Origin of Species Variorum

Comparison with 1866

find that the seedlings were miserably dwarfed and utterly sterile, and that they behaved in all other respects like ordinary hybrids. He might then maintain that he had actually proved, in accordance with the common view, that his two varieties were as good and as distinct species as any in the world; but he would be completely mistaken.

The facts now given on dimorphic and trimorphic plants are of importance, because they show us, firstly, that the physiological test of lessened fertility, both in first crosses and in hybrids, is no safe criterion of specific distinction; secondly, because we are thus led to infer, as previously remarked, that there must be some unknown law or bond connecting the infertility both of illegitimate unions and of first crosses, with the infertility of their illegitimate and hybrid offspring; thirdly, because we find, and this seems to me of especial im- portance, that two or three forms of the same species may exist and may differ in no respect, except in certain characters in their reproductive organs,—such as in the relative lengths of the stamens and pistils, in the size, form, and colour of the pollen-grains, in the structure of the stigma, and in the number and size of the seeds. ↑ ↑ With these differences and with no others, either in organisation or constitution, between the several forms, which are all hermaphrodites, we find that their illegitimate unions and their illegitimate progeny are more or less sterile, and closely resemble in a whole series of relations the first unions and the hybrid offspring of distinct species. We are thus led to infer that the sterility of species when crossed and of their hybrid progeny is likewise in all probability exclusively due to similar differences confined to their reproductive systems. ↑ We are indeed led to his same conclusion from considering reciprocal crosses between the same two species, in which the male of one cannot be united, or can be united with great difficulty, with the female of the other species, whilst the converse cross can be effected with perfect facility; for this difference in the facility of making reciprocal crosses and in the fertility of their offspring must be attributed either to the male or to the female element in the one species having been differentiated, with reference to the other sexual element, in a higher degree than in the second species. That excellent observer, Gärtner, likewise came on general grounds to the same conclusion, namely, that species when crossed are sterile owing to differences confined to their reproductive systems.

Finally, we are naturally led to inquire for what useful end have plants been rendered reciprocally dimorphic and trimorphic? A wide-spread analogy clearly gives us the answer as far as the immediate cause is concerned, namely, to prevent the pollen of each flower acting on the stigma of that flower. We see this effected in a host of flowers by the most curious mechanical contrivances, as I have shown with Orchids, and as could be shown with many plants of many other orders. There are also numerous plants, called dicho-gamous by C. K. Sprengel, in which the pollen and stigma are never mature at the same time, so that these plants can never fertilise themselves. There are many flowers, which, though they have their stigmas and pollen mature together, and which do not present any obstacle to self-impregnation, yet nevertheless are almost always fertilised by surrounding varieties when growing in the vicinity, as shown by the character of their seedlings. Then, again, we have many flowers with separated sexes borne on distinct plants, or on the same, which inevitably prevents self-fertilisation. Lastly, in accordance with the great principle prevailing throughout nature, of the same end being gained by the most diversified means, we find in dimorphic and trimorphic plants, in which self-fertilisation is not checked by any of the above-specified means, that this has been effected by the pollen of each flower, and consequently of all the flowers of the same form, having been rendered more or less impotent on their own stigmas; so that its action is easily and wholly obliterated by pollen habitually brought by insects from other individuals and forms of the same species.

In searching for the cause of dimorphism and trimorphism in plants, we may, in my opinion, safely go one step further, and conclude that the pollen has been prevented acting on the stigma of the same flower, in order to give vigour to the offspring by leading to the union of two distinct individuals. But on this view it is not a little remarkable that the end has been gained, in the case of dimorphic and trimorphic plants, at the expense of all the plants of the same form being rendered more or less sterile when united, and producing more or less sterile offspring. With respect to the steps by which it is probable that plants have been rendered dimorphic and trimorphic, want of space prevents my entering on the subject; but I will add that there is no special difficulty in this having been effected through variability, through the good gained by the prepotency of one sort of pollen over another, and through the accumulative action of natural selection.

Fertility of Varieties when Crossed, and of their Mongrel Offspring .

It may be urged, as a most forcible argu- ment, that there must be some essential distinction between species and varieties, and that there must be some error in all the foregoing remarks, inasmuch as varieties, however much they may differ

The facts now given on dimorphic and trimorphic plants are .. important, because they show us, first, that the physiological test of lessened fertility, both in first crosses and in hybrids, is no safe criterion of specific distinction; secondly, because we may conclude that there is some unknown bond which connects the infertility .. of illegitimate unions with that of their illegitimate offspring, and we are led to extend the same view to first crosses and hybrids; thirdly, because we find, and this seems to me of especial .. importance, that two or three forms of the same species may exist and may differ in no respect whatever, either in structure or in constitution, relatively to external conditions, and yet be sterile when united in certain ways. ↑ For we must remember that it is the union of the sexual elements of individuals of the same form, for instance, of two long-styled forms, which results in sterility; whilst it is the union of the sexual elements proper to two distinct forms which is fertile. Hence the case appears at first sight exactly the reverse of what occurs, in the ordinary unions of the individuals of the same species and with crosses between distinct species. It is, however, doubtful whether this is really so; but I will not enlarge on this obscure subject. ↑ We may, however, infer as probable from the consideration of dimorphic and trimorphic plants, that the sterility of distinct species when crossed and of their hybrid progeny, depends exclusively on the nature of their sexual elements, and not on any difference in their structure or general constitution. We are also led to this same conclusion by considering reciprocal crosses, ... in which the male of one species cannot be united, or can be united with great difficulty, with the female of a second species, whilst the converse cross can be effected with perfect facility. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. ... .. .. .. .. .. .. .. That excellent observer, Gärtner, likewise concluded that species when crossed are sterile owing to differences confined to their reproductive systems. ↑

Fertility of Varieties when Crossed , and of their Mongrel Offspring , not universal .

It may be urged, as an overwhelming argument, that there must be some essential distinction between species and varieties, ... inasmuch as the latter, however much they may differ