[146] No one has supposed that the tendency need be "strong": it has only to be persistent.
[147] Of course it must be observed that degeneration of complexity involves also degeneration of size, so that a more correct statement of the case would be--Why, under the cessation of selection, does an organ of extreme complexity degenerate much more rapidly than one of much less complexity? For example, under domestication the brains of rabbits and ducks appear to have been reduced in some cases by as much as 50 per cent.
(Darwin, and Sir J. Crichton Browne.) But if it is possible to attribute this effect--or part of it--to an artificial selection of stupid animals, I give in the text an example occurring under nature. Many other cases, however, might be given to show the general rule, that under cessation of selection complexity of structure degenerates more rapidly--and also more thoroughly--than size of it. This, of course, is what Mr. Galton and I should expect, seeing that the more complex a structure the greater are the number of points for deterioration to invade when the structure is no longer "protected by selection." (On the other hand, of course, this fact is opposed to the view that degeneration of useless structures below the "birth-mean" of the first generations, is exclusively due to the reversal of selection; for economy of growth, deleterious effect of weight, and so forth, ought to affect size of structure _much more_ than complexity of it.) But I choose the above case, partly because Professor Lloyd Morgan has himself alluded to "the eyes of crustacea," and partly because Professor Ray Lankester has maintained that the loss of these eyes in dark caves is due to the reversal of selection, as distinguished from the cessation of it. In view of the above parenthesis it will be seen that the point is not of much importance in the present connexion; but it appears to me that cessation of selection must here have had at least the larger share in the process of atrophy.
For while the economy of nutrition ought to have removed the relatively large _foot-stalks_ as rapidly as the _eyes_, I cannot see that there is any advantage, other than the economy of nutrition, to be gained by the rapid loss of hard-coated _eyes_, even though they have ceased to be of use.
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 a.s.sisted 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 ma.s.s.
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 a.s.sociated 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.
APPENDIX II.
ON CHARACTERS AS ADAPTIVE AND SPECIFIC.
It is the object of this Appendix to state, more fully than in the text, the opinions with regard to this subject which have been published by the two highest authorities on the theory of natural selection--Darwin and Professor Huxley. I will take first the opinion of Professor Huxley, quoted _in extenso_, and then consider it somewhat more carefully than seemed necessary in the text.
As far as I am aware, the only occasion on which Professor Huxley has alluded to the subject in question, is in his obituary notice of Darwin in the _Proceedings of the Royal Society_, Vol. XLIV, No. 269, p. xviii.
The allusion is to my paper on _Physiological Selection_, in the _Journal of the Linnaean Society_, Zool. Vol. XIX, pp. 337-411. But it will be observed that the criticism has no reference to the theory which it is the object of that paper to set forth. It refers only to my definition of the theory of natural selection as primarily a theory of the origin, or c.u.mulative development, of adaptations. This criticism, together with my answer thereto at the time, is conveyed in the following words.
"Every variety which is selected into a species is favoured and preserved in consequence of being, in some one or more respects, better adapted to its surroundings than its rivals. In other words, every species which exists, exists in virtue of adaptation, and whatever accounts for that adaptation accounts for the existence of the species. To say that Darwin has put forward a theory of the adaptation of species, but not of their origin, is therefore to misunderstand the first principles of the theory. For, as has been pointed out, it is a necessary consequence of the theory of selection that every species must have some one or more structural or functional peculiarities, in virtue of the advantage conferred by which it has fought through the crowd of its compet.i.tors, and achieved a certain duration. In this sense, it is true that every species has been "originated" by selection."
Now, in the first place, I have nowhere said that "Darwin has put forward a theory of the adaptation of species, but not of their origin." I said, and continue to say, that he has put forward a theory of _adaptations in general_, and that where such adaptations appertain to species only (i.e. are peculiar to particular species), the theory becomes "_also_ a theory of the origin of the species which present them." The only possible misunderstanding, therefore, which can here be alleged against me is, that I fail to perceive it as a "necessary consequence of the theory of selection that _every_ species _must_ have some one or more structural or functional _peculiarities_" of an adaptive or utilitarian kind.
Now, if this is a misunderstanding, I must confess to not having had it removed by Mr. Huxley"s exposition.
The whole criticism is tersely conveyed in the form of two sequent propositions--namely, "Every species which exists, exists in virtue of adaptation; and whatever accounts for that adaptation accounts for the existence of the species." My answer is likewise two-fold.
First, I do not accept the premiss; and next, even if I did, I can show that the resulting conclusion would not overturn my definition. Let us consider these two points separately, beginning with the latter, as the one which may be most briefly disposed of.
I. Provisionally conceding that "every species which exists, exists in virtue of adaptation," I maintain that my definition of the theory of natural selection still holds good. For even on the basis of this concession, or on the ground of this a.s.sumption, the theory of natural selection is not shown to be "_primarily_" a theory of the origin of species. It follows, indeed, from the a.s.sumption--is, in fact, part and parcel of the a.s.sumption--that all species have been originated by natural selection; but why? _Only because natural selection has originated those particular adaptive features in virtue of which (by the hypothesis) species exist as species._ It is only in virtue of having created these features that natural selection has created the species presenting them--just as it has created genera, families, orders, &c., in virtue of _other_ adaptive features extending through progressively wider areas of taxonomic division. Everywhere and equally this principle has been "primarily" engaged in the evolution of adaptations, and if one result of its work has been that of enabling the systematist to trace lines of genetic descent under his divisions of species, genera, and the rest, such a result is but "secondary" or "incidental."
In short, it is "_primarily_" a theory of adaptations _wherever these occur_, and only becomes "_also_" or "_incidentally_" a theory of species in cases where adaptations happen to be restricted in their occurrence to organic types of a certain order of taxonomic division.
II. Hitherto, for the sake of argument, I have conceded that, in the words of my critic, "it is a necessary consequence of the theory of selection that every species must have some one or more structural or functional peculiarities" of an adaptive kind. But now I will endeavour to show that this statement does not "follow as a necessary consequence" from "the theory of selection."
Most obviously "it follows" from the theory of selection that "every variety which is selected into a species is favoured and preserved in consequence of being, in some one or more respects, better adapted to its surroundings than its rivals." This, in fact, is no more than a re-statement of the theory itself. But it does _not_ follow that "every species which exists, exists in virtue of adaptation" _peculiar to that species_; i.e. that every species which exists, exists _in virtue of having been "selected_." This may or may not be true as a matter of fact: as a matter of logic, the inference is not deducible from the selection theory. Every variety which is "_selected into_" a species must, indeed, present some such peculiar advantage; but this is by no means equivalent to saying, "in other words," that every variety which _becomes_ a species must do so. For the latter statement imports a completely new a.s.sumption--namely, that every variety which _becomes_ a species must do so because it has been "_selected into_" a species.
In short, what we are here told is, that if we believe the selection principle to have given origin to some species, we must further believe, "as a necessary consequence," that it has given origin to all species.
The above reply, which is here quoted _verbatim_ from _Nature_, Vol. 38, p. 616-18, proceeded to show that it does not belong to "the first principles of the theory of natural selection" to deny that no other cause than natural selection can possibly be concerned in the origin of species; and facts were given to prove that such unquestionably has been the case as regards the origin of "local" or "permanent" _varieties_.
Yet such varieties are what Darwin correctly terms "incipient" species, or species in process of taking _origin_. Therefore, if Professor Huxley"s criticism is to stand at all, we must accept it "as a necessary consequence of the theory of selection," that every such _variety_ "which exists, exists in virtue of adaptation"--a statement which is _proved_ to be untrue by the particular cases forthwith cited. But as this point has been dealt with much more fully in the text of the present treatise, I shall sum up the main points in a few words.
The criticism is all embodied in two propositions--namely, (_a_) that the theory of natural selection carries with it, as a "necessary consequence," the doctrine that survival of the fittest has been the cause of the origin of _all_ species; and (_b_) that therefore it amounts to one and the same thing whether we define the theory as a theory of species or as a theory of adaptations. Now, as a mere matter of logical statement, it appears to me that both these propositions are unsound. As regards the first, if we hold with Darwin that other causes have co-operated with natural selection in the origination of some (i.
e. many) species, it is clearly no part of the theory of natural selection to a.s.sume that none of these causes can ever have acted independently. In point of fact, as we have seen in the foregoing chapters, such has probably and frequently been the case under the influences of isolation, climate, food, s.e.xual selection, and laws of growth; but I may here adduce some further remarks with regard to yet another possible cause. If the Lamarckian principles are valid at all, no reason can be shown why in some cases they may not have been competent _of themselves_ to induce morphological changes of type by successive increments, until a trans.m.u.tation of species is effected by their action alone--as, indeed, Weismann believes to have been the case with all the species of Protozoa[148]. That such actually has often been the case also with numberless species of Metozoa, is the belief of the neo-Lamarckians; and whether they are right or wrong in holding this belief, it is equally certain that, _as a matter of logical reasoning_, they are not compelled by it to profess any _disbelief_ in the agency of natural selection. They may be mistaken as to the facts, as Darwin in a lesser degree may have been similarly mistaken; but just as Darwin has nowhere committed himself to the statement that _all_ species must _necessarily_ have been originated by natural selection, so these neo-Lamarckians are perfectly logical in holding that _some_ species may have been wholly caused by the inheritance of acquired characters, as _other_ species may have been wholly caused by the natural selection of congenital characters. In short, unless we begin by a.s.suming (with Wallace and against Darwin) that there _can be no other cause_ of the origin of species than that which is furnished by natural selection, we have no basis for Professor Huxley"s statement "that every species has been originated by selection"; while, if we do set out with this a.s.sumption, we end in a mere tautology. What ought to be done is to prove the validity of this a.s.sumption; but, as Professor Huxley makes no attempt to do this, his criticism amounts to mere begging of the question.
[148] Since the above was written Professor Weismann has transferred this doctrine from the Protozoa to their ancestors.
And now, as regards the second point (_b_), even if we grant the a.s.sumption that natural selection is the only possible cause of the origin of species--or, which is the same thing, that every species has been originated by natural selection,--is it likewise the same thing whether we define the theory of natural selection as a theory of species or as a theory of adaptations? Professor Huxley"s criticism endeavours to show that it is; but a little consideration is enough to show that it is not. What does follow from the a.s.sumption is, that, _so far as specific characters are concerned_, it is one and the same thing to say that the theory is a theory of species, and to say that it is a theory of adaptations. But specific characters are not conterminous with adaptive characters; for innumerable adaptive characters are not distinctive of species, but of genera, families, orders, cla.s.ses, and sub-kingdoms. Therefore, if it is believed (as, of course, Professor Huxley believes) that the theory in question explains the evolution of all adaptive characters, obviously it is not one and the same thing to define it indifferently as a theory of species or as a theory of adaptations.
Now, all this is not merely a matter of logic chopping. On the contrary, the question whether we are to accept or to reject the deduction that all species must necessarily have owed their origin to natural selection, is a question of no small importance to the general theory of evolution. And our answer to this question must be determined by that which we give to the ulterior question--Is the theory of natural selection to be defined as a theory of species, or as a theory of adaptations?
We now pa.s.s on to our consideration of Darwin"s opinion touching the question, as stated by himself,--"The doctrine of utility, how far true?" As I cannot ascertain that Darwin has anywhere expressed an opinion as to whether natural selection has been necessarily concerned in the origin of all _species_, the issue here is as to whether he held this with regard to all _specific characters_. It will be remembered that while opposing this doctrine as erroneous both in logic and in fact, I have represented that it is not a doctrine which Darwin sanctioned; but, on the contrary, that it is one which he expressly failed to sanction, by recognizing the frequent inutility of specific characters. Mr. Wallace, on the other hand, alleges that Darwin did believe in the universal--as distinguished from the general--utility of such characters. And he adds that he has "looked in vain in Mr. Darwin"s works" for any justification of my statements to the contrary[149].
Therefore I will endeavour to show that Mr. Wallace"s search has not been a very careful one.
[149] _Darwinism_, p. 131. He says:--"I have looked in vain in Mr.
Darwin"s works for any such acknowledgement" (i.e. "that a large proportion of specific distinctions must be conceded useless to the species presenting them").
We must remember, however, that it was not until the appearance of my paper on _Physiological Selection_, four years after Darwin"s death, that the question now in debate was raised. Consequently, he never had occasion to deal expressly with this particular question--viz. whether "the doctrine of utility" has any _peculiar_ reference to _specific_ characters--as he surely would have done had he entertained the important distinction between specific and all other characters which Mr. Wallace now alleges that he did entertain. But, be this as it may, we cannot expect to find in Darwin"s writings any express allusion to a question which had not been raised until 1886. The most we can expect to find are scattered sentences which prove that the distinction in question was never so much as present to his mind,--i. e. never occurred to him as even a possible distinction.
I will first take the pa.s.sages which Mr. Wallace himself supplies from among those which I had previously indicated.
"But when, from the nature of the organism and of the conditions, modifications have been induced which are unimportant for the welfare of the _species_, they may be, and apparently often have been, transmitted in nearly the same state to numerous, otherwise modified, descendants[150]."
[150] _Origin of Species_, p. 175. Italics mine.
On this pa.s.sage Mr. Wallace remarks that the last five words "clearly show that such characters are usually not "specific," in the sense that they are such as distinguish species from one another, but are found in numerous allied species." But I cannot see that the pa.s.sage shows anything of the sort. What to my mind it does show is, (_a_) that Mr.
Darwin repudiated Mr. Wallace"s doctrine touching the _necessary_ utility of _all_ specific characters: (_b_) that he takes for granted the contrary doctrine touching the inutility of _some_ specific characters: (_c_) that without in this place alluding to the proportional number of useless specific characters, he refers their origin in some cases to "the nature of the organism" (i.e. "spontaneous variability" due to internal causes), and in other cases to "the conditions" (i.e. variability induced by external causes): (_d_) that when established as a specific character by heredity, such a useless character was held by him not to tend to become obsolete by the influence of natural selection or any other cause; but, on the contrary, to be "transmitted in nearly the same state to numerous, otherwise modified, descendants"--or progeny of the species in genera, families, &c.: (_e_) and, therefore, that useless characters which are now distinctive of genera, families, &c., were held by him frequently, if not usually, to point to uselessness of origin, when first they arose as merely specific characters. Even the meaning which Mr. Wallace reads into this pa.s.sage must imply every one of these points; and therefore I do not see that he gains much by apparently seeking to add this further meaning--viz. that in Darwin"s opinion there must have been some una.s.signable reason preventing the occurrence of useless specific characters in cases where species are _not_ destined to become the parents of genera.
Moreover, any such meaning is out of accordance with the context from which the pa.s.sage is taken. For, after a long consideration of the question of utility, Darwin sums up,--"We thus see that with plants many morphological changes may be attributed to the laws of growth and the interaction of parts, _independently of natural selection_." And then he adds,--"From the fact of the above characters being _unimportant for the welfare of the species_, any slight variations which occurred in them _would not have been augmented through natural selection_." Again, still within the same pa.s.sage, he says, while alluding to the causes other than natural selection which lead to changes of specific characters,--"If the _unknown cause_ were to act almost uniformly for a length of time, we may infer that the result would be almost uniform; and in this case _all_ the individuals of the _species_ would be modified in the same manner." For my own part I do not understand how Mr. Wallace can have overlooked these various references to _species_, all of which occur on the very page from which he is quoting. The whole argument is to show that "many morphological changes may be attributed to the laws of growth and the inter-action of parts [_plus_ external conditions of life], independently of natural selection"; that such non-adaptive changes, when they occur as "specific characters," may, if the species should afterwards give rise to genera, families, &c., become distinctive of these higher divisions. But there is nothing here, or in any other part of Darwin"s writings, to countenance the inconsistent notion which Mr. Wallace appears to entertain,--viz. that species which present useless characters are more apt to give rise to genera, families, &c., than are species which do not present such characters.
The next pa.s.sage which Mr. Wallace quotes, with his comments thereon, is as follows. The italics are his.
""Thus a large yet undefined extension may safely be given to the direct and indirect results of natural selection; but I now admit, after reading the essay of Nageli on plants, and the remarks by various authors with respect to animals, more especially those recently made by Professor Broca, that in the earlier editions of my Origin of Species I perhaps attributed too much to the action of natural selection, or the survival of the fittest. I have altered the fifth edition of the Origin so as to confine my remarks to adaptive changes of structure; _but I am convinced, from the light gained during even the last few years, that very many structures which now appear to be useless, will hereafter be proved to be useful, and will therefore come within the range of natural selection_. Nevertheless I did not formerly consider sufficiently the existence of structures which, as far as we can at present judge, are neither beneficial nor injurious; and this I believe to be one of the greatest oversights as yet detected in my work."
Now it is to be remarked that neither in these pa.s.sages nor in any of the other less distinct expressions of opinion on this question, does Darwin ever admit that "specific characters"--that is, the particular characters which serve to distinguish one species from another--are ever useless, much less that "a large proportion of them" are so, as Mr. Romanes makes him "freely acknowledge." On the other hand, in the pa.s.sage which I have italicised he strongly expresses his view that much of what we suppose to be useless is due to our ignorance; and as I hold myself that, as regards many of the supposed useless characters, this is the true explanation, it may be well to give a brief sketch of the progress of knowledge in transferring characters from the one category to the other[151]."
[151] _Darwinism_, p. 132.
It is needless to continue this quotation, because of course no one is disputing that an enormous number of specific characters whose utility is unknown are nevertheless useful, and therefore due to natural selection. In other words, the question is not--Are there not many useful specific characters whose utility is unknown? but--Does it follow from the theory of natural selection that all specific characters must necessarily be useful? Well, it appears to me that without going further than the above pa.s.sage, which Mr. Wallace has quoted, we can see clearly enough what was Darwin"s opinion upon the subject. He did not believe that it followed _deductively_ from his theory that all specific characters must necessarily be useful; and therefore he regarded it as a question of _fact_--to be determined by induction as distinguished from deduction--in what proportional number of cases they are so. Moreover he gives it as his more matured opinion, that, "as far as we can at present judge" (i.e. from the present state of observation upon the subject: if, with Mr. Wallace, his judgement were _a priori_, why this qualification?), he had not previously sufficiently considered the existence of non-adaptive characters--and this he ended by believing was one of the greatest oversights as yet detected in his work. To me it has always seemed that this pa.s.sage is one of the greatest exhibitions of candour, combined with solidity of judgement, that is to be met with even in the writings of Darwin. There is no talk about any deductive "necessity"; but a perfect readiness to allow that causes other than natural selection may have been at work in evoking non-adaptive characters, so that the fifth edition of the _Origin of Species_ was altered in order to confine the theory of natural selection to "adaptive changes"--i.e. to const.i.tute it, as I have said in other words, "a theory of the origin, or c.u.mulative development, of _adaptations_."
If to this it be said that in the above pa.s.sage there is no special mention of _species_, the quibble would admit of a three-fold reply. In the first place, the quibble in question had never been raised. As already stated, it is only since the appearance of my own paper on _Physiological Selection_ that anybody ever thought of drawing a distinction between species and genera, such that while all specific characters must be held necessarily useful, no such necessity extends to generic characters. In the second place, that Darwin must have had specific characters (as well as generic) in his mind when writing the above pa.s.sage, is rendered unquestionable by the fact that many of the instances of inutility adduced by Nageli and Broca have reference to specific characters. Lastly, as shown in the pa.s.sages previously quoted from the sixth edition of the _Origin of Species_, Darwin attributed the origin of useless generic characters to useless specific characters; so that Mr. Wallace really gains nothing by his remark that specific characters are not specially mentioned in the present pa.s.sage.
Once more:--
"Darwin"s latest expression of opinion on this question is interesting, since it shows he was inclined to return to his earlier view of the general, or universal, utility of specific characters[152]."
[152] _Darwinism_, p. 142.
This "latest expression of opinion," as I shall immediately prove, shows nothing of the kind--being, in fact, a mere re-statement of the opinion everywhere and at all times expressed by Darwin, touching the caution that must be observed in deciding, _with respect to individual cases_, whether an apparently useless specific character is to be regarded as really useless. Moreover, at no time and in no place did Darwin entertain any "view of the general, or universal, utility of specific characters." But the point now is, that if (as was the case) Darwin "inclined" to depart more and more from his earlier view of the highly _general_ utility of specific characters; and if (as was not the case) he ended by showing an inclination "_to return_" to this earlier view; what becomes of the whole of Mr. Wallace"s contention against which this Appendix is directed, namely, _that Darwin never entertained any other view than that of the "general, or universal, utility of specific characters_"?
The "latest expression of opinion" which Mr. Wallace quotes, occurs in a letter written to Professor Semper in 1878. It is as follows:--
"As our knowledge advances, very slight differences, considered by systematists as of no importance in structure, are continually found to be functionally important; and I have been especially struck with this fact in the case of plants, to which my observations have of late years been confined. Therefore it seems to me rather rash to consider the slight differences between representative species, for instance those inhabiting the different islands of the same archipelago, as of no functional importance, and as not in any way due to natural selection[153]."
[153] _Life and Letters_, vol. iii. p. 161.