Darwin, and After Darwin Volume Ii Part 4

[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.

In our own organization there is a reflex mechanism which ensures the prompt withdrawal of the legs from any source of irritation supplied to the feet. For instance, even after a man has broken his spine in such a manner as totally to interrupt the functional continuity of his spinal cord and brain, the reflex mechanism in question will continue to retract his legs when his feet are stimulated by a touch, a burn, &c.

This responsive action is clearly an adaptive action, and, as the man neither feels the stimulation nor the resulting movement, it is as clearly a reflex action. The question now is as to the mode of its origin and development.

I will not here dwell upon the argument from co-adaptation, because this may be done more effectually in the case of more complicated reflex actions, but will ask whether we can reasonably hold that this particular reflex action--comparatively simple though it is--has ever been of selective value to the human species, or to the ancestors thereof? Even in its present fully-formed condition it is fairly questionable whether it is of any adaptive _value_ at all. The movement performed is no doubt an adaptive _movement_; but is there any occasion upon which the reflex mechanism concerned therein can ever have been of adaptive _use_? Until a man's legs have been paralyzed as to their voluntary motion, he will always promptly withdraw his feet from any injurious source of irritation by means of his conscious intelligence.

True, the reflex mechanism secures an almost inappreciable saving in the time of response to a stimulus, as compared with the time required for response by an act of will; but the difference is so exceedingly small, that we can hardly suppose the saving of it in this particular case to be a matter of any adaptive--much less selective--importance. Nor is it more easy to suppose that the reflex mechanism has been developed by natural selection for the purpose of replacing voluntary action when the latter has been destroyed or suspended by grave spinal injury, paralysis, coma, or even ordinary sleep. In short, even if for the sake of argument we allow it to be conceivable that any single human being, ape, or still more distant ancestor, has ever owed its life to the possession of this mechanism, we may still be certain that not one in a million can have done so. And, if this is the case with regard to the mechanism as now fully constructed, still more must it have been the case with regard to all the previous stages of construction. For here, without elaborating the point, it would appear that a process of construction by survival of the fittest alone is incomprehensible.

On the other hand, of course, the theory of use-inheritance furnishes a fully intelligible--whether or not a true--explanation. For those nerve-centres in the spinal cord which co-ordinate the muscles required for retracting the feet are the centres used by the will for this purpose. And, by hypothesis, the frequent use of them for this purpose under circ.u.mstances of stimulation which render the muscular response appropriate, will eventually establish an organic connexion between such response and the kind of stimulation to which it is appropriate--even though there be no utilitarian reason for its establishment[43]. To invert a phrase of Aristotle, we do not frequently use this mechanism because we have it (seeing that in our normal condition there is no necessity for such use); but, by hypothesis, we have it because we have frequently used its several elements in appropriate combination.

[43] It may be said, with regard to this particular reflex, that it may perhaps be, so to speak, a mechanical accident, arising from the contiguity of the sensory and motor roots in the cord.

But as this suggestion cannot apply to other reflexes presently to be adduced, it need not be considered.

I will adduce but one further example in ill.u.s.tration of these general principles--pa.s.sing at once from the foregoing case of comparative simplicity to one of extreme complexity.

There is a well-known experiment on a brainless frog, which reveals a beautiful reflex mechanism in the animal, whereby the whole body is enabled continually to readjust its balance on a book (or any other plane surface), as this is slowly rotated on a horizontal axis. So long as the book is lying flat, the frog remains motionless; but as soon as the book is tilted a little, so that the frog is in danger of slipping off, all the four feet begin to crawl up the hill; and the steeper the hill becomes, the faster they crawl. When the book is vertical, the frog has reached the now horizontal back, and so on. Such being the facts, the question is--How can the complicated piece of machinery thus implied have been developed by natural selection? Obviously it cannot have been so by any of the parts concerned having been originally distributed among different individuals, and afterwards united in single individuals by survival (i.e. free intercrossing) of the fittest. In other words, the case is obviously one of co-adaptation, and not one of the blending of adaptations. Again, and no less obviously, it is impossible that the co-adaptation can have been _gradually developed_ by natural selection, because, in order to have been so, it must by hypothesis have been of some degree of use in every one of its stages; yet it plainly cannot have been until it had been fully perfected in all its astonis.h.i.+ng complexity[44].

[44] Of course it will be observed that the question is not with regard to the development of all the nerves and muscles concerned in this particular process. It is as to the development of the co-ordinating centres, which thus so delicately respond to the special stimuli furnished by variations of angle to the horizon. And it is as inconceivable in this case of reflex action, as it is in almost every other case of reflex action, that the highly specialized machinery required for performing the adaptive function can ever have had its origin in the performance of any other function. Indeed, a noticeable peculiarity of reflex mechanisms as a cla.s.s is the highly specialized character of the functions which their highly organized structures subserve.

Lastly, not only does it thus appear impossible that during all stages of its development--or while as yet incapable of performing its intricate function--this nascent mechanism can have had any adaptive value; but even as now fully developed, who will venture to maintain that it presents any selective value? As long as the animal preserves its brain, it will likewise preserve its balance, by the exercise of its intelligent volition. And, if the brain were in some way destroyed, the animal would be unable to breed, or even to feed; so that natural selection can never have had any _opportunity_, so to speak, of developing this reflex mechanism in brainless frogs. On the other hand, as we have just seen, we cannot perceive how there can ever have been any _raison d'etre_ for its development in normal frogs--even if its development were conceivably possible by means of this agency. But if practice makes perfect in the race, as it does in the individual, we can immediately perceive that the constant habit of correctly adjusting its balance may have gradually developed, in the batrachian organization, this non-necessary reflex[45].

[45] We meet with a closely a.n.a.logous reflex mechanism in brainless vertebrata of other kinds; but these do not furnish such good test cases, because the possibility of natural selection cannot be so efficiently attenuated. The perching of brainless birds, for instance, at once refers us to the roosting of sleeping birds, where the reflex mechanism concerned is clearly of high adaptive value. Therefore such a case is not available as a test, although the probability is that birds have inherited their balancing mechanisms from their sauropsidian ancestors, where it would have been of no such adaptive importance.

And, of course, this example--like that of withdrawing the feet from a source of stimulation, which a frog will do as well as a man--does not stand alone. Without going further a-field than this same animal, any one who reads, from our present point of view, Goltz's work on the reflex actions of the frog, will find that the great majority of them--complex and refined though most of them are--cannot conceivably have ever been of any use to any frog that was in undisturbed possession of its brain.

Hence, not to occupy s.p.a.ce with a reiteration of facts all more or less of the same general kind, and therefore all presenting identical difficulties to ultra-Darwinian theory, I shall proceed to give two others which appear to me of particular interest in the present connexion, because they furnish ill.u.s.trations of reflex actions in a state of only partial development, and are therefore at the present moment demonstrably useless to the animal which displays them.

Many of our domesticated dogs, when we gently scratch their sides and certain other parts of the body, will themselves perform scratching movements with the hind leg of the same side as that upon which the irritation is being supplied. According to Goltz[46], this action is a true reflex; for he found that it is performed equally well in a dog which has been deprived of its cerebral hemispheres, and therefore of its normal volition. Again, according to Haycraft[47], this reflex is congenital, or not acquired during the life-time of each individual dog.

Now, although the action of scratching is doubtless adaptive, it appears to me incredible that it could ever have become organized into a congenital reflex by natural selection. For, in order that it should, the scratching away fleas would require to have been a function of selective value. Yet, even if the irritation caused by fleas were supposed to be so far fatal in the struggle for existence, it is certain that they would always be scratched away by the conscious intelligence of each individual dog; and, therefore, that no advantage could be gained by organizing the action into a reflex. On the other hand, if acquired characters are ever in any degree transmitted, it is easy to understand how so frequently repeated an action should have become, in numberless generations of dogs, congenitally automatic.

[46] _Pfluger's Archiv_, Bd. xx. s. 23 (1879).

[47] _Brain_, part xlviii, pp. 516-19 (1889).--There is still better proof of this in the case of certain rodents. For instance, observing that rats and mice are under the necessity of very frequently scratching themselves with their hind-feet, I tried the experiment of removing the latter from newly-born individuals--i.e. before the animals were able to co-ordinate their movements, and therefore before they had ever even attempted to scratch themselves. Notwithstanding that they were thus dest.i.tute of individual experience with regard to the benefit of scratching, they began their scratching movements with their stumps as soon as they were capable of executing co-ordinated movements, and afterwards continued to do so till the end of their lives with as much vigour and frequency as unmutilated animals. Although the stumps could not reach the seats of irritation which were bent towards them, they used to move rapidly in the air for a time sufficient to have given the itching part a good scratch, had the feet been present--after which the animals would resume their sundry other avocations with apparent satisfaction. These facts showed the hereditary response to irritation by parasites to be so strong, that even a whole life-time's experience of its futility made no difference in the frequency or the vigour thereof.

So much for the general principle of selective value as applied to this particular case. And similarly, of course, we might here repeat the application of all the other general principles, which have just been applied in the two preceding cases. But it is only one of these other general principles which I desire in the present case specially to consider, for the purpose of considering more closely than hitherto the difficulty which this principle presents to ultra-Darwinian theory.

The difficulty to which I allude is that of understanding how all the stages in the _development_ of a reflex action can have been due to natural selection, seeing that, before the reflex mechanism has been sufficiently elaborated to perform its function, it cannot have presented any degree of utility. Now the particular force of the present example, the action of scratching--as also of the one to follow--consists in the fact that it is a case where a reflex action is not yet completely organized. It appears to be only in course of construction, so that it is neither invariably present, nor, when it is present, is it ever fully adapted to the performance of its function.

That it is not invariably present (when the brain is so) may be proved by trying the simple experiment on a number of puppies--and also of full-grown dogs. Again, that even when it is present it is far from being fully adapted to the performance of its function, may be proved by observing that only in rare instances does the scratching leg succeed in scratching the place which is being irritated. The movements are made more or less at random, and as often as not the foot fails to touch the body at any place at all. Hence, although we have a "prophecy" of a reflex action well designed for the discharge of a particular function, at present the machinery is not sufficiently perfected for the adequate discharge of that function. In this important respect it differs from the otherwise closely a.n.a.logous reflex action of the frog, whereby the foot of the hind leg is enabled to localize with precision a seat of irritation on the side of the body. But this beautiful mechanism in the frog cannot have sprung into existence ready formed at any historical moment in the past history of the phyla. It must have been the subject of a more or less prolonged evolution, in some stage of which it must presumably have resembled the now nascent scratching reflex of the dog, in making merely abortive attempts at localizing the seat of irritation--supposing, of course, that some physiologist had been there to try the experiment by first removing the brain. Now, even if one could imagine it to be, either in the frog or in the dog, a matter of selective importance that so exceedingly refined a mechanism should have been developed for the sole purpose of inhibiting the bites of parasites--which in every normal animal would certainly be discharged by an _intentional_ performance of the movements in question,--even if, in order to save an hypothesis at all costs, we make so violent a supposition as this, still we should do so in vain. For it would still remain undeniably certain that the reflex mechanism is _not_ of any selective value. Even now the mechanism in the dog is not sufficiently precise to subserve the only function which occasionally and abortively it attempts to perform. Thus it has all the appearance of being but an imitating shadow of certain neuro-muscular adjustments, which have been habitually performed in the canine phyla by a volitional response to cutaneous irritation. Were it necessary, this argument might be strengthened by observing that the reflex action is positively _improved_ by removal of the brain.

The second example of a nascent reflex in dogs which I have to mention is as follows.

Goltz found that his brainless dogs, when wetted with water, would shake themselves as dry as possible, in just the same way as normal dogs will do under similar circ.u.mstances. This, of course, proves that the shaking movements may be performed by a reflex mechanism, which can have no other function to perform in the organization of a dog, and which, besides being of a highly elaborate character, will respond only to a very special kind of stimulation. Now, here also I find that the mechanism is congenital, or not acquired by individual experience. For the puppies on which I experimented were kept indoors from the time of their birth--so as never to have had any experience of being wetted by rain, &c.--till they were old enough to run about with a full power of co-ordinating their general movements. If these young animals were suddenly plunged into water, the shock proved too great: they would merely lie and s.h.i.+ver. But if their feet alone were wetted, by being dipped in a basin of water, the puppies would soon afterwards shake their heads in the peculiar manner which is required for shaking water off the ears, and which in adult dogs const.i.tutes the first phase of a general shaking of the whole body.

Here, then, we seem to have good evidence of all the same facts which were presented in the case of the scratching reflex. In the first place, co-adaptation is present in a very high degree, because this shaking reflex in the dog, unlike the skin-twitching reflex in the horse, does not involve only a single muscle, or even a single group of muscles; it involves more or less the co-ordinated activity of many voluntary muscles all over the body. Such, at any rate, is the case when the action is performed by the intelligent volition of an adult dog; and if a brainless dog, or a young puppy, does not perform it so extensively or so vigorously, this only goes to prove that the reflex has not yet been sufficiently developed to serve as a subst.i.tute for intelligent volition--i.e. that it is _useless_, or a mere organic shadow of the really adaptive substance. Again, even if this nascent reflex had been so far developed as to have been capable of superseding voluntary action, still we may fairly doubt whether it could have proved of selective value. For it is questionable whether the immediate riddance of water after a wetting is a matter of life and death to dogs in a state of nature. Moreover, even if it were, every individual dog would always have got rid of the irritation, and so of the danger, by means of a _voluntary_ shake--with the double result that natural selection has never had any opportunity of gradually building up a special reflex mechanism for the purpose of securing a shake, and that the canine race have not had to wait for any such unnecessary process. Lastly, such a process, besides being unnecessary, must surely have been, under any circ.u.mstances, impossible. For even if we were to suppose--again for the sake of saving an hypothesis at any cost--that the presence of a fully-formed shaking reflex is of selective value in the struggle for existence, it is perfectly certain that all the stages through which the construction of so elaborate a mechanism must have pa.s.sed could not have been, under any circ.u.mstances, of any such value.

But, it is needless to repeat, according to the hypothesis of use-inheritance, there is no necessity to suppose that these incipient reflex mechanisms _are_ of any value. If function produces structure in the race as it does in the individual, the voluntary and frequently repeated actions of scratching and shaking may very well have led to an organic integration of the neuro-muscular mechanisms concerned. Their various parts having been always co-ordinated for the performance of these actions by the intelligence of innumerable dogs in the past, their co-adapted activity in their now automatic responses to appropriate stimuli presents no difficulty. And the consideration that neither in their prospectively more fully developed condition, nor, _a fortiori_, in their present and all previous stages of evolution, can these reflex mechanisms be regarded as presenting any selective--or even so much as any adaptive--value, is neither more nor less than the theory of use-inheritance would expect.

Thus, with regard to the phenomena of reflex action in general, all the facts are such as this theory requires, while many of the facts are such as the theory of natural selection alone cannot conceivably explain.

Indeed, it is scarcely too much to say, that most of the facts are such as directly contradict the latter theory in its application to them.

But, be this as it may, at present there are only two hypotheses in the field whereby to account for the facts of adaptive evolution. One of these hypotheses is universally accepted, and the only question is whether we are to regard it as _alone_ sufficient to explain _all_ the facts. The other hypothesis having been questioned, we can test its validity only by finding cases which it is fully capable of explaining, and which do not admit of being explained by its companion hypothesis. I have endeavoured to show that we have a large cla.s.s of such cases in the domain of reflex action, and shall next endeavour to show that there is another large cla.s.s in the domain of instinct.