Volume Ii Part 16 (1/2)

and ”genera” in the same respects. Yet, in accordance with the general theory of evolution, it is plainly as impossible to draw any such line in the one case as it is to do so in the other. Just as fixed varieties are what Darwin called ”incipient species,” so are species incipient genera, genera incipient families, and so on. Evolutionists must believe that the process of evolution is everywhere the same. Nevertheless, while admitting all this, the school of Huxley contradicts itself by alleging some unintelligible exception in the case of ”species,” while the school of Wallace presses this exception so as to embrace ”specific characters.” Indeed Mr. Wallace, while maintaining that all specific characters must necessarily be useful, maintains at the same time that any number of varietal characters on the one hand, and a good half of generic characters on the other, are probably useless. Thus he contradicts his argument from the ”constancy of specific characters”

(seeing that generic characters are still more constant), as later on we saw that he contradicts his deductive generalization touching their necessary utility, by giving a non-utilitarian explanation of whole mult.i.tudes of specific characters. I need not, however, again go over the ground so recently traversed; but will conclude by once more recurring to the only explanation which I have been able to devise of the otherwise inexplicable fact, that in regard to this subject so many naturalists still continue to entangle themselves in the meshes of absurdity and contradiction.

The only conceivable explanation is, that these naturalists have not yet wholly divested themselves of the special creation theory. Although professing to have discarded the belief that ”species” are ”definite ent.i.ties,” differing in kind from ”varieties” on the one hand and from ”genera” on the other, these writers are still imbued with a vague survival of that belief. They well know it to belong to the very essence of their new theory that ”species” are but ”p.r.o.nounced varieties,” or, should we prefer it, ”incipient genera”; but still they cannot altogether escape the pre-Darwinian conception of species as organic units, whose single mode of origin need not extend to other taxonomic groups, and whose characters therefore present some exceptional significance to the scientific naturalist. So to speak, such divinity doth still hedge a species, that even in the very act of declaring it but an idol of their own creation, these naturalists bow before their fetish as something that is unique--differing alike in its origin and in its characters from the varieties beneath and the genera above. The consequence is that they have endeavoured to reconcile these incompatible ideas by subst.i.tuting the principle of natural selection for that of super-natural creation, where the particular case of ”species” is concerned. In this way, it vaguely seems to them, they are able to save the doctrine of some one mode of origin as appertaining to species, which need not ”necessarily” appertain to any other taxonomic division. All other such divisions they regard, with their pre-Darwinian forefathers, as merely artificial constructions; but, likewise with these forefathers, they look upon species as natural divisions, proved to be such by a single and necessary mode of origin. Hence, Mr. Wallace expressly defines a species with reference to this single and necessary mode of origin (_see_ above, p. 235), although he must be well aware that there is no better, or more frequent, proof of it in the case of species, than there is in that of somewhat less p.r.o.nounced types on the one hand (fixed varieties), or of more p.r.o.nounced types on the other (genera, families, &c.). Hence, also, the theory of natural selection is defined as _par excellence_ a theory of the origin of species; it is taken as applying to the particular case of the origin of species in a peculiarly stringent manner, or in a manner which does not apply to the origin of any other groups. And I believe that an important accessory reason of the continuance of this view for more than thirty years after the publication of the _Origin of Species by means of Natural Selection_, is to be found in the t.i.tle of that work. ”Natural Selection” has thus become verbally a.s.sociated with ”Origin of Species,”

till it is thoughtlessly felt that, in some way or another, natural selection must have a peculiar reference to those artificially delineated forms which stand anywhere between a fixed variety and a so-called genus. This verbal a.s.sociation has no doubt had the effect of still further preserving the traditional halo of mystery which clings to the idea of a ”species.” Hence it comes that the t.i.tle which Darwin chose--and, looking to the circ.u.mstances of the time, wisely chose--for his great work, has subsequently had the effect of fostering the very idea which it was the object of that work to dissipate, namely, that species are peculiar ent.i.ties, which differ more or less in origin or kind from all other taxonomic groups. The full t.i.tle of this work is--_The Origin of Species by means of Natural Selection: or the Preservation of Favoured Races in the Struggle for Life_. Now, supposing that instead of this its author had chosen some such t.i.tle as the following:--_The Origin of Organic Types by means of Adaptive Evolution: or Survival of the Fittest Forms in the Struggle for Life_. Of course this would have been a bad subst.i.tute from various points of view; but could any objection have been urged against it from our present point of view? I do not see that there could. Yet, if such had been the t.i.tle, I have little doubt that we should never have heard of those great generalizations with regard to species and specific characters, the futility of which it has been the object of these chapters to expose.

In conclusion, it only remains to reiterate that in thus combating what appears to me plainly erroneous deductions from the theory of natural selection, I am in no wise combating that theory itself. On the contrary, I hope that I am rendering it no unimportant service by endeavouring to relieve it of a parasitic growth--an accretion of false logic. Regarding as I do the theory of natural selection as, primarily, a theory of the origin (or c.u.mulative development) of adaptations, I see in merely non-adaptive characters--be they ”specific” or other--a comparatively insignificant cla.s.s of phenomena, which may be due to a great variety of incidental causes, without any further reference to the master-principle of natural selection than that in the presence of this principle none of these non-adaptive characters can be actively deleterious. But that there may be ”any number of indifferent characters” it is no part of the theory of natural selection to deny; and all attempts to foist upon it _a priori_ ”deductions” opposed alike to the facts of nature and to the logic of the case, can only act to the detriment of the great generalization which was expressly guarded from such fallacies by the ever-careful judgement of Darwin.

APPENDICES AND NOTES

APPENDIX I.

ON PANMIXIA.

There are several points of considerable theoretical importance connected with Panmixia, which were omitted from the text, in order to avoid distracting attention from the main issue which is there under consideration. These side issues may now be appropriately presented in the form in which they were published in _Nature_, March 13, 1890[140].

After stating, in almost the same words, what has already been said in Chapter X, this paper proceeds, with the exception of a few verbal alterations, as follows.

[140] Vol. xli. p. 438.

”There is, however, one respect in which Professor Weismann's statement of the principle of panmixia differs from that which was considered by Mr. Darwin; and it is this difference of statement--which amounts to an important difference of theory--that I now wish to discuss.

”The difference in question is, that while Professor Weismann believes the cessation of selection to be capable of inducing degeneration down to the almost complete disappearance of a rudimentary organ, I have argued that, _unless a.s.sisted 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 argument (which is given in detail in the _Nature_ articles of 1873-1874) is, that panmixia depends for its action upon fortuitous variations round an ever-diminis.h.i.+ng average--the average thus diminis.h.i.+ng 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 therefore, as long as the force of heredity persists unimpaired, fortuitous 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 foreshadowed, the balance between the positive force of heredity and the negative effects of promiscuous variability will most likely be arrived at 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 diminis.h.i.+ng 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 80 or 90 per cent. of the original size, cessation of selection will reduce the organ through the 10 or 20 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 circ.u.mstances 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, occupying s.p.a.ce, 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 a.s.sisted 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 diminis.h.i.+ng 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 (or second) reason I found in the consideration that, however enduring we may suppose the force of heredity to be, we cannot 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 useful, and thus firmly stamped upon the organization by natural selection causing them to be strongly inherited in the first instance. 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 distinctive of the higher taxonomic divisions--i.e. of organs which were longest in building up, and therefore longest sustained in a state of working efficiency.

”Thus, upon the whole, my view of the facts of degeneration remains the same as it was when first published in these columns seventeen years ago, and may be summarized as follows.

”The cessation of selection when working alone (as it probably does during the first centuries of its action upon structures or colours which do not entail any danger to, or perceptible drain upon, the nutritive resources of the organism) cannot cause degeneration below, probably, some 10 to 20 per cent. But if from the first the cessation of selection has been a.s.sisted 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-diminis.h.i.+ng 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 cessation 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, 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 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 added characters, the force of heredity will be less than in that of more anciently added 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. In cases of newly added 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 a.s.sisted by a failure in the force of heredity; seeing that such newly added characters will not be so strongly inherited as are the more ancient characters distinctive of higher taxonomic groups.

”Let us now turn to Weismann's view of degeneration. First of all, he has omitted to perceive that 'panmixia' alone (if una.s.sisted either by reversed selection or an inherent diminis.h.i.+ng 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 100 to 5, instead of from 100 to 90 or 80, which, for the reasons above given, appeared (and still appears) to me about the most that this principle 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 Professor Weismann; but the oversight is rendered remarkable by the fact that he _does_ invoke the aid of reversed selection _in order to explain the final disappearance of 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 Professor Weismann's statement, which is of more importance than the first. For the place at which he does invoke the a.s.sistance of reversed selection is exactly the place at which reversed selection must necessarily have ceased to act. This place, as already explained, is where an obsolescent organ has become rudimentary, or, as above supposed, reduced to 5 per cent. of its original size; and the reason why he invokes the aid of reversed selection at this place is in order to save his doctrine of 'the stability of germ-plasm.' That the force of heredity should finally become exhausted if no longer _maintained_ by the _presence_ of selection, is what Darwin's theory of perishable gemmules would lead us to expect, while such a fact would be fatal to Weismann's theory of an imperishable germ-plasm. Therefore he seeks to explain the eventual failure of heredity (which is certainly a fact) by supposing that after the point at which the cessation of selection alone can no longer act (and which his first oversight has placed some 80 per cent. too low), the reversal of selection will begin to act directly against the force of heredity as regards the diminis.h.i.+ng organ, until such direct action of reversed selection will have removed the organ altogether. Or, in his own words, 'The complete disappearance of a rudimentary organ can only take place by the operation of natural selection; this principle will lead to its diminution, inasmuch as the disappearing structure takes the place and the nutriment of other useful and important organs.' That is to say, the rudimentary organ finally disappears, not because the force of heredity is finally exhausted, but because natural selection has begun to utilize this force against the continuance of the organ--always picking out those congenital variations of the organ which are of smallest size, and thus, by its now _reversed_ action, _reversing_ the force of heredity as regards the organ.

”Now the oversight here is in not perceiving that the smaller the disappearing structure becomes, the less hold must 'this principle'

of reversed selection retain upon it. As above observed, during the earlier stages of reduction (or while co-operating with the cessation of selection) the reversal of selection will be at its _maximum_ of efficiency; and, as the process of diminution continues, a point must eventually be reached at which the reversal of selection can no longer act. Take the original ma.s.s of a now obsolescent organ in relation to that of the entire organism of which it then formed a part to be represented by the ratio 1:100.

For the sake of argument we may a.s.sume that the ma.s.s of the organism has throughout remained constant, and that by 'ma.s.s' in both cases is meant capacity for absorbing nutriment, causing weight, occupying s.p.a.ce, and so forth. Now, we may further a.s.sume that when the ma.s.s of the organ stood to that of its organism in the ratio of 1:100, natural selection was strongly reversed with respect to the organ. But when this ratio fell to 1:1000, the activity of such reversal must have become enormously diminished, even if it still continued to exercise any influence at all. For we must remember, on the one hand, that the reversal of selection can only act as long as the presence of a diminis.h.i.+ng organ continues to be so injurious that variations in its size are matters of life and death in the struggle for existence; and, on the other hand, that natural selection in the case of the diminis.h.i.+ng organ does not have reference to the presence and the absence of the organ, but only to such variations in its ma.s.s as any given generation may supply. Now, the process of reduction does not end even at 1:1000.

It goes on to 1:10,000, and eventually 1:8. Consequently, however great our faith in natural selection may be, a point must eventually come for all of us at which we can no longer believe that the reduction of an obsolescent organ is due to reversed selection. And I cannot doubt that if Professor Weismann had sufficiently considered the matter, he would not have committed himself to the statement that 'the complete disappearance of a rudimentary organ can only take place by the operation of natural selection.'

”According to my view, the complete disappearance of a rudimentary organ can only take place by the _cessation_ of natural selection, which permits the eventual exhaustion of heredity, when heredity is thus simply left to itself. During all the earlier stages of reduction, the cessation of selection was a.s.sisted in its work by the reversal of selection; but when the rudiment became too small for such a.s.sistance any longer to be supplied, the rudiment persisted in that greatly reduced condition until the force of heredity with regard to it was eventually worn out. This appears to me, as it appeared in 1873, the only reasonable conclusion that can be drawn from the facts. And it is because this conclusion is fatal to Professor Weismann's doctrine of the permanent 'stability' of germ-plasm, while quite in accordance with all theories which belong to the family of pangenesis, that I deem the facts of degeneration of great importance as tests between these rival interpretations of the facts of heredity. It is on this account that I have occupied so much s.p.a.ce with the foregoing discussion; and I shall be glad to ascertain whether any of the followers of Professor Weismann are able to controvert these views.

”GEORGE J. ROMANES.”