"Were it not for the very general occurrence of some degree of sterility between even closely allied species and were it not also for the fact, that closely allied species are not always—or even generally—separated from one another by geographical barriers, we might reasonably attribute all cases of species-formation by independent variability to the prevention of intercrossing by geographical barriers or by migration. But it is evident that these two facts can no more be explained by the influence of geographical barriers, or by migration, than they can be by the influence of natural selection.
"Now, of all parts of those variable objects which we call organisms, the most variable is the reproductive system; and the variations may be either in the direction of increased or diminished fertility. Consequently, variations in the way of greater or less sterility 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, whenever it 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 the variation. If, however, the variation should be such that, while showing some degree of sterility with the parent form, it continues to be as fertile as before within the limits of the varietal form, it would neither be swamped by intercrossing nor die out on account of sterility.
"For example, suppose the variation in the reproductive system is such that the season of flowering, or of pairing, becomes either advanced or retarded. Whether this variation be "spontaneous," or due to change of food, climate, habitat, etc., does not signify. The only point we need attend to is that some individuals, living on the same geographical area as the rest of their species, have demonstrably varied in their reproductive systems, so that they are perfectly fertile inter se, while absolutely sterile with the rest of their species. By inheritance there would thus arise a variety living on the same geographical area as its parent form, and yet prevented from intercrossing with that form by a barrier quite as effectual as a thousand miles of ocean; the only difference would be that the barrier, instead of being geographical, is physiological. And now, of course, the two sections of the physiologically divided species would be able to develop independent histories of their own without intercrossing; even though they are living together on the same geographical area, their physiological isolation would lead to their taking on distinct specific characters by independent variations, [or homogamy,] just as is the case with sections of a species when separated from each other by geographical isolation.
"To state this suggestion in another form, it enables us to regard many, if not most, species as the records of variations in the reproductive systems of 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 happen to arise in the reproductive system in the way here suggested, they must 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 new variety would take rank as a true species.
"The principle thus briefly sketched in some respects resembles, and in other respects differs from, the principle of natural selection, or survival of the fittest. For the sake of convenience, therefore, and in order to preserve analogies with already existing terms, I have called this principle Physiological Selection, or Segregation of the Fit.
"Let it be noted that we are not concerned either with the causes or the degrees of the particular kind of variation on which this principle depends. Not with the causes, because in this respect the theory of physiological selection is in just the same position as that of natural selection: it is enough for both that the needful variations are provided, without its being incumbent on either to explain the causes which in all cases underlie them. Neither are we concerned with the degrees of sterility which the variation in question may in any particular case supply. For whether the degree of sterility with the parent form be originally great or small, the result of it will be in the long run the same: the only difference will be that in the latter case a greater number of generations would be required in order to separate the varietal from the parent form. [In other words, homogamy due to such physiological isolation is cumulative.]
"The object of this paper being that of furnishing a general answer to criticisms on the hypothesis of physiological selection, I will not occupy space by detailing evidence of that hypothesis, further than is needful for the object just mentioned.[3] This evidence abundantly proves that the particular kind of variation which the theory of physiological selection requires does take place, (a) in individuals, (b) in races, and (c) in species. Next, the evidence goes on to show that the facts of organic nature are such as they ought to be, supposing it true that this variation has played any considerable part in the differentiation of specific types. In particular, it is shown that the general association between the one primary, or relatively constant, specific distinction (mutual sterility), and the innumerable secondary, or relatively variable, distinctions (slight morphological changes which may effect any parts of any organisms), of itself indicates that the former has been the original condition to the occurrence of the latter, in all cases where free intercrossing has not been otherwise prevented. For even in cases where the secondary distinctions may be supposed to have induced the primary,—or where morphological changes taking place in other parts of an organic type have exercised a reflex influence on the reproductive system, such that the changed organism is no longer fertile with its unchanged parent form,—even in such cases the theory of physiological selection is available to explain the association in question. For even in these cases, notwithstanding that the secondary changes are historically the prior changes, they still depend for their preservation on the principles of physiological selection. These principles have, in all such cases, selected the particular kinds of secondary distinction which have proved themselves capable of so reacting on the reproductive system as to bring about the primary distinction, and thus to protect themselves against the destructive power of free intercrossing."
[3] The evidence, so far as yet published, may be read by any one who cares to purchase the original paper, which can be obtained from the Linnean Society in a separate form.
Now for Mr. Wallace's criticism of this theory, as presented in his recently published work on "Darwinism."
Briefly put, he furnishes a numerical calculation, showing that when "the physiological peculiarity is not correlated with any external differences of form or color, or with inherent peculiarities of likes or dislikes leading to any choice as to pairing," even when so large a proportion as ten per cent. of the exceptional variety arises every year in the midst of the species, "it is unable to increase its numbers much above its starting-point, and remains wholly dependent on the continued renewal of the variety for its existence beyond a few years."