Here we perceive that Mr. Wallace, after having adopted the theory of physiological selection in its main elements, next proceeds to supplement that theory (as Mr. Gulick and myself had previously done), by showing how greatly the principle of physiological selection must be assisted by any association with other forms of isolation, or segregate breeding. The only difference between Mr. Wallace and ourselves here is, that while he instances but three or four forms of segregate breeding (or homogamy) with which physiological selection may be associated, I had previously considered several others in addition to these, while Mr. Gulick had gone into the matter still more exhaustively. Therefore, here as elsewhere, I can only account for the character of Mr. Wallace's criticism by supposing that he read our papers inattentively in the first instance, and was afterwards influenced by "unconscious memory" in his subsequent cogitations upon the problem of cross-sterility.
And now, finally, in order to show this still more completely, I may quote the whole paragraph which concludes his long discussion of that problem:
"The preceding argument, it will be seen, depends entirely upon the assumption that some amount of infertility characterises the distinct varieties which are in process of differentiation into species; and it may be objected that of such infertility there is no proof. This is admitted: but it is urged that facts have been adduced which render such infertility probable, at least in some cases, and this is all that is required. It is by no means necessary that all varieties should exhibit incipient infertility, but only some varieties; for we know that, of the innumerable varieties that occur, but few become developed into distinct species; and it may be that the absence of infertility, to obviate the effects of intercrossing, is one of the usual causes of their failure. All I have attempted to show is, that when incipient infertility does occur in correlation with other varietal differences, that infertility can be, and in fact must be, increased by natural selection; and this, it appears to me, is a decided step in advance in the solution of the problem."
This serves to convey a very accurate summary of the whole "preceding argument"; and it is likewise an admirably concise restatement of the theory of physiological selection. The only points in it to which I object—considered as an epitome of my own paper—are as follows. First, Mr. Wallace has not proved quite so good an advocate as he might have proved, had he looked more closely into the evidence "that some amount of infertility characterises the distinct varieties which are in process of differentiation into species." For although he says, properly enough, that his "preceding argument"—i. e., the theory of physiological selection—"depends entirely upon the assumption" that such infertility does "characterise distinct varieties which are in process of differentiation into species"; still he is wrong in saying it is "admitted" that in favor of this assumption there is "no proof" beyond what he has himself "urged" in the way of "facts which render such infertility probable": there are many other facts which not only render such infertility probable, but prove it to be actual. Secondly, although I quite agree with Mr. Wallace in holding that natural selection must often, as I said in my original paper, "co-operate" with physiological selection, still I must point out that the particular form of segregate breeding to which he here alludes is not natural selection at all; but (as explained in the foot-note to page 15) physiological selection pure and simple. My objections, however, with regard to these two points have no reference to the validity of Mr. Wallace's restatement of my views; and the fact that this restatement has been given with the most incomprehensible unconsciousness that it is a restatement, does not appear to me to detract from the significance of the argumentative suicide in which his entire criticism is thus found to terminate.[7]
[7] I am the more surprised that Mr. Wallace did not perceive his almost complete adoption of my views in this latest publication of his own, because I had previously had occasion to point out a partial adoption of them in an earlier publication of his on the same subject. The following is what I said upon that occasion—viz., in the Nineteenth Century, January, 1888:
"One very obvious and probably frequent instance of what may be termed collective variation in the reproductive system—or a variation due to a common cause acting on many individuals simultaneously—is actually quoted from my paper by Mr. Wallace himself, namely, changes in the season of flowering or of pairing, which insure that any section of a species so affected shall be fertile only within itself. Collective variation of this kind may be directly due to the incidence of some common cause, such as changed conditions of life with respect to food, climate, station, etc.; or, as in the case of bud-variation, it may be due to a single "sport" affecting all the blossoms growing upon the same branch. But besides such direct action of a common cause, it is easy to see that natural selection, use and disuse, etc., by operating in the production of organic changes elsewhere, may not unfrequently react on the sexual system indirectly, and so induce the sexual change required in a number of individuals simultaneously."
Now, in his Darwinism, Mr. Wallace again reproduces this instance of "physiological selection," without even yet appearing to perceive that both in my original paper upon the subject and in my answer to his criticism as above quoted, I adduce this particular instance of physiological selection as a typical one. Therefore, when he now says:—"Another mode of isolation is brought about by the variety—either owing to habits, climate, or constitutional change—breeding at a slightly different time from the parent species: this is known to produce complete isolation in the case of many varieties of plants": he is merely restating what I have repeatedly given as an unquestionable case of physiological selection.
With the self-destruction of this criticism I am left without any other to answer; and I should not have occupied so much space in dealing with this one, were it not that the high estimation in which Mr. Wallace is so deservedly held by all other naturalists is calculated to render almost incredible the peculiar position to which he has eventually gravitated with reference to my views—professing hostility on the one hand, while reproducing them as original on the other. The misunderstanding of my ideas which this state of matters represents, might have led me to wonder whether I could possibly have rendered my meaning more clear in the first instance, were it not that this misunderstanding extends in an even greater measure to Mr. Gulick's paper than it does to mine. For seeing that the whole criticism is founded on the erroneous idea that our theory supposes physiological selection always to act alone, the misconception becomes positively ludicrous in its relation to Mr. Gulick's views; seeing that, as previously stated, Mr. Gulick not only agrees with me in holding that physiological selection must be greatly fortified by being associated with any other form of homogamy, but even goes so far as to agree with Mr. Wallace that, unless it is so fortified, it can never act at all. So that, as far as physiological selection is concerned, Mr. Gulick's theory is precisely identical with that of Mr. Wallace, and differs from his statement of it only in recognising a number of forms of homogamy, in addition to natural selection, sexual selection, etc., with which the principle of physiological selection may be associated.