Origin of Species Variorum

Comparison with 1869

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; for it could .. have been of no direct advantage to an individual animal to breed badly with another individual of a different variety, and thus to leave few offspring; consequently such individuals could not have been preserved or selected. Or take the case of two species which in their present state when crossed, produce few and sterile offspring; now, what is there which could favour 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 towards 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. With sterile neuter insects we have reason to believe that modifications in their structure have been slowly accumulated by natural selection, from an advantage having been thus indirectly given to the community to which they belonged over other communities of the same species; but an individual animal, if rendered slightly sterile when crossed with some other variety, would not thus indirectly give any advantage to its nearest relatives or to any other individuals of the same variety, thus leading to their preservation. From these considerations I infer, as far as animals are concerned, that the various degrees of lessened fertility which occur with species when crossed cannot have been slowly accumulated by means of natural selection. ↑

With plants, it is possible that the case may be different. With very many kinds, insects constantly bring pollen from neighbouring plants of the same or of other varieties to the stigma of each flower; and with some this is effected by the wind. Now, if the pollen of any one variety should become by spontaneous variation in ever so slight a degree prepotent over the pollen of other varieties, so that, when deposited by any means on the stigmas of the flowers of its own variety, it obliterated the effects of previously placed pollen of other varieties, this would certainly be an advantage to the variety; for it would thus escape being bastardised and deteriorated in character. And the more prepotent the pollen could be rendered through natural selection the greater the advantage would be. We know from the researches of Gärtner that prepotency of this kind always accompanies the sterility which follows from crossing distinct species; but we do not know whether prepotency is a consequence of sterility, or sterility a consequence of prepotency. If the latter view be correct, we may infer that, as the prepotency became stronger through natural selection, from being advantageous to a species in process of formation, so the sterility consequent on prepotency would at the same time be augmented; and the final result would be various degrees of sterility, such as actually occur with our existing species when crossed. This same view might be extended to animals if the female before each birth received several males, so that the sexual element of the prepotent male of her own variety obliterated all effects from the access of previous males of other varieties; but we have no reason to believe, at least with terrestrial animals, that this is the case; as most males and females pair for each birth, and some few for life.

On the whole we may conclude that with animals the sterility of crossed species has not been slowly augmented through natural selection; and as this sterility follows the same general laws in the vegetable as in the animal kingdom, it is improbable, though apparently possible, that crossed plants should have been rendered sterile by a different process from animals. From this consideration, and remembering that species which have never co-existed in the same country, and which therefore could not have profited by having been rendered mutually infertile, yet are sterile when crossed; and bearing in mind that in reciprocal crosses between the same two species there is sometimes the widest difference in the resulting degrees of sterility, we must give up the belief that natural selection has come into play; and we are driven to our former proposition, that the sterility of first crosses, and indirectly of hybrids, is simply incidental on unknown differences in the reproductive systems of the parent-species.

We may now try and look a little closer at the probable nature of these differences, which induce sterility in first crosses, as well as in hybrids. Pure species and hybrids differ, as already remarked, in the state of their reproductive organs; but from what will presently follow on reciprocally dimorphic and trimorphic plants, it would appear as if some unknown bond or law existed, which causes the young from a union not fully fertile to be themselves more or less infertile. ↑

In the case of first crosses between pure species, the greater or less difficulty in effecting an union and in obtaining offspring apparently depends on several distinct causes. There must sometimes be a

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, produce few and sterile offspring; now, what is there which could favour 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 towards 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. With sterile neuter insects we have reason to believe that modifications in their structure and fertility have been slowly accumulated by natural selection, from an advantage having been thus indirectly given to the community to which they belonged over other communities of the same species; but an individual animal not belonging to a social community, if rendered slightly sterile when crossed with some other variety, would not thus itself gain any advantage or indirectly give any advantage to the other individuals of the same variety, thus leading to their preservation. ↑

But it would be superfluous to discuss this question in detail; for with plants we have conclusive evidence that the sterility of crossed species must be due to some principle, quite independent of natural selection. Both Gärtner and Kölreuter have proved that in genera including numerous species, a series can be formed from species which when crossed yield fewer and fewer seeds, to species which never produce a single seed, but yet are affected by the pollen of certain other species, for the germen swells. It is here manifestly impossible to select the more sterile individuals, which have already ceased to yield seeds; so that this acme of sterility, when the germen alone is affected, cannot have been gained through selection; and from the laws governing the various grades of sterility being so uniform throughout the animal and vegetable kingdoms, we may infer that the cause, whatever it may be, is the same or nearly the same in all cases.

We will now look a little closer at the probable nature of the differences between species which induce sterility in first crosses and in hybrids. In the case of first crosses, ... the greater or less difficulty in effecting an union and in obtaining offspring apparently depends on several distinct causes. There must sometimes be a