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Show 182 RYBlUDISl\J. CHAP. XIX. after a considerable interval of time, placed on the same stigma, its action is so strongly prepotent that it generally annihilates the effect of the foreign pollen; so it is with the pol~en of the several forms of the same species, for legitimate pollen IS strongly prepotent over illegitimate pollen, when both are placed on the same stigma. I ascertained this by fertilising severa~ .flowers, first illegitimately, and twenty-four hours afterwards ~eg1ttmately, with pollen taken from a peculiarly colo.ured vanety, and a~l the seedlings were similarly coloured ; this shows that the legitimate pollen, though applied twenty-four hours subsequ~ntly, had wholly destroyed or prevented the ac~ion of .the pre:wusly applied illegitimate pollen. Again, as, m m~bng r.e01procal crosses between the same two species, there IS occaswnall y a 0-reat difference in the result, so something analogous occurs ~~ith dimorphic plants; for a short-styled cowslip (P. veris) yields more seed when fertilised by the long-styl~d form, and less seed when fertilised by its own form, compared with a long-styled cowslip when fertilised in the two corresponding methods. In all these respects the forms of the same undoubted species, when illegitimately united, behave in exactly the same manner as do two distinct species when crossed. This led me carefully to observe during four years many seedlings, raised from several illegitimate unions. The chief result is that these illegitimate plants, as they may be called, are not fully fertile. It is possible to raise from dimorphic species, both long-styled and shortstyled illegitimate plants, and from trimorphic plants all three illegitimate forms. These can then be properly united in a legitimate manner. vVhen this is done, there is no apparent reason why they should not yield as many seeds as did their parents when legitimately fertilised. But such is not the case; they are all infertile, but in various degrees ; some being so utterly and incurably sterile that they did not yield during four seasons a single seed or even seed-capsule. rrhese illegitimate plants, which are so sterile, although united with each other in a legitimate manner, may be strictly compared with hybrids when crossed inter se, and it is well known how sterile these latter generally are. vVhen, on the other hand, a hybrid is crossed with either pure parent-species, the sterility is usually much lessened: and so it is when an illegitimate plant is fertilised by CILAI'. XIX. RYBRIDIS:i\L 183 a legitimate plant. In the same manner as the sterility of hybrids does not always run r arallel with the difficulty of making the first cross between the two parent species, so the sterility of certain illegitimate plants was unusually great, whilst the sterility of the union from which they were derived was by no means great. vVith hybrids raised from the same seed-capsule the degree of sterility is innately variable, so it is in a marked manner \vith illegitimate plants. Lastly, many hybrids are profuse and persistent flowerers, whilst other and more sterile hybrids produce few flowers, and- are weak, miserable dwarfs; exactly similar cases occur with the illegitimate offspring of various dimorphic and trimorphic plants. Altogether there is the closest identity in character and behaviour between illegitimate plants and hybrids. It is hardly an exaggeration to maintain that the former are hybrids, but produced within the limits of the same species by the improper union of eertain forms, whilst ordinary hybrids are produced :::'rom an improper union between so-called distinct species. We have already seen that there is the closest similarity in all respects between first illegitimate unions, and first crosses between distinct species. rrhis will perhaps be made more fully apparent by an illustration: we may suppose that a botanist found two well-marked varieties (and such oecur) of the long-styled form of the trimorphic Lythrum salicaria, and that he determined to try by crossing whether they were specifically distinct. He would find that they yielded only about one-fifth of the proper number of seed, and that they behaved in all the other abovespecified respects as if they had been two distinct species. But to make the case sure, he would raise plants from his supposed hybridised seed, and he would 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 complete! y mistaken. The facts now given on dimorphic and trimorphic plants are important, because they show us, firstly, that the physiological |