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Show 218 HYBRIDISM. [CHAP. VIIJ.. produce either few or no offspring. liybrids, on the other hand, have their reproductive organs functionally impotent, as may be clearly seen in the state of the male element in both plants and animals ; though the organs themselves ~re perfect in structure, as far as the microscope reveals. In the first case the two sexual elements which go to form the embryo are perfect; in the second case they are either not at all developed, or are imperfectly developed. This distinction is important, when the cause of the sterility, 'vhich is common to the two cases has to be conside;ed. The dist~n.ctio~ has probably bee~ slurred over, owing_ to the sterihty In both cases being looked on as .a spemal endowment, beyond the province of our reasoning powers. The fertility of varieties, that is of the forms known or believed to have descended from common parents when interc:osse.d, and likewise the fertili~y of their :rdongrel offspring, Is, on my theory, of equal Importance with the sterility of species ; for it seems to make a broad and clear distinction between varieties and species. First, for the sterility of species when crossed and of their hybr~d offspring. It is impossible to study the several memoirs and works of those two conscientious and admirable observers, Kolreuter and Gartner who almost devoted their lives to this subject, without l>eing deeply i~npressed .:With the high generality of some degree of sterihty. Kolreuter makes the rule universal ; but then he cuts the knot, for in ten cases in which he found two forms, considered by most authors as distinct species q_ui~e fertil~ together, he unhesitatingly ranks them as va~ rieties. 9-artner also, ~nakes ~~e rule equally universal; and ~e disputes t~e entire ferhhty of l{olreuter's ten cases. But In these and In many other cases, Gartner is obliged carefully to count the seeds, in order to show that there is any degree of sterility. He always compares the maxi4 mum number of seeds produced by two species when crossed and by their hybrid offspring, with the average number produced by both pure-parent species in a state of nature. But a serious cause of error seems to me to be here introduced: a plant to be hybridised must be cas- CHAP. VIII.) STERILITY. 219 trated, and, what is often more important, must be se· eluded in order to prevent pollen being brought to it by insects fron1 other plants. Nearly all the plants experimentised on by Gartner were potted, and apparently were kept in a cha1nber in his house. That these processes are • often injurious to the fertility of a plant cannot be doubt· ed; for Gartner gives in his table about a score of cases of plants which he castrated, and artificially fertilised with their own pollen, and (excluding all cases such as the Leguminosrn, in which there is an acknowledged difficulty in the manipulation) half of these twenty plants had their fertility in some degree impaired. Moreover, as Gartner during several years repeatedly crossed the primrose and co·wslip, which we have such good reasons to believe to be varieties, and only once or twice succeeded in getting fertile seed; as he found the common red and blue pimpernels (Anagallis arvensis and coorulea), which the best botanists ranlc as varieties, absolutely sterile together; and as he cnme to the same conclusion in several other analogous cases; it seems to n1e that we may well be pennitted to doubt whether many other species are really so sterile, when intercrossed, as Gartner believes. It is certain, on the one hand, that the sterility of various species when crossed is so different in degree and graduates away so insensibly, and, on the other hand, that the fertility of pure species· is so easily affected by various circumstances, that for all practical purposes it is most difficult to say where perfect fertility ends and sterility begins. I think no better evidence of this can be required than that the two most experienced observers who have ever lived, nan1ely, Kolreuter and Gartner, should have arrived at diametrically opposite conclusions in regard to the very sarne species. It is also most instructive to compare- but I have not space here to enter on details-the evidence advanced by our best botanists on the question whether certain doubtful forms should be ranked as species or varieties, with the evidence from fertility adduced by different hybridisers, or by the same author, from experiments made during different years. It can thus be shown that neither sterility nor fertility affords any clear |
