Volume Ii Part 4 (1/2)
[38] _Essays on Heredity_, vol. i. p. 389.
[For further treatment of the subject under discussion _see_ Weismann, _The All-sufficiency of Natural Selection_ (Contemp.
Rev. Sept. and Oct. 1893), and _The Effect of External Influences upon Development_. ”Romanes Lecture” 1894, and Spencer, _Weismannism once more_ (Cont. Rev. Oct. 1894). C. Ll.
M.]
So much for what Weismann has said touching this matter. But the matter has also been dealt with both by Darwin and by Wallace. Darwin very properly distinguishes between the fallacy that ”with animals such as the giraffe, of which the whole structure is admirably co-ordinated for certain purposes, it has been supposed that all the parts must have been simultaneously modified[39],” and the sound argument that the co-ordination itself cannot have been due to natural selection alone.
This important distinction may be rendered more clear as follows.
[39] _Variation_, &c., vol. ii. p. 206.
The facts of artificial selection prove that immense modifications of structure may be caused by a c.u.mulative blending in the same individuals of characters which were originally distributed among different individuals. Now, in the parallel case of natural selection the characters thus blended will usually--if not invariably--be of an adaptive kind; and their eventual blending together in the same individuals will be due to free intercrossing of the most fit. But this _blending of adaptations_ is quite a different matter from the _occurrence of co-ordination_. For it belongs to the essence of co-ordination that each of the co-ordinated parts should be dest.i.tute of adaptive value _per se_: the adaptation only begins to arise if all the parts in question occur a.s.sociated together in the same individuals _from the very first_. In this case it is obvious that the a.n.a.logy of artificial selection can be of no avail in explaining the facts, since the difficulty presented has nothing to do with the blending in single individuals of adaptations previously distributed among different individuals; it has to do with the simultaneous appearance in single individuals of a co-adaptation of parts, none of which could ever have been of any adaptive value had it been previously distributed among different individuals. Consequently, where Darwin comes to consider this particular case (or the case of co-adaptation as distinguished from the blending of adaptations), he freely invokes the aid of the Lamarckian principles[40].
[40] E. g. _Origin of Species_, p. 178.
Wallace, on the other hand, refuses to do this, and says that ”the best answer to the difficulty” of supposing natural selection to have been the only cause of co-adaptation may be ”found in the fact that the very thing said to be impossible by variation and natural selection, has been again and again affected by variation and artificial selection[41].”
This a.n.a.logy (which Darwin had already and very properly adduced with regard to the _blending of adaptations_) he enforces by special ill.u.s.trations; but he does not appear to perceive that it misses the whole and only point of the ”difficulty” against which it is brought.
For the case which his a.n.a.logy sustains is not that which Darwin, Spencer, Broca and others, mean by _co-adaptation_: it is the case of a blending of _adaptations_. It is not the case where adaptation is _first initiated in spite of intercrossing_, by a fortuitous concurrence of variations each in itself being without adaptive value: it is the case where adaptation is _afterwards increased by means of intercrossing_, through the blending of variations each of which has always been in itself of adaptive value.
[41] _Darwinism_, p. 418.
From this I hope it will be apparent that the only way in which the ”difficulty” from co-adaptation can be logically met by the ultra-Darwinian school, is by denying that the phenomenon of co-adaptation (as distinguished from the blending of adaptations) is ever to be really met with in organic nature. It may be argued that in all cases where co-adaptation _appears_ to occur, closer examination will show that the facts are really due to a blending of adaptations.
The characters A + B + C + D, which are now found united in the same organism, and, as thus united, all conspiring to a common end, may originally have been distributed among different organisms, where they _severally_ subserved some other ends--or possibly the same end, though in a less efficient manner. Obviously, however, in this case their subsequent combination in the same organism would not be an instance of co-adaptation, but merely of an advantageous blending together of already existing adaptations. This argument, or rejoinder, has in point of fact been adopted by Professor Meldola, he believes that all cases of seeming co-adaptation are thus due to a mere blending of adaptations[42]. Of course, if this position can be maintained, the whole difficulty from co-adaptation would lapse. But even then it would lapse on the ground of _fact_. It would not have been overturned, or in any way affected, by Wallace's _argument_ from artificial selection.
For, in that event, no such argument would be required, and, if adduced, would be irrelevant, since no one has ever alleged that there is any difficulty in understanding the mere confluence of adaptations by free-intercrossing of the best adapted.
[42] _Nature_, vol. xliii. pp. 410, 557; vol. xliv. pp. 7, 29. I say ”adopted,” because I had objected to his quoting the a.n.a.logy of artificial selection, and stated, as above, that the only way to meet Mr. Spencer's ”difficulty” was to deny the fact of co-adaptation as ever occurring in any case. It then appeared that Professor Meldola agreed with me as to this. But I do not yet understand why, if such were his view, he began by endorsing Mr. Wallace's a.n.a.logy from artificial selection--i.
e. confusing the case of co-adaptation with that of the blending of adaptations. If any one denies the fact of co-adaptation, he cannot a.s.sist his denial by arguing the totally different fact that adaptations may be blended by free intercrossing; for this latter fact has never been questioned, and has nothing to do with the one which he engaged in disputing.
Now, if we are agreed that the only question in debate is the question of fact whether or not co-adaptation ever occurs in nature, it appears to me that the best field for debating the question is furnished by the phenomena of reflex action. I can well perceive that the instances adduced by Broca and Spencer in support of their common argument--such as the giraffe, the elk, &c.--are equivocal. But I think that many instances which may be adduced of reflex action are much more to the point. _For it belongs to the very nature of reflex action that it cannot work unless all parts of the machinery concerned are already present, and already co-ordinated, in the same organism._ It would be useless, in so far as such action is concerned if the afferent and efferent nerves, the nerve-centre, and the muscles organically grouped together, were not all present from the very first in the same individuals, and from the very first were not co-ordinated as a definite piece of organic machinery.
With respect to reflex actions, therefore, it is desirable to begin by pointing out how widely the adaptations which they involve differ from those where no manufacture, so to speak, of special machinery is required. Thus, it is easy to understand how natural selection alone is capable of gradually acc.u.mulating congenital variations in the direction of protective colouring; of mimicry; of general size, form, mutual correlation of parts as connected with superior strength, fleetness, agility, &c.; of greater or less development of particular parts, such as legs, wings, tails, &c. For in all such cases the adaptation which is in process of acc.u.mulation is from its very commencement and throughout each of its subsequent stages, of _use_ in the struggle for existence.
And inasmuch as all the individuals of each successive generation vary round the specific mean which characterized the preceding generation, there will always be a sufficient number of individuals which present congenital variations of the kind required for natural selection to seize upon, without danger of their being swamped by free intercrossing--as Mr. Wallace has very ably shown in his _Darwinism_.
But this law of averages can apply only to cases where single structures--or a single group of correlated structures--are already present, and already varying round a specific mean. The case is quite different where a _co-ordination_ of structures is required for the performance of a _previously non-existent_ reflex action. For some, at least, of these structures must be _new_, as must also be the function which all of them first conspire to perform. Therefore, neither the new elements of structure, nor the new combination of structures, can have been previously given as varying round a specific mean. On the contrary, a very definite piece of machinery, consisting of many co-ordinated parts, must somehow or other be originated in a high degree of working efficiency, before it can be capable of answering its purpose in the prompt performance of a particular action under particular circ.u.mstances of stimulation. Lastly, such pieces of machinery are always of a highly delicate character, and usually involve so immensely complex a co-ordination of mutually dependent parts, that it is only a physiologist who can fully appreciate the magnitude of the distinction between ”adaptations” of this kind, and ”adaptations” of the kind which arise through natural selection seizing upon congenital variations as these oscillate round a specific mean.
Or the whole argument may be presented in another form, under three different headings, thus:--
In the first place, it will be evident from what has just been said, that such a piece of machinery as is concerned in even the simplest reflex action cannot have occurred in any considerable number of individuals of a species, _when it first began to be constructed_. On the contrary, if its _origin_ were dependent on congenital variations alone, the needful co-adaptation of parts which it requires can scarcely have happened to occur in more than a very small percentage of cases--even if it be held conceivable that by such means alone it should ever have occurred at all. Hence, instead of preservation and subsequent improvement having taken place _in consequence of_ free intercrossing among all individuals of the species (as in the cases of protective colouring, &c., where adaptation has no reference to any mechanical co-adaptation of parts), they must have taken place _in spite of_ such intercrossing.
In the second place, adaptations due to organic machineries of this kind differ in another all-important respect from those due to a summation of adaptive characters which are already present and already varying round a specific mean. The latter depend for their summation upon the fact--not merely, as just stated, that they are already present, already varying round a specific mean, and therefore owe their progressive evolution to free intercrossing, but also--_that they admit of very different degrees of adaptation_. It is only because the degree of adaptation in generation B is superior to that in generation A that _gradual improvement_ in respect of adaptation is here possible. In the case of protective resemblance, for example, a very imperfect and merely accidental resemblance to a leaf, to another insect, &c., may at the first start have conferred a sufficient degree of adaptive imitation to count for something in the struggle for life; and, if so, the basis would be given for a progressive building up by natural selection of structures and colours in ever-advancing degrees of adaptive resemblance. There is here no necessity to suppose--nor in point of fact is it ever supposed, since the supposition would involve nothing short of a miracle--that such extreme perfection in this respect as we now so frequently admire has originated suddenly in a single generation, as a collective variation of a congenital kind affecting simultaneously a large proportional number of individuals. But in the case of a reflex mechanism--which may involve even greater marvels of adaptive adjustment, and _all_ the parts of which must occur in the same _individuals_ to be of any use--it _is_ necessary to suppose some such sudden and collective origin in some very high degree of efficiency, if natural selection has been the only principle concerned in afterwards perfecting the mechanism. For it is self-evident that a reflex action, from its very nature, cannot admit of any great differences in its degrees of adaptation: if it is to work at all, so as to count for anything in the struggle for life, it must already be given in a state of working efficiency. So that, unless we invoke either the doctrine of ”prophetic types” or the theory of sudden creations, I confess I do not see how we are to explain either the origin, or the development, of a reflex mechanism by means of natural selection alone.
Lastly, in the third place, _even when reflex mechanisms have been fully formed_, it is often beyond the power of sober credence to believe that they now are, or ever can have been, of selective value in the struggle for existence, as I will show further on. And such cases go to fortify the preceding argument. For if not conceivably of selective value even when completely evolved, much less can they conceivably have been so through all the stages of their complex evolution back to their very origin. Therefore, supposing for the present that there are such cases of reflex action in nature, neither their origin nor their development can conceivably have been due to natural selection alone. The Lamarckian factors, however, have no reference to degrees of adaptation, any more than they have to degrees of complexity. No question of value, as selective or otherwise, can obtain in their case: neither in their case does any difficulty obtain as regards the co-adaptation of severally useless parts.
Now, if all these distinctions between the Darwinian and Lamarckian principles are valid--and I cannot see any possibility of doubt upon this point--strong evidence in favour of the latter would be furnished by cases (if any occur) where structures, actions, instincts, &c., although of some adaptive value, are nevertheless plainly not of selective value. According to the ultra-Darwinian theory, no such cases ought ever to occur: according to the theory of Darwin himself, they ought frequently to occur. Therefore a good test, or criterion, as between these different theories of organic evolution is furnished by putting the simple question of fact--Can we, or can we not, show that there are cases of adaptation where the degree of adaptation is so small as to be incompatible with the supposition of its presenting a selective value? And if we put the wider question--Are there any cases where the co-adaptation of severally useless parts has been brought about, when even the resulting whole does not present a selective value?--then, of course, we impose a still more rigid test.
Well, notwithstanding the difficulty of proving such a negative as the absence of natural selection where adaptive development is concerned, I believe that there are cases which conform to both these tests simultaneously; and, moreover, that they are to be found in most abundance where the theory of use-inheritance would most expect them to occur--namely, in the province of reflex action. For the very essence of this theory is the doctrine, that constantly a.s.sociated use of the same parts for the performance of the same action will progressively organize those parts into a reflex mechanism--no matter how high a degree of co-adaptation may thus be reached on the one hand, or how low a degree of utilitarian value on the other.
Having now stated the general or abstract principles which I regard as const.i.tuting a defence of the Lamarckian factors, so far as this admits of being raised on grounds of physiology, we will now consider a few concrete cases by way of ill.u.s.tration. It is needless to multiply such cases for the mere purpose of ill.u.s.tration. For, on reading those here given, every physiologist will at once perceive that they might be added to indefinitely. The point to observe is, the relation in which these samples of reflex action stand to the general principles in question; for there is nothing unusual in the samples themselves. On the contrary, they are chosen because they are fairly typical of the phenomena of reflex action in general.