"_It appears probable that disuse has been the main agent in rendering organs rudimentary_ [italics mine]. It would at first lead by slow steps to the more and more complete reduction of a part, until at last it has become rudimentary--as in the case of the eyes of animals inhabiting dark caverns, and of the wings of birds inhabiting oceanic islands, which have seldom been forced by beasts of prey to take flight, and have ultimately lost the power of flying. Again, an organ, useful under certain conditions, might become injurious under others, as _with the wings of beetles living on small and exposed islands_; and in this case natural selection will have aided in reducing the organ, until it was rendered harmless and rudimentary [italics mine]." {266}
So that just as an undefined amount of use and disuse was introduced on the earlier page to supplement the effects of natural selection in respect of the wings of beetles on small and exposed islands, we have here an undefined amount of natural selection introduced to supplement the effects of use and disuse in respect of the identical phenomena. In the one pa.s.sage we find that natural selection has been the main agent in reducing the wings, though use and disuse have had an appreciable share in the result; in the other, it is use and disuse that have been the main agents, though an appreciable share in the result must be ascribed to natural selection.
Besides, who has seen the uncles and aunts going away with the uniformity that is necessary for Mr. Darwin"s contention? We know that birds and insects do often get blown out to sea and perish, but in order to establish Mr. Darwin"s position we want the evidence of those who watched the reduction of the wings during the many generations in the course of which it was being effected, and who can testify that all, or the overwhelming majority, of the beetles born with fairly well-developed wings got blown out to sea, while those alone survived whose wings were congenitally degenerate. Who saw them go, or can point to a.n.a.logous cases so conclusive as to compel a.s.sent from any equitable thinker?
Darwinians of the stamp of Mr. Thiselton Dyer, Professor Ray Lankester, or Mr. Romanes, insist on their pound of flesh in the matter of irrefragable demonstration. They complain of us for not bringing forward someone who has been able to detect the movement of the hour-hand of a watch during a second of time, and when we fail to do so, declare triumphantly that we have no evidence that there is any connection between the beating of a second and the movement of the hour-hand. When we say that rain comes from the condensation of moisture in the atmosphere, they demand of us a rain-drop from moisture not yet condensed. If they stickle for proof and cavil on the ninth part of a hair, as they do when we bring forward what we deem excellent instances of the transmission of an acquired characteristic, why may not we, too, demand at any rate some evidence that the unmodified beetles actually did always, or nearly always, get blown out to sea, during the reduction above referred to, and that it is to this fact, and not to the masterly inactivity of their fathers and mothers, that the Madeira beetles owe their winglessness? If we begin stickling for proof in this way, our opponents would not be long in letting us know that absolute proof is unattainable on any subject, that reasonable presumption is our highest certainty, and that crying out for too much evidence is as bad as accepting too little. Truth is like a photographic sensitized plate, which is equally ruined by over and by under exposure, and the just exposure for which can never be absolutely determined.
Surely if disuse can be credited with the vast powers involved in Mr. Darwin"s statement that it has probably "been the main agent in rendering organs rudimentary," no limits are a.s.signable to the acc.u.mulated effects of habit, provided the effects of habit, or use and disuse, are supposed, as Mr. Darwin supposed them, to be inheritable at all. Darwinians have at length woke up to the dilemma in which they are placed by the manner in which Mr. Darwin tried to sit on the two stools of use and disuse, and natural selection of accidental variations, at the same time. The knell of Charles-Darwinism is rung in Mr. Wallace"s present book, and in the general perception on the part of biologists that we must either a.s.sign to use and disuse such a predominant share in modification as to make it the feature most proper to be insisted on, or deny that the modifications, whether of mind or body, acquired during a single lifetime, are ever transmitted at all. If they can be inherited at all, they can be acc.u.mulated. If they can be acc.u.mulated at all, they can be so, for anything that appears to the contrary, to the extent of the specific and generic differences with which we are surrounded. The only thing to do is to pluck them out root and branch: they are as a cancer which, if the smallest fibre be left unexcised, will grow again, and kill any system on to which it is allowed to fasten. Mr. Wallace, therefore, may well be excused if he casts longing eyes towards Weismannism.
And what was Mr. Darwin"s system? Who can make head or tail of the inextricable muddle in which he left it? The Origin of Species in its latest shape is the reduction of hedging to an absurdity. How did Mr. Darwin himself leave it in the last chapter of the last edition of the Origin of Species? He wrote:--
"I have now recapitulated the facts and considerations which have thoroughly convinced me that species have been modified during a long course of descent. This has been effected chiefly through the natural selection of numerous, successive, slight, favourable variations; aided in an important manner by the inherited effects of the use and disuse of parts, and in an unimportant manner--that is, in relation to adaptive structures whether past or present--by the direct action of external conditions, and by variations which seem to us in our ignorance to arise spontaneously. It appears that I formerly underrated the frequency and value of these latter forms of variation, as leading to permanent modifications of structure independently of natural selection."
The "numerous, successive, slight, favourable variations" above referred to are intended to be fortuitous, accidental, spontaneous.
It is the essence of Mr. Darwin"s theory that this should be so.
Mr. Darwin"s solemn statement, therefore, of his theory, after he had done his best or his worst with it, is, when stripped of surplusage, as follows:--
"The modification of species has been mainly effected by acc.u.mulation of spontaneous variations; it has been aided in an important manner by acc.u.mulation of variations due to use and disuse, and in an unimportant manner by spontaneous variations; I do not even now think that spontaneous variations have been very important, but I used once to think them less important than I do now."
It is a discouraging symptom of the age that such a system should have been so long belauded, and it is a sign of returning intelligence that even he who has been more especially the alter ego of Mr. Darwin should have felt constrained to close the chapter of Charles-Darwinism as a living theory, and relegate it to the important but not very creditable place in history which it must henceforth occupy. It is astonishing, however, that Mr. Wallace should have quoted the extract from the Origin of Species just given, as he has done on p. 412 of his Darwinism, without betraying any sign that he has caught its driftlessness--for drift, other than a desire to hedge, it a.s.suredly has not got. The battle now turns on the question whether modifications of either structure or instinct due to use or disuse are ever inherited, or whether they are not. Can the effects of habit be transmitted to progeny at all?
We know that more usually they are not transmitted to any perceptible extent, but we believe also that occasionally, and indeed not infrequently, they are inherited and even intensified.
What are our grounds for this opinion? It will be my object to put these forward in the following number of the Universal Review.
The Deadlock in Darwinism: Part II {271}
At the close of my article in last month"s number of the Universal Review, I said I would in this month"s issue show why the opponents of Charles-Darwinism believe the effects of habits acquired during the lifetime of a parent to produce an effect on their subsequent offspring, in spite of the fact that we can rarely find the effect in any one generation, or even in several, sufficiently marked to arrest our attention.
I will now show that offspring can be, and not very infrequently is, affected by occurrences that have produced a deep impression on the parent organism--the effect produced on the offspring being such as leaves no doubt that it is to be connected with the impression produced on the parent. Having thus established the general proposition, I will proceed to the more particular one--that habits, involving use and disuse of special organs, with the modifications of structure thereby engendered, produce also an effect upon offspring, which, though seldom perceptible as regards structure in a single, or even in several generations, is nevertheless capable of being acc.u.mulated in successive generations till it amounts to specific and generic difference. I have found the first point as much as I can treat within the limits of this present article, and will avail myself of the hospitality of the Universal Review next month to deal with the second.
The proposition which I have to defend is one which no one till recently would have questioned, and even now those who look most askance at it do not venture to dispute it unreservedly; they every now and then admit it as conceivable, and even in some cases probable; nevertheless they seek to minimize it, and to make out that there is little or no connection between the great ma.s.s of the cells of which the body is composed, and those cells that are alone capable of reproducing the entire organism. The tendency is to a.s.sign to these last a life of their own, apart from, and unconnected with that of the other cells of the body, and to cheapen all evidence that tends to prove any response on their part to the past history of the individual, and hence ultimately of the race.
Professor Weismann is the foremost exponent of those who take this line. He has naturally been welcomed by English Charles-Darwinians; for if his view can be sustained, then it can be contended that use and disuse produce no transmissible effect, and the ground is cut from under Lamarck"s feet; if, on the other hand, his view is unfounded, the Lamarckian reaction, already strong, will gain still further strength. The issue, therefore, is important, and is being fiercely contested by those who have invested their all of reputation for discernment in Charles-Darwinian securities.
Professor Weismann"s theory is, that at every new birth a part of the substance which proceeds from parents and which goes to form the new embryo is not used up in forming the new animal, but remains apart to generate the germ-cells--or perhaps I should say "germ- plasm"--which the new animal itself will in due course issue.
Contrasting the generally received view with his own, Professor Weismann says that according to the first of these "the organism produces germ-cells afresh again and again, and that it produces them entirely from its own substance." While by the second "the germ-cells are no longer looked upon as the product of the parent"s body, at least as far as their essential part--the specific germ- plasm--is concerned; they are rather considered as something which is to be placed in contrast with the tout ensemble of the cells which make up the parent"s body, and the germ-cells of succeeding generations stand in a similar relation to one another as a series of generations of unicellular organisms arising by a continued process of cell-division." {274a} On another page he writes:--
"I believe that heredity depends upon the fact that a small portion of the effective substance of the germ, the germ-plasm, remains unchanged during the development of the ovum into an organism, and that this part of the germ-plasm serves as a foundation from which the germ-cells of the new organism are produced. There is, therefore, continuity of the germ-plasm from one generation to another. One might represent the germ-plasm by the metaphor of a long creeping root-stock from which plants arise at intervals, these latter representing the individuals of successive generations."
{274b}
Mr. Wallace, who does not appear to have read Professor Weismann"s essays themselves, but whose remarks are, no doubt, ultimately derived from the sequel to the pa.s.sage just quoted from page 266 of Professor Weismann"s book, contends that the impossibility of the transmission of acquired characters follows as a logical result from Professor Weismann"s theory, inasmuch as the molecular structure of the germ-plasm that will go to form any succeeding generation is already predetermined within the still unformed embryo of its predecessor; "and Weismann," continues Mr. Wallace, "holds that there are no facts which really prove that acquired characters can be inherited, although their inheritance has, by most writers, been considered so probable as hardly to stand in need of direct proof."
{275}
Professor Weismann, in pa.s.sages too numerous to quote, shows that he recognizes this necessity, and acknowledges that the non- transmission of acquired characters "forms the foundation of the views" set forth in his book, p. 291.
Professor Ray Lankester does not commit himself absolutely to this view, but lends it support by saying (Nature, December 12, 1889): "It is hardly necessary to say that it has never yet been shown experimentally that _anything_ acquired by one generation is transmitted to the next (putting aside diseases)."
Mr. Romanes, writing in Nature, March 13, 1890, and opposing certain details of Professor Weismann"s theory, so far supports it as to say that "there is the gravest possible doubt lying against the supposition that any really inherited decrease is due to the inherited effects of disuse." The "gravest possible doubt" should mean that Mr. Romanes regards it as a moral certainty that disuse has no transmitted effect in reducing an organ, and it should follow that he holds use to have no transmitted effect in its development.
The sequel, however, makes me uncertain how far Mr. Romanes intends this, and I would refer the reader to the article which Mr. Romanes has just published on Weismann in the Contemporary Review for this current month.
The burden of Mr. Thiselton Dyer"s controversy with the Duke of Argyll (see Nature, January 16, 1890, et seq.) was that there was no evidence in support of the transmission of any acquired modification. The orthodoxy of science, therefore, must be held as giving at any rate a provisional support to Professor Weismann, but all of them, including even Professor Weismann himself, shrink from committing themselves to the opinion that the germ-cells of any organisms remain in all cases unaffected by the events that occur to the other cells of the same organism, and until they do this they have knocked the bottom out of their case.
From among the pa.s.sages in which Professor Weismann himself shows a desire to hedge I may take the following from page 170 of his book:--
"I am also far from a.s.serting that the germ-plasm which, as I hold, is transmitted as the basis of heredity from one generation to another, is absolutely unchangeable or totally uninfluenced by forces residing in the organism within which it is transformed into germ-cells. I am also compelled to admit it as conceivable that organisms may exert a modifying influence upon their germ-cells, and even that such a process is to a certain extent inevitable. The nutrition and growth of the individual must exercise some influence upon its germ-cells . . . "
Professor Weismann does indeed go on to say that this influence must be extremely slight, but we do not care how slight the changes produced may be, provided they exist and can be transmitted. On an earlier page (p. 101) he said in regard to variations generally that we should not expect to find them conspicuous; their frequency would be enough, if they could be acc.u.mulated. The same applies here, if stirring events that occur to the somatic cells can produce any effect at all on offspring. A very small effect, provided it can be repeated and acc.u.mulated in successive generations, is all that even the most exacting Lamarckian will ask for.
Having now made the reader acquainted with the position taken by the leading Charles-Darwinian authorities, I will return to Professor Weismann himself, who declares that the transmission of acquired characters "at first sight certainly seems necessary," and that "it appears rash to attempt to dispense with its aid." He continues:--
"Many phenomena only appear to be intelligible if we a.s.sume the hereditary transmission of such acquired characters as the changes which we ascribe to the use or disuse of particular organs, or to the direct influence of climate. Furthermore, how can we explain instinct as hereditary habit, unless it has gradually arisen by the acc.u.mulation, through heredity, of habits which were practised in succeeding generations?" {277}
I may say in pa.s.sing that Professor Weismann appears to suppose that the view of instinct just given is part of the Charles-Darwinian system, for on page 389 of his book he says "that many observers had followed Darwin in explaining them [instincts] as inherited habits."
This was not Mr. Darwin"s own view of the matter. He wrote:--
"If we suppose any habitual action to become inherited--and I think it can be shown that this does sometimes happen--then the resemblance between what originally was a habit and an instinct becomes so close as not to be distinguished. . . . But it would be the most serious error to suppose that the greater number of instincts have been acquired by habit in one generation, and then transmitted by inheritance to succeeding generations. It can be clearly shown that the most wonderful instincts with which we are acquainted, namely, those of the hive-bee and of many ants, could not possibly have been thus acquired."--[Origin of Species, ed.
1859, p. 209.]
Again we read: "Domestic instincts are sometimes spoken of as actions which have become inherited solely from long-continued and compulsory habit, but this, I think, is not true."--Ibid., p. 214.
Again: "I am surprised that no one has advanced this demonstrative case of neuter insects, against the well-known doctrine of inherited habit, as advanced by Lamarck."--[Origin of Species, ed. 1872, p.
233.]
I am not aware that Lamarck advanced the doctrine that instinct is inherited habit, but he may have done so in some work that I have not seen.
It is true, as I have more than once pointed out, that in the later editions of the Origin of Species it is no longer "the _most_ serious" error to refer instincts generally to inherited habit, but it still remains "a serious error," and this slight relaxation of severity does not warrant Professor Weismann in ascribing to Mr.
Darwin an opinion which he emphatically condemned. His tone, however, is so off-hand, that those who have little acquaintance with the literature of evolution would hardly guess that he is not much better informed on this subject than themselves.
Returning to the inheritance of acquired characters, Professor Weismann says that this has never been proved either by means of direct observation or by experiment. "It must be admitted," he writes, "that there are in existence numerous descriptions of cases which tend to prove that such mutilations as the loss of fingers, the scars of wounds, etc., are inherited by the offspring, but in these descriptions the previous history is invariably obscure, and hence the evidence loses all scientific value."
The experiments of M. Brown-Sequard throw so much light upon the question at issue that I will quote at some length from the summary given by Mr. Darwin in his Variation of Animals and Plants under Domestication. {279} Mr. Darwin writes:--
"With respect to the inheritance of structures mutilated by injuries or altered by disease, it was until lately difficult to come to any definite conclusion." [Then follow several cases in which mutilations practised for many generations are not found to be transmitted.] "Notwithstanding," continues Mr. Darwin, "the above several negative cases, we now possess conclusive evidence that the effects of operations are sometimes inherited. Dr. Brown-Sequard gives the following summary of his observations on guinea-pigs, and this summary is so important that I will quote the whole:--
""1st. Appearance of epilepsy in animals born of parents having been rendered epileptic by an injury to the spinal cord.
""2nd. Appearance of epilepsy also in animals born of parents having been rendered epileptic by the section of the sciatic nerve.
""3rd. A change in the shape of the ear in animals born of parents in which such a change was the effect of a division of the cervical sympathetic nerve.
""4th. Partial closure of the eyelids in animals born of parents in which that state of the eyelids had been caused either by the section of the cervical sympathetic nerve or the removal of the superior cervical ganglion.
""5th. Exophthalmia in animals born of parents in which an injury to the restiform body had produced that protrusion of the eyeball.