generation down to the. almost complete disappearance of a 

 rudimentary organ, I have argued that, unless assisted by some 

 other principle, it can at most only reduce the degenerating 

 organ to considerably above one-half its original size — or probably 

 not through so much as one-quarter. The ground of this argu- 

 ment (which is given in detail in the Nature articles before 

 alluded to) is, that panmixia depends for its action upon 

 fortuitous variations round an ever-diminishing average — the 

 average thus diminishing because it is no longer sustained by 

 natural selection. But although no longer sustained by natural 

 selection, it does continue to be sustained by heredity ; and there- 

 fore, as long as the force of heredity persists unimpaired, fortui- 

 tous variations alone — or variation which is no longer controlled 

 by natural selection — cannot reduce the dwindling organ to so 

 much as one-half of its original size ; indeed, as above fore- 

 shadowed, the balance between the positive force of heredity 

 and the negative effects of promiscuous variability will probably 

 be arrived at considerably above the middle line thus indicated. 

 Only if for any reason the force of heredity begins to fail, can 

 the average round which the cessation of selection works become 

 a progressively diminishing average. In other words, so long 

 as the original force of heredity as regards the useless organ 

 remains unimpaired, the mere withdrawal of selection cannot 

 reduce the organ much below the level of efficiency above 

 which it was previously maintained by the presence of selection. 

 If we take this level to be 70 per cent, of the original size, 

 cessation of selection will reduce the organ through the 30 per 

 cent , and there leave it fluctuating about this average, unless 

 for any reason the force of heredity begins to fail — in which 

 case, of course, the average will progressively fall in proportion 

 to the progressive weakening of this force. 



Now, according to my views, the force of heredity under such 

 circumstances is always bound to fail, and this for two reasons. 

 In the first place, it must usually happen that when an 

 organ becomes useless, natural selection as regards that 

 organ will not only cease, but become reversed. For 

 the organ is now absorbing nutriment, causing weight, oc- 

 cupying space, and so on, uselessly. Hence, even if it be 

 not also a source of actual danger, "economy of growth" will 

 determine a reversal of selection against an organ which is now 

 not merely useless, but deleterious. And this degenerating 

 influence of the reversal of selection will throughout be assisted 

 by the cessation of selection, which will now be always acting 

 round a continuously sinking average. Nevertheless, a point 

 of balance will eventually be reached in this case, just as it was 

 in the previous case where the cessation of selection was 

 supposed to be working alone. For, where the reversal of 

 selection has reduced the diminishing organ to so minute a size 

 that its presence is no longer a source of detriment to the 

 organism, the cessation of selection will carry the reduction a 

 small degree further ; and then the organ will remain as a 

 "rudiment." And so it will remain permanently, unless there 

 be some further reason why the still remaining force of heredity 

 should be abolished. This further reason I found in the con- 

 sideration that, however enduring we may suppose the force of 

 heredity to be, it would be unreasonable to suppose that it is 

 actually everlasting ; and, therefore, that we may reasonably 

 attribute the eventual disappearance of rudimentary organs to the 

 eventual failure of heredity itself. In support of this view there is 

 the fact that rudimentary organs, although very persistent, are 

 not everlasting. That they should be very persistent is what we 

 should expect, if the hold which heredity has upon them is great 

 in proportion to the time during which they were originally use- 

 ful, and so firmly stamped upon the organization by natural 

 selection causing them to be strongly inherited in the first in- 

 stance. Thus, for example, we might expect that it would be 

 more difficult finally to eradicate the rudiment of a wing than 

 the rudiment of a feather ; and accordingly we find it a general 

 rule that long-enduring rudiments are rudiments of organs dis- 

 tinctive of the higher taxonomic divisions — i.e. of organs which 

 were longest in building up in the first place, and longest sus- 

 tained in a state of working efficiency in the second place. 

 Again, that rudimentary organs, although in such cases very 



persistent, should not be everlasting, is also what we should 

 expect, unless (like Weismann) we have some argumentative 

 reason to sustain the doctrine that the force of heredity is 

 inexhaustible, so that never in any case can it become enfeebled 

 by a mere lapse of time — a doctrine the validity of which in the 

 present connection I will consider later on. 



Thus, upon the whole, my view of the facts of degeneration 

 remains the same as it was when first published in these columns 

 sixteen years ago, and may be summarized as follows. 



The cessation of selection when working alone (as it 

 probably does work in our domesticated animals, and during 

 the first centuries of its working upon structures or colours 

 which do not entail any danger to, or perceptible drain upon 

 the nutritive resources of, the organism) cannot cause degenera- 

 tion below, probably, some 20 to 30 per cent. But if from 

 the first the cessation of selection has been assisted by the 

 reversal of selection (on account of the degenerating structure 

 having originally been of a size sufficient to entail a perceptible 

 drain on the nutritive resources of the organism, having now 

 become a source of danger, and so forth), the two principles 

 acting together will continue to reduce the ever-diminishing 

 structure down to the point at which its presence is no longer a 

 perceptible disadvantage to the species. When that point is 

 reached, the reversal of selection will terminate, and the cessa- 

 tion of selection will not then be able of itself to reduce the 

 organ through more than at most a very few further percentages 

 of its original size. But, after this point has been reached, the 

 now total absence of selection, either for or against the organ, 

 will sooner or later entail this further and most important 

 consequence — viz. a failure of heredity as regards the organ. 

 So long as the organ was of use, its efficiency was constantly 

 maintained by the presence of selection — which is merely another 

 way of saying that selection was constantly maintaining the force 

 of heredity as regards that organ. But as soon as the organ 

 ceased to be of use, selection ceased to maintain the force of 

 heredity ; and thus, sooner or later, that force began to waver 

 or fade. Now it is this wavering or fading of the force of 

 heredity, thus originally due to the cessation of selection, that in 

 turn co-operates with the still continued cessation of selection 

 (panmixia) in reducing the structure below the level where its 

 reduction was left by the actual reversal of selection. So that 

 from that level downwards the cessation of selection and the 

 consequent failing of heredity act and react in their common 

 work of causing obsolescence. In the case of newly acquired 

 characters the force of heredity will be lass than in that of more 

 anciently acquired characters ; and thus we can understand the 

 long endurance of "vestiges" characteristic of the higher 

 taxonomic divisions, as compared with those characteristic of the 

 lower. But in all cases, if time enough be allowed, under the 

 cessation of selection the force of heredity will eventually fall to 

 zero, when the hitherto obsolescent structure will finally become 

 obsolete.^ 



Let us now turn to Weismann's view of degeneration. First 

 of all, he has omitted to perceive that "panmixia" alone (if 

 unassisted either by reversed selection or an inherent diminish- 

 ing of the force of heredity) cannot reduce a functionless organ, 

 to the condition of a rudiment. Therefore he everywhere 

 represents panmixia (or the mere cessation of selection) as of 

 itself sufficient to cause degeneration, say from loo to 5, instead 

 of from 100 to 80 or 70, which, for the reasons above given, ap- 

 peared (and still appears) to me about the most that this principle 

 alone can accomplish, so long as the original force of heredity 

 continues unimpaired. No doubt we have here what must be 

 regarded as a mere oversight on the part of Prof. Weismann ;. 

 but the oversight is rendered remarkable by the fact that he does 

 invoke the aid of reversed selection /// order to explain the final 

 disappearance op a rudiment. Yet it is self-evident that the 

 reversal of selection must be much more active during the initial 

 than during the final stages of degeneration, seeing that, ex 

 hypothesi, the greater the degree of reduction which has been 

 attained the less must be the detriment arising from any useless- 

 expenditure of nutrition, &c. 



And this leads me to a second oversight in Prof. Weismann's 

 statement, which is of more importance than the first. For the 



'^ It may not be needless to add that in the case of newly acquired and 

 comparatively trivial characters, with regard to which reversal of selection is 

 not likely to take place (e.g. slight differences of colour between allied species), 

 cessation of selection is likely to be very soon assisted by a failure in the 

 force of heredity ; seeing that such newly acquired characters will not be so 

 strongly inherited as are the more ancient characters distinctive of higher 

 taxonomic groups. 



