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occurs with illegitimate unions, as I ascertained in the following manner: I fertilized illegitimately a long-styled common Cowslip with pollen from the same form, and exactly twenty-four hours afterwards I fertilized the same stigmas legitimately with pollen from a short-styled dark-red Polyanthus. I should state that I raised many seedlings from crossed Cowslips and the Polyanthus, and know their peculiar appearance. I further know by the test of the fertility of the mongrels, as well inter se as with both parent forms, that the Polyanthus is a variety of the Cowslip, and not of the Primrose (P. vulgaris) as some authors have supposed. Now from the long-styled Cowslip, fertilized in the manner just described, I raised thirty seedlings, every one of which had flowers coloured more or less red, so that the legitimate Polyanthus-pollen wholly obliterated the influence of the illegitimate Cowslip-pollen

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sometimes much dwarfed in stature, and have so weak a constitution that they are liable to premature death; and we have seen exactly parallel cases with the illegitimate seedlings of Lythrum and Primula. Many hybrids are the most persistent and profuse flowerers, as are some illegitimate plants. When a hybrid is crossed by either pure parent form, it is notoriously much more fertile than when crossed inter se or by another hybrid; so when an illegitimate plant is fertilized by a legitimate plant, it is more fertile than when fertilized inter se or by another illegitimate plant. When two species are crossed and they produce numerous

* This has been remarked by many experimentalists in effecting crosses between distinct species; and in regard to illegitimate unions I have given a striking illustration in the case of Primula veris in a foot-note to my paper on Lythrum, in Proc Linn. Soc. vol. viii. (1864) p. 180.

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had anticipated in my paper, p. 86) that a vast majority of the pollen-grains which adhered to the base of the proboscis were large-sized and had come from the long stamens of the short-styled form, and were thus placed ready to fertilize the stigma of the long-styled form. On the other hand, on the middle, and near the tip of the proboscis, a very large proportion of the pollen-grains were of the small size, and had come from the short stamens of the long-styled form. My son caught, also, a moth (Cucullia verbasci) hovering over the bed, and I found on its proboscis a similar distribution of the two kinds of pollen-grains. I give these facts as a further illustration of the importance of the relative lengths of the stamens and pistil.

† Verhand. des naturhist. Vereins, 5. Jahrgang, 1848, S. 11, 13.

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* Lagerstrœmia Indica, one of the Lythraceæ, is strangely variable in its stamens—I presume in part due to its growth in a hothouse. The most perfect flowers produced with me five very long stamens with thick flesh-coloured filaments and green pollen, and from nineteen to twenty-nine short stamens with yellow pollen; but many flowers produced only one, two, three, or four long stamens with green pollen, which in some of the anthers was wholly replaced by yellow pollen; one anther offered the singular case of half, or one cell being filled with bright green, and the other cell with bright yellow pollen. One petal had a furrow near its base, which contained pollen. According to analogy with Lythrum, this species would produce three forms; if so, the above plant was a mid-styled form: it was quite sterile with its own two kinds of pollen.

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and 4 short-styled. As in this latter case the two parent plants, whilst under the net, did not produce a sufficiency of pollen, I committed, through forgetfulness, a capital error, and took some pollen from an adjoining uncovered long-styled plant. Now I have found on the proboscis of humble-bees of two species and of a moth (Cucullia), which were caught sucking the flowers of the Cowslip, an abundance of pollen of both forms. Hence, by taking the anthers of the uncovered long-styled plant, which probably had been visited by insects, the flowers under the net might have accidentally received a few grains from the short-styled form. Whether the appearance of the four short-styled plants in this set of seedlings may thus be accounted for I know not; but it is the sole exception which has occurred with me of a long-styled form of any plant, when self-fertilized, failing to produce the same form. Dr. Hildebrand, however, states that, out of 17 plants of P. sinensis derived from the self-fertilized long-styled form, three were short-styled. Altogether, in the first lot of seedlings, consisting of four generations, and in the second lot, 152 plants were raised, and all were long-styled with the exception of the just-mentioned four short-styled plants.

From the first seeds sown I raised from a self-fertilized short-

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namely, 23 seeds. As a standard of comparison I must add that during the same three seasons 44 flowers borne by legitimate short-styled plants were self-fertilized, and yielded 26 capsules; so that the fact of the 27 flowers on the illegitimate plant having produced only one capsule proves how sterile it was. To show that the conditions of life were favourable, I should here state that numerous plants of this and other species of Primula all produced an abundance of capsules whilst growing in the same soil and situation as the present and following plants. The sterility in the above illegitimate short-styled plant depended on both the male and female organs being in a deteriorated condition. This was manifestly the case with the pollen; for many of the anthers were shrivelled or contabescent. Nevertheless some of the anthers contained pollen, with which I succeeded in fertilizing some flowers on the illegitimate long-styled plants immediately to be described. Four flowers on this same short-styled plant were likewise legitimately fertilized with pollen from one of the following long-styled plants; but only one capsule was produced, containing 26 seeds; and this is a very low number for a legitimate union.

With respect to the five illegitimate long-styled plants, derived

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[F1742 offprint title page]

(Extracted from the Linnean Society's Journal.-Botany, vol. x.)

 ON THE CHARACTER AND HYBRID-LIKE NATURE OF THE OFFSPRING FROM THE ILLEGITIMATE UNIONS OF DIMORPHIC AND TRIMORPHIC PLANTS.

BY CHARLES DARWIN, M.A., F.R.S., &c.

[verso]

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TABLE OF CONTENTS.

Preliminary explanation.	Primula Auricula, equal-styled var. of.
Lythrum Salicaria, various illegitimate unions and character of offspring.	Primula vulgaris, transmission of form and fertility of illegitimate offspring.
Concluding remarks on Lythrum.	Primula veris, transmission of form and fertility of illegitimate offspring.
Oxalis rosea, transmission of form.	Equal-styled red variety of P. veris.
Primula sinensis.	Pulmonaria, transmission of form.
Summary on the transmission of form, constitution, and fertility of the illegitimate offspring of P. sinensis.	Conclusions in regard to the illegitimate offspring of trimorphic and dimorphic plants.
Equal-styled variety of P. sinensis.

VARIOUS plants, which I have called dimorphic and trimorphic, have been described by me in papers read before this Society*. But it may be convenient to recall as briefly as possible the meaning of these terms. Dimorphic species consist of two forms, which naturally exist in about equal numbers: in the long-styled form the pistil is always longer, and the stamens (excepting in the case of Linum grandiflorum) are shorter than in the other form. Conversely, in the short-styled form the pistil is shorter and the stamens longer than in the long-styled form. In the latter the pollen-grains are almost always of larger size than in the short-styled form. The sexual union of the two distinct forms is necessary for full fertility: such unions I formerly called heteromorphic; but, for reasons which will immediately appear, it is more convenient to speak of them as legitimate, and the offspring thus produced, as ordinarily occurs in nature, as legitimate. When long-styled or short-styled plants are impregnated with their own-form pollen, the union is not fully fertile, or is even absolutely barren. Such unions, and the offspring raised from them, may be called illegitimate. Thus two legitimate and two illegitimate unions can be effected.

With trimorphic species, the case is more complex. There are three forms, which differ greatly in the length of the pistil; and in each form two sets of stamens exist, differing in length, in the

* "On the two Forms, or Dimorphic Condition, in the species of Primula," Journal of Proceedings, Bot. vol. vi. (1862) p. 77. "On Linum," ibid. vol. vii, (1864) p. 69. "On Lythrum salicaria," ibid. vol. viii. 1864, p 169. See Correspondence vol. 16.

¹ An edited version of this article was later published in Forms of flowers (1877).

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size of the pollen-grains, and often in colour. The stamens are graduated in length, so that one of the two sets in two of the forms is equal in length to the pistil in the third form. For instance, in the long-styled form the pistil equals in length the longer set of stamens in the mid-styled and short-styled forms. In all three forms the union is fully fertile and legitimate only when the pistil is impregnated with pollen from the stamens which equal it in length. Thus the long-styled form can be legitimately fertilized only by the longer stamens of the mid-styled or short-styled form; it can be illegitimately fertilized by its own two sets of stamens, and by the shorter stamens of both the mid-styled and short-styled forms; so that the long-styled form can be fertilized legitimately in two ways and illegitimately in four ways. The same holds good with the mid-styled and short-styled forms; hence with trimorphic species eighteen unions are possible, of which six are legitimate, and produce legitimate offspring, and twelve are illegitimate and produce illegitimate offspring.

I will give the results of my experiments on the illegitimate offspring of various dimorphic and trimorphic plants in full detail, partly because the observations are extremely troublesome, and will not probably soon be repeated (thus I have been compelled to count under the microscope above 20,000 seeds of Lythrum Salicaria), but chiefly because much light is thus indirectly thrown on the important subject of hybridism.

LYTHRUM SALICARIA.

I will commence with this trimorphic species. Of the twelve illegitimate unions, two were completely barren; and I succeeded in raising seedlings from only six, or doubtfully from seven, of the remaining ten illegitimate unions. The experiments are arranged in classes according to the parentage of the plants. In each case I give the average number of seeds per capsule, generally taken from ten capsules, which, according to my experience, is a nearly sufficient number. I give also in each case the maximum number of seeds in any one capsule; and this is a useful point of comparison with the normal standard—that is, with the number of seeds produced by legitimate plants when legitimately fertilized. I give likewise in each case the minimum number. When the maximum and minimum differ greatly, and no remark is made on the subject, it may be understood that the extremes are so closely connected by intermediate figures that the average is fair. Large

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allied species cannot be crossed, or can only be crossed with the greatest difficulty. We are led to this conclusion still more forcibly by considering the great difference which often exists in the facility of crossing reciprocally the same two species; for it is clear in this case that the result must depend on the nature of the sexual elements, the male element of the one species acting freely on the female element of the other, but not so in the reversed direction. And now we see that this same conclusion is independently and strongly fortified by considering the illegitimate unions and offspring of trimorphic and dimorphic plants. In so complex and obscure a subject as hybridism it is no slight gain to arrive at a definite conclusion, namely, that we must look exclusively to a functional difference in the sexual elements, as the cause of the sterility of species when first crossed, and of their hybrid offspring. It was this consideration which led me to make so many and such laborious observations as have been recorded in this paper, and which justify, I think, their publication.