In regard to reciprocal crosses Darwin makes the following important statements: “The diversity of the result in reciprocal crosses between the same two species was long ago observed by Kölreuter. To give an instance: Mirabilis jalapa can easily be fertilized by the pollen of M. longiflora, and the hybrids thus produced are sufficiently fertile; but Kölreuter tried more than two hundred times, during eight following years, to fertilize reciprocally M. longiflora with the pollen of M. jalapa, and utterly failed.”
A formal interpretation of this difference can be easily imagined. The infertility in one direction may be due to some physical difficulty met with in penetrating the stigma, or style. For instance, the tissue in one species may be too compact, or the style too long. Pflüger, who carried out a large number of experiments by cross-fertilizing different species of frogs, reached the conclusion that the spermatozoa having small and pointed heads could cross-fertilize more kinds of eggs, than could the spermatozoa with large blunt heads. This is probably due to the ability of the smaller spermatozoa to penetrate the jelly around the eggs, or the pores in the surface of the egg itself. But there are also other sides to this question, as recent results have shown, for, even if a foreign spermatozoon can enter an egg, it does not follow that the development of the egg will take place. Here the difficulty is due to some obscure processes in the egg itself. Now that we know more of the nicely balanced combinations that take place during fertilization of the egg, and during the process of cell division, we can easily see that if the processes were in the least different in the two species it might be impossible to combine them in a single act.
“Now do these complex and singular rules indicate that species have been endowed with sterility simply to prevent their becoming confounded in nature? I think not. For why should the sterility be so extremely different in degree, when various species are crossed, all of which we must suppose it would be equally important to keep from blending together?”
“The foregoing rules and facts, on the other hand, appear to me clearly to indicate that the sterility both of first crosses and of hybrids is simply incidental or dependent on unknown differences in their reproductive systems; the differences being of so peculiar and limited a nature, that, in reciprocal crosses between the same two species, the male sexual element of the one will often freely act on the female sexual element of the other, but not in a reversed direction.”
Does Darwin give here a satisfactory answer to the difficulty that he started out to explain away? On the whole, the reader will admit, I think, that he has fairly met the situation, in so far as he has shown that there is no absolute line of demarcation between the power of intercrossing of varieties and races, and of species. It is also extremely important to have found that the difficulties increase, so to speak, even beyond the limits of the species; since species, belonging to different genera, are as a rule more difficult to intercross than when they belong to the same genus. The further question, as to whether there are differences in respect to the power of intercrossing between different kinds of varieties, such as those dependent on selection of fluctuating variations, of local conditions, of mutations, etc., is far from being settled at the present time.
That this property of species is useful to them, in the somewhat unusual sense that it keeps them from freely mingling with other species, is true; but, as has been said, this would be a rather peculiar kind of adaptation. If, however, it be claimed that this property is useful to species, as Darwin himself claims, then, as he also points out, it is a useful acquirement that cannot have arisen through natural selection. It is not difficult to show why this must be so. If two varieties were to some extent at the start less fertile, inter se, than with their own kind, the only way in which they could become more infertile through selection would be by selecting those individuals in each generation that are still more infertile, but the forms of this sort would, ex hypothese, become less numerous than the descendants of each species itself, which would, therefore, supplant the less fertile ones.
Darwin’s own statement in regard to this point is as follows:—
“At one time it appeared to me probable, as it has to others, that the sterility of first crosses and of hybrids might have been slowly acquired through the natural selection of slightly lessened degrees of fertility, which, like any other variation, spontaneously appeared in certain individuals of one variety when crossed with those of another variety. For it would clearly be advantageous to two varieties or incipient species, if they could be kept from blending, on the same principle that, when man is selecting at the same time two varieties, it is necessary that he should keep them separate.
“In considering the probability of natural selection having come into action, in rendering species mutually sterile, the greatest difficulty will be found to lie in the existence of many graduated steps from slightly lessened fertility to absolute sterility. It may be admitted that it would profit an incipient species, if it were rendered in some slight degree sterile when crossed with its parent form or with some other variety; for thus fewer bastardized and deteriorated offspring would be produced to commingle their blood with the new species in process of formation. But he who will take the trouble to reflect on the steps by which this first degree of sterility could be increased through natural selection to that high degree which is common with so many species, and which is universal with species which have been differentiated to a generic or family rank, will find the subject extraordinarily complex. After mature reflection it seems to me that this could not have been effected through natural selection. Take the case of any two species which, when crossed, produced few and sterile offspring; now, what is there which could favor the survival of those individuals which happened to be endowed in a slightly higher degree with mutual infertility, and which thus approached by one small step toward absolute sterility? Yet an advance of this kind, if the theory of natural selection be brought to bear, must have incessantly occurred with many species, for a multitude are mutually quite barren.”
Darwin points out the interesting parallel existing between the results of intercrossing, and those of grafting together parts of different species.