Professor G. J. Romanes has adduced it as one of the difficulties which can alone be overcome by his theory of physiological selection[49].
This, however, is a misapprehension. I have by no means represented that the difficulty in question can alone be overcome by this theory. What I have represented is, that it can be overcome by any of the numerous forms of isolation which I named, and of which physiological selection is but one. And although, where common areas are concerned, I believe that the physiological form of isolation is the most important form, this is a very different thing from entertaining the supposition which Mr. Wallace here assigns to me.
I may take this opportunity of correcting a somewhat similar misunderstanding which has been more recently published by Professor W. A. Herdman, of Liverpool; and as the case which he gives is one of considerable interest in itself, I will quote his remarks in extenso. In his Opening Address to the Liverpool Biological Society, Professor Herdman said:—
Some of you will doubtless remember that in last year's address, while discussing Dr. Romanes' theory of physiological selection, I quoted Professor Flemming Jenkin's imaginary case of a white man wrecked upon an island inhabited by negroes, given as an illustration of the supposed swamping effect by free intercrossing of a marked variety with the parent species. I then went on to say in criticism of the result at which Jenkin arrived, viz. that the characteristics of the white man would be stamped out by intercrossing with the black:—
"Two influences have, I think, been ignored, viz. atavism, or reversion to ancestral characters, and the tendency of the members of a variety to breed with one another. Keeping to the case described above, I should imagine that the numbers of intelligent young mulattoes produced in the second, third, fourth, and few succeeding generations would to a large extent intermarry, the result of which would be that a more or less white aristocracy would be formed on the island, including the king and all the chief people, the most intelligent men and the bravest warriors. Then atavism might produce every now and then a much whiter individual—a reversal to the characteristics of the ancestral European—who, by being highly thought of in the whitish aristocracy, would have considerable influence on the colour and other characteristics of the next generation. Now such a white aristocracy would be in precisely the same circumstances as a favourable variety competing with its parent species," &c.
You may imagine then my pleasure when, a few months after writing the above, I accidentally found, in a letter[50] written by the celebrated African traveller Dr. David Livingstone to Lord Granville, and dated "Unyanyembe, July 1st, 1872," the following passage:—
"About five generations ago, a white man came to the highlands of Basañgo, which are in a line east of the watershed. He had six attendants, who all died, and eventually their headman, called Charura, was elected chief by the Basañgo. In the third generation he had sixty able-bodied spearmen as lineal descendants. This implies an equal number of the other sex. They are very light in colour, and easily known, as no one is allowed to wear coral beads such as Charura brought except the royal family. A book he brought was lost only lately. The interest of the case lies in its connexion with Mr. Darwin's celebrated theory on the 'origin of species,' for it shows that an improved variety, as we whites modestly call ourselves, is not so liable to be swamped by numbers as some have thought."
Here we have a perfect fulfilment of what I last year, in ignorance of this observation of Livingstone's, predicted as being likely to occur in such a case. We have the whitish aristocracy in a dominant condition, and evidently in a fair way to spread their characteristics over a larger area and give rise to a marked variety, and it had clearly struck Livingstone fourteen years before the theory of physiological selection had been heard of, just as it must strike us now, as an instance telling strongly against the "swamping" argument as used by Flemming Jenkin and Romanes.
Here we have a curious example of one writer supporting the statements of another, while appearing to be under the impression that he is controverting those statements. Both Professor Herdman's imaginary case, and its realization in Livingstone's account, go to show "the tendency of the members of a variety to breed with one another." This is what I have called "psychological selection," and, far from "ignoring" it, I have always laid stress upon it as an obviously important form of isolation or prevention of free intercrossing. But it is a form of isolation which can only occur in the higher animals, and, therefore, the whole of Professor Herdman's criticism is merely a restatement of my own views as already published in the paper which he is criticizing. For all that his argument goes to prove is, first, the necessity for some form of isolation if the overwhelming effects of intercrossing are to be obviated; and, secondly, the manifest consequence that where the psychological form is unavailable (as in many of the lower animals and in all plants), some other form must be present if divergent evolution is taking place on a common area.
Seeing that so much misunderstanding has been shown with reference to my views on "the swamping effects of intercrossing," and seeing also that this misunderstanding extends quite as much to Mr. Gulick's views as to my own, I will here supply brief extracts from both our original papers, for the double purpose of showing our complete agreement, and of leaving it to be judged whether we can fairly be held responsible for the misunderstanding in question. After having supplied these quotations, I will conclude this historical sketch by considering what Mr. Wallace has said in reply to the views therein presented. I will transcribe but a single passage from our papers, beginning with my own.
Any theory of the origin of species in the way of descent must be prepared with an answer to the question, Why have species multiplied? How is it that, in the course of evolution, species have not simply become transmuted in linear series instead of ramifying into branches? This question Mr. Darwin seeks to answer "from the simple circumstance that the more diversified the descendants from any one species becomes in structure, constitution, and habits, by so much will they be better enabled to seize on many and widely diversified places in the economy of nature, and so be enabled to increase in numbers." And he proceeds to illustrate this principle by means of a diagram, showing the hypothetical divergence of character undergone by the descendants of seven species. Thus, he attributes divergence of character exclusively to the influence of natural selection.
Now, this argument appears to me unassailable in all save one particular; but this is a most important particular: the argument wholly ignores the fact of intercrossing with parent forms. Granting to the argument that intercrossing with parent forms is prohibited, and nothing can be more satisfactory. The argument, however, sets out with showing that it is in limited areas, or in areas already overstocked with the specific form in question, that the advantages to be derived from diversification will be most pronounced. It is where they "jostle each other most closely" that natural selection will set a premium upon any members of the species which may depart from the common type. Now, inasmuch as this jostling or overcrowding of individuals is a needful condition to the agency of natural selection in the way of diversifying character, must we not feel that the general difficulty from intercrossing previously considered is here presented in a special and aggravated form? At all events, I know that, after having duly and impartially considered the matter, to me it does appear that unless the swamping effects of intercrossing with the parent form on an overcrowded area is in some way prevented to begin with, natural selection could never have any material supplied by which to go on with. Let it be observed that I regard Mr. Darwin's argument as perfectly sound where it treats of the divergence of species, and of their further divergence into genera; for in these cases the physiological barrier is known to be already present. But in applying the argument to explain the divergence of individuals into varieties, it seems to me that here, more than anywhere else, Mr. Darwin has strangely lost sight of the formidable difficulty in question; for in this particular case so formidable does the difficulty seem to me, that I cannot believe that natural selection alone could produce any divergence of specific character, so long as all the individuals on an overcrowded area occupy that area together. Yet, if any of them quit that area, and so escape from the unifying influence of free intercrossing, these individuals also escape from the conditions which Mr. Darwin names as those that are needed by natural selection in order to produce divergence. Therefore, it appears to me that, under the circumstances supposed, natural selection alone could not produce divergence; the most it could do would be to change the whole specific type in some one direction, and thus induce transmutation of species in a linear series, each succeeding member of which might supplant its parent form. But in order to secure diversity, multiplication, or ramification of species, it appears to me obvious that the primary condition required is that of preventing intercrossing with parent forms at the origin of each branch, whether the prevention be from the first absolute, or only partial.
Now for Mr. Gulick, a portion of whose more lengthy discussion of the subject, however, is all that I need quote:—