This correlation between similar variations and infertility with other members of the species not so modified might arise in three ways: the two phenomena might occur simultaneously; or the variation might arise first and then the infertility with reference to the rest of the species; or infertility might appear first and then variations might ensue. If partial infertility, with the rest of the species, appeared first, then what would happen? There would be no suppression of the sexual passion, and those variants would be as ready to unite as the others would be. They would interbreed with the average of the race, but they would have no offspring, or only a few; and their peculiarity would soon disappear through the inability to reproduce itself. If, on the other hand, the similar variation in structure appeared first, the isolating cause—viz., infertility with the rest-would be absent; the variants would intermarry with the rest of the species, and in the absence of prepotent power the variations would be swallowed up in the race. Hence it seems to me quite clear that in order that this principle should be effective the structural variation and the infertility with the rest of the species should arise simultaneously in the same organism, and moreover that the infertility should be complete from the first. If there was only a slight degree of infertility this would not prevent intercrossing; and this slight variation might easily disappear as so many others do by the principle of regression to the average of the race. But what cause can we regard as efficient to produce at once a change of structure and absolute infertility with the rest of the species? Assuredly such a modification could not arise from the mere union of two normal individuals. We must find its cause in some external influence brought to bear upon the parturient system in many individuals. Mr. Darwin argues that the

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same causes which led to sterility may lead to other modifications of structure; and surely it is admissible to believe that the reverse may also take place, and that similar variants may possess fertility inter se and infertility with the rest of the species. But if we grant, for the sake of argument, that this correlation is possible, so far as the variation due to sexual reproduction is concerned, there is no conceivable reason why such correlation should occur except in the most casual way. Mr. Catchpole assumes that this correlation may arise in only two individuals, and that these individuals will marry one another.* But if the accident of birth is the only cause of this correlation, these two individuals may be living far apart and may never meet. In this case they have a potential power of becoming the ancestors of a new species; but it will avail nothing in the actual life. If they do unite, it would probably be on account of their similar variation which led them to segregate themselves, in which case their infertility with others would not be of any use. Many similar variants, all of whom were only fertile inter se, would not avail if they occurred at random, unless they could be brought together by segregation or some other means. If they were not, the variants would die out either because they did not find partners or because they intermarried with those with whom they would be infertile.

But if the infertility of variants with the rest of the species be due to some definite cause, acting on a given number of individuals, this will probably arise from the fact that, living together in the same locality, they are subject to the same modifying influences. And inasmuch. as they are already living together by the hypothesis, they will intermarry with one another; and then their

*Nature. vol. xxxi., p. 4.

infertility with the rest of the species will serve as a protection against indiscriminate interbreeding should opportunity occur for that, through the invasion of some of the normal forms; or through their encountering such on the outskirts of the locality in which they live.

Mr. Meldona proposes to himself the question, “Can physiological selection work independently of Natural Selection?" and he decides that it cannot.

"Let us suppose, for the sake of argument, that among the individuals of a species there arise certain varieties which are fertile inter se, but sterile with the parent form. There would thus arise a new race which could not be swamped by intercrossing with the predominant form, and the one species would practically be resolved into two-the parent form being still in the ascendency as regards numbers. But the competition is always most severe between the most closely related forms, and unless the new form (arising by inter-racial amixia) possessed some distinct advantage over the old one, it would as surely be exterminated by the overwhelming majority of the parent type, as it would be by intercrossing in the absence of amixia. Physiological selection thus appears to me to be as subordinate to natural selection as sexual selection, correlated variability, the law of homology, or any other of the Darwinian factors. The expression used by Mr. Romanes for his new factor-the 'Segregation of the Fit-seems to imply fitness for something, presumably for the conditions of life, and if the survival of the 'fit' race is determined by Natural Selection, then I venture to think that Natural Selection must still be regarded as the theory of the origin of species and as something more than a theory of the origin of adaptations."—(Nature. vol. xxxiv., pp. 384-5.)

This argument is based upon a belief in the intenser struggle for existence between closely allied forms and upon the idea of utility as the condition of survival in the case of such struggle. But in some cases, at any rate, closely allied species do not compete with one another in an internecine strife. Sometimes they live together harmoniously. And if we look to local or climatic variations, we shall find that in some cases, at any rate, the

modification is one that takes place apart altogether from the usefulness of the transmutation.

It follows, from what has been said, that if the tacit assumptions which underlie the theory of Pure Darwinism be accepted as true, there are other modes of isolating similar variants besides that of Natural Selection. But as these assumptions are not true, this argumentum ad hominem is, in effect, a mere war of words, when we are brought face to face with the facts of nature. It is difficult to understand how variations which are necessarily associated with reproduction should be isolated by the methods which we have mentioned. It is easy to see how useful these modes of isolation may be in assisting the effect of the direct action of changed conditions. This discussion will not be fruitless, if it leads to the conviction that physical separation, social segregation, and physiological differentiation, under certain circumstances, are efficient agents for isolating similar variants, and that they produce in nature a selection which is not Natural Selection, and which Natural Selection cannot control.

CHAPTER VII.

MIXED DARWINISM.

THE TRANSFORMING AND THE SELECTING INFLUENCE OF

CHANGED CONDITIONS.

"Tempora mutantur nos et mutamur in illis."

"In unison at times,

And then apart again,

And both in one have brought us hither."

-PALGRAVE. The Reign of Law.

By the phrase, mixed Darwinism, we must understand the affirmation that there are many possible methods by which the transmutation of species can be brought about. We have already shown that if there is a transmutation of species by Natural Selection, there is nothing inconsistent with that fact in supposing that there may be other methods also. For if there are variations necessarily associated with reproduction which require selection, there may be variations occasionally associated with reproduction which do not require the principle of selection. If the principle of selection be necessary, there may be other modes of selection in nature besides that accomplished by life and death. And in point of fact, we find that such phenomena do exist, and, so far, the belief in mixed Darwinism is perfectly reasonable, supposing that there is such a thing as Natural Selection at all.

But it is not of these matters that we think chiefly when we speak of mixed Darwinism. That phrase more