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Show 264 CONCLUDING REMARKS CIIAP. VI. manner will be nearly or completely lost. He was led to this view by observing that Diptera frequently carried pollen from the long-sty led flowers of Hottonia to the stigma of the same form, and that this illegitimate union was not nearly so sterile as the conesponding union in other heterosty led species. But this conclusion is directly opposed by some other cases, for instance by that of Lin~~m grandiflorum; for here the long-styled form is utterly barren with its own-form pollen, although from the position of the anthers this pollen is invariably applied to the stigma. It is obvious that with heterostyled dimorphic plants the two female and. the two male organs differ in power ; for if the same kind of pollen be placed on the stigmas of the two forn1s, and again if the two kinds of pollen be placed on the stigmas of the same form, the results are in each case widely different. Nor can we see how this differentiation of the two female and two male orga.ns could have been effected merely through each kind of pollen Leing habitually placed on one of the two stigmas. ·Another view seems at first sight probable, namely, that an incapacity to be fertilised in certain ways has been specially acquired by heterostyled plants. vVe may suppose that our varying species was sOinewhat sterile (as is often the case) with pollen from its own stamens, whether these were long or short ; and that such sterility was transferred to all . the individuals with pistils and stamens of the same length, so that these became incapable of intercrossing freely; but that such sterility was eliminated in the case of the individuals which differed in the length of their pistils and stamens. It is, however, incredible that so peculiar a form of mutual infertility should have been specially CHAP. VI. ON HETEROS'J1YLED PLANTS. 265 acquired unless it were highly beneficial to the species; and although it may be beneficial to an individual plant to be sterile with its own pollen, cross-fertilisation · being thus ensured, how can it be any ad vantage to a plant to be sterile with half its brethren, that is, with all the individuals belonging to the same form? Moreover, if the sterility of the unions between plants of the same form had been a special acquirement, we might have expected that the longstyled form fertilised by the long-styled would have been sterile in the same degree as the short-sty led fertilised by the short-styled; but this is hardly ever the case. On the contrary, there is sometimes the widest difference in this respect, as between the two illegitimate unions of Pulmonaria angustifolia and of Hottonia palustris. It is a more probable view that the male and female organs in two sets of individuals have been by some means specially adapted for reciprocal action; and that the sterility between the individuals of the same set or form is an incidental and purposeless result. The meaning of the term "incidental " may be illustrated by the greater or less difficulty in grafting or budding together two plants belonging to distinct species; for as this capacity is quite immaterial to the welfare of either, it cannot have been specially acquired, and must be the incidental result of differences in their vegetative systems. But how the sexual elements of heterostyled plants came to differ from what they were whilst the species was homostyled, and how they became co-adapted in two sets of individuals, are very obscure points. We know that in the two forms of our existing heterostyled plants the pistil always differs, and the s~amens generally differ in length ; so does the stigma in ·structure, |