[Enclosure]

Natural Selection of Organic Types

The only serious difficulty which the theory of natural selection has yet encountered, is that which was originally raised & admirably rendered by a writer in the North British Review.⁸ This difficulty, as all who are in any degree acquainted with the literature of the subject must be well aware, is that of the swamping influence of intercrossing upon individual variations. Of course this difficulty could be removed if any cause could be suggested sufficient to account for numerous individuals varying in the same direction at the same time. A paragraph in First Principles, although not originally written with any such purpose, seems in this way partly to remove the difficulty. (See pp. 444–447.) ⁹ The substance of this paragraph is, that geological changes, by successively altering in a given area the conditions of life under which the organisms upon it exist, successively modify the character of these organisms in virtue of the direct influence of the changes themselves, & altogether apart from the indirect influence of natural selection. Now, although I cannot agree with M^r. Spencer when he says, “in the absence of these successive changes of conditions, ‘natural selection’ would effect comparatively little”;¹⁰ still I think that such successive changes might in some cases materially assist the influence of natural selection, by causing similar variations to arise in a number of individuals simultaneously.

The question as to the relative importance of the direct action of the environment in calling forth responsive changes in the organism, & the indirect action of survival of the fittest, is a question which the present generation can scarcely hope to see answered. I may observe, however, that to Darwinists in general the chief value of the passage above aluded to will be felt to consist in the increase which it affords to the sphere of natual selection’s agency. For, in whatever degree geological changes exert an influence of the direct kind, they must also increase, in a high proportional degree, the indirect influence of natural selection, by increasing at the same time the variability of the organisms & the severity of the struggle for existance to which the organisms are exposed. Thus, remembering that increase of area does not necessarilly belong to geological change, I believe that the principal effect of the latter in modifying species is probably that of intensifying the influence of natural selection, & hence that the converse of the above-quoted sentence is most likely to be true, namely, that in the absence of natural selection, geological changes would effect comparatively little.

Still, as it is manifestly desirable to seek for additional causes which may be supposed to mitigate the swamping influence of intercrossing upon incipient variations, I venture to suggest the following possibilities, which although of a purely speculative character, nevertheless to my mind appear entitled to rank as à priori probabilities of no inconsiderable weight.

For the sake of clearness, let us suppose that England and Scotland are slowly rising, that they are seperated from one another by a narrow strait, & that each of the two resulting islands contains a peculiar variety of the same terrestrial species. Let us further suppose that of these two varieties the more northern one is the better fitted to survive; & for the sake of allowing the smallest amount of strength to the influence of natural selection, & so the greatest amount of strength to the swamping influence of intercrossing, let us lastly suppose that the superiority of the northern type over the southern one is of a kind which natural selection cannot effect at all when acting in the ordinary way—i.e. that the superiority of the type is of such a kind that it confers no benefit upon the individual. Now the question is, when the elevation of the British Islands proceeds sufficiently far to unite England with Scotland & the two varieties are thus allowed to intermingle, is there any possibility of the superior variety supplanting the inferior? Of course at first sight it seems that the only answer open to us is an unequivocal negative; for the only probality seems to be that the two types would merely intermingle upon their common border-land, & that in time they would undergo more or less of mutual diffusion. That is to say, survival of the fittest appearing to have no basis of operation supplied to it—the superiority of the northern type being supposed to confer no benefit upon its constituent individuals,—amalgamation of the two varieties by free intercrossing appears to be the sole possibility. And so it undoubtedly is, if we grant that survival of the fittest can only act on individuals. I believe, however, that survival of the fittest may possibly act in the interest of varieties, quite independently of benefit to individuals, or even in opposition to such benefit. To take the simplest case I can think of: great expenditure in reproduction is certainly of benefit to such species as exhibit this character, while it is no less certainly detrimental to their constituent individuals.

How then was the character acquired? There seems to be only one answer possible, viz. that throughout the life-history of the species, those individuals which produced most young had always the best chance of leaving offspring to inherit a peculiarity, which, although detrimental to themselves, was notwithstanding of benefit to their species. I say this is the simplest case I can think of; let us then apply it to our illustration, & suppose that the advantage which the Scottish type has over the English one consists in its possessing a higher degree of fertility. Now, when the two types begin to emigrate into each other’s territory, it is manifest that the northern one would have more penetrating power, so to speak, than would the southern one; for the superiority of the former over the latter would assert itself in two ways at the same time: it would prevent immigration of the English type, & it would assist emigration of the Scottish one; for whenever any representatives of the English type passed into Scotland & there crossed with endemic type, the resulting progeny would not have so good a chance of perpetuating their mixed chararacters as would any of the other progenies around it have of perpetuating their pure character; while, contrariwise, whenever an individual of the Scottish breed crossed with English blood on English soil, the progeny would have the very best chance of perpetuating their hybrid type; & so in all proportions of admixture. Hence, if we suppose the struggle for existence to be tolerably severe, it is quite conceivable that the diffusive power of the one variety over the other might be very considerable. But now, would not the Scottish type, after it had affected the English one over a comparatively narrow band of territory, become lost by intercrossing? Not necessarily. Imagine a band of country 50 miles wide, stretching across the North of England & occupied by the offspring of the various intercrosses between English & Scottish blood. Above the northern limit of this band—i.e., slightly north of the border-line between the two countries—the Scottish type is still quite pure; the diffusive power of the English one having been but slight. On the other hand, the further South we go from the border-line, the more diluted does the Scottish type become. What then should we expect to ensue? Surely that the Scottish type should penetrate still further into England. Each of the types is subject to a greater or less degree of pressure—each would multiply itself indefinitely if only supplied with a large enough area free from opposition,— Hence it seems but natural that the Scottish type should extend itself in the line of the least resistance—i.e. progressively creep over the 50-mile band of territory, in its advance both purifying the previously mixed breed, & extending the latter further south. In this way I think there might be a continual gravitation of the one variety into the area previously occupied by the other—the rapidity of the process being determined by the intensity of the struggle for existence in relation to the intensity of what we may term the specific pressure. And that the line of least resistance in such a case would probably be well pronounced is evident, if we remember how keen the struggle for existence generally is between closely allied types.

In this way I think that the Northern variety might either extirpate the Southern one altogether, or supplant it over a large portion of its territory. During this process, however, new variations would be arising in the Northern type, spread as it now is over a larger area, & subject to numerous & important changes in its conditions of life. Some of these variations would almost certainly be in the direction of increased fertility; & if in any one part of the now greatly increased area occupied by the Northern type, a comparatively large number of individuals varied in this direction at the same time (in consequence of increased food or other local influence) so as to give rise to a local variety in this respect; then this local variety might serve as a new centre from which there might radiate a further modification of the entire specific type. And by a series of such processes taking place either successively or simultaneously, one species might sooner or later become transmuted into another; for, as survival of the fittest is in this case selecting large numbers of individuals collectively, specific characters might be changed by a comparatively small number of stages.

How far this theory may seem probable at first sight, I do not know; but we must remember that, as a matter of fact, most species do exhibit local varieties; that even slight geographical changes must frequently bring strongly pronounced varieties into collision; & that in such cases numerous species might be thus undergoing transmutation at the same time. In some cases, too, a small amount of sterility (or in the case of the higher animals, of sexual aversion) between members of the two varieties, would greatly assist the process. Moreover, the example which for the sake of simplicity I have chosen—viz. fertility—is not in other respects a good one; for it depends so much upon the mere doctine of chances. If, however, natural selection ever operates in the way now suggested, it must do so most energetically when in association with the principle acting in the ordinary way; for then the two methods of action would mutually assist each other.

G. J. Romanes