Darwin, and After Darwin, Volume 3 of 3 / Post-Darwinian Questions: Isolation and Physiological Selection

by George John Romanes

Of all parts of those variable objects which we call organisms, the most variable is the reproductive system; and the variations may carry with them functional changes, which may be either in the direction of increased or of diminished fertility. Consequently variations in the way of greater or less fertility frequently take place, both in plants and animals; and probably, if we had adequate means of observing this point, we should find that there is no one variation more common. But of course where infertility arises—whether as a result of changed conditions of life, or, as we say, spontaneously—it immediately becomes extinguished, seeing that the individuals which it affects are less able (if able at all) to propagate and to hand on the variation. If, however, the variant, while showing some degree of infertility with the parent form, continues to be as fertile as before when mated with similar variants, under these circumstances there is no reason why such differential fertility should not be perpetuated.

Stated in another form this suggestion enables us to regard many, if not most, species as the records of variations in the reproductive systems of their ancestors. When variations of a non-useful kind occur in any of the other systems or parts of organisms, they are, as a rule, immediately extinguished by intercrossing. But whenever they arise in the reproductive system in the way here suggested, they tend to be preserved as new natural varieties, or incipient species. At first the difference would only be in respect of the reproductive systems; but eventually, on account of independent variation, other differences would supervene, and the variety would take rank as a true species.

Now we must remember that physiological isolation is not like those other forms of isolation (e.g. geographical) which depend for their occurrence on accidents of the environment, and which may therefore take place suddenly in a full degree of completeness throughout a large section of a species. Physiological isolation depends upon distinctive characters belonging to organisms themselves; and it would be opposed to the whole theory of descent with progressive modification to imagine that absolute sterility usually arises, in a single generation between two sections of a perfectly fertile species. Therefore evolutionists must believe that in most, if not in all cases—could we trace the history, say of any two species, which having sprung from a single parent stock on a common area, are now absolutely sterile with one another—we should find that this mutual sterility had been itself a product of gradual evolution. Starting from complete fertility within the limits of a single parent species, the infertility between derivative or divergent species, at whatever stage in their evolution this began to occur, must usually at first have been well-nigh imperceptible, and thenceforth have proceeded to increase stage by stage.

But, if it be true that physiological isolation between genetically allied groups must usually itself have been the product of a gradual evolution; and if, when fully evolved, it constitutes a condition of the first importance to any further differentiation of these groups (by preventing fusion again into one group, more or less resembling the original parent form), do we not perceive at least a strong probability that in the lower stages of its evolution such mutual infertility must have acted as a segregating influence between the diverging types, in a degree proportional to its own development? The importance of mutual sterility as a condition to divergent evolution is not denied, when this sterility is already present in an absolute degree; and we have just seen that, before it can have attained to this absolute degree it must presumably, and as a rule, itself have been the subject of a gradual development. Does it not therefore become, on merely antecedent grounds, in a high degree probable, that from the moment of its inception this isolating agency must have played the part of a segregating cause, in a degree proportional to that of its completeness as a physiological character?

Whoever answers this question in the affirmative will have gone most of the way towards accepting, on merely antecedent grounds, the theory of physiological selection. And therefore it is that I have begun this statement of the theory by introducing it upon these grounds, thereby hoping to show how extremely simple—how almost self-evident—is the theory which it will now be my endeavour to substantiate. I may here add that the theory was foreshadowed by Mr. Belt in 1874[16], clearly enunciated in its main features by Mr. Catchpool in 1884[17], and very fully thought out by Mr. Gulick during a period of about fifteen years, although he did not publish until a year after the appearance of my own paper in 1886[18].

I must next proceed to state some of the leading features of physiological selection in further detail.

It has already been shown that Darwin clearly perceived that the very general occurrence of some degree of infertility between allied species cannot possibly be attributed to the direct agency of natural selection. His explanation was that the slight structural modifications entailed by the transformation of one specific type into another, so react upon the highly delicate reproductive system of the changing type as to render it in some degree infertile with its parent type. Now the theory of physiological selection begins by traversing this view. It does not, however, deny that in some cases the morphological may be the prior change; but it strenuously denies that this must be so in all cases. Indeed, according to my statement in 1886, the theory inclines to the view that, as a rule, the physiological change is prior. At the same time, the theory, as I have always stated it, maintains that it is immaterial whether, "in the majority of instances," the physiological change has been prior to the morphological, or vice versa; since in either case the physiological change will equally make for divergence of character.

To show this clearly the best way will be to consider the two cases separately, taking first that in which the physiological change has priority. In this case our theory regards any morphological changes which afterwards supervene as due to the independent variability which will sooner or later arise under the physiological isolation thus secured. But to whatever causes the subsequent morphological changes may be due, the point to notice is that they are as a general rule, consequent upon the physiological change. For in whatever degree such infertility arises between two sections of a species occupying the same area, in that degree is their interbreeding prevented, and, therefore, opportunity is given for a subsequent divergence of type, whether by the influence of independent variability alone, or also by that of natural selection, as now acting more or less independently on each of the partially separated groups. In short, all that was said in the foregoing chapters with respect to isolation in general, here applies to physiological isolation in particular; and by supposing such isolation to have been the prior change, we can as well understand the subsequent appearance of morphological divergence on continuous areas, as in other forms of isolation we can understand such divergence on discontinuous areas, seeing that even a moderate degree of cross-infertility may be as effectual for purposes of isolation as a high mountain-chain, or a thousand miles of ocean.

Here, then, are two sharply-defined theories to explain the very general fact of there being some greater or less degree of cross-infertility between allied species. The older, and hitherto current theory, supposes the cross-infertility to be but an accident of specific divergence, which, therefore, has nothing to do with causing the divergence. The newer theory, on the other hand, supposes the cross-infertility to have often been a necessary condition to the divergence having begun at all. Let us now consider which theory has most evidence in its favour.

First of all we have to notice the very general occurrence of the fact in question. For when we include the infertility of hybrids, as well as first crosses, the occurrence of some degree of infertility between allied species is so usual that Mr. Wallace recommends experiments to ascertain whether careful observation might not prove, even of species which hybridize, "that such species, when crossed with their near allies, do always produce offspring which are more or less sterile inter se[19]." This seems going too far, but nevertheless it is the testimony of a highly competent naturalist to the very general occurrence of an association between the morphological differentiation of species and the fact of a physiological isolation. Now I regard it as little short of self-evident that this general association between mutual infertility and innumerable secondary, or relatively variable morphological distinctions, is due to the former having been an original and a necessary condition to the occurrence of the latter, in cases where intercrossing has not been otherwise prevented.