Darwin, and After Darwin, Volume 2 of 3 / Post-Darwinian Questions: Heredity and Utility

by George John Romanes

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.

The facts of artificial selection prove that immense modifications of structure may be caused by a cumulative 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 destitute of adaptive value per se: the adaptation only begins to arise if all the parts in question occur associated together in the same individuals from the very first. In this case it is obvious that the analogy 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].

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 analogy (which Darwin had already and very properly adduced with regard to the blending of adaptations) he enforces by special illustrations; 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 analogy 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.

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.

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 accumulating 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 accumulation 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 circumstances 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.