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Show 230 HYBIUDISM, [CHAP. VIII. sarne species? Why should som~ species .cro~s with facil~ ity, and yet p1~oduce very s~enle hybr1ds, and other species cross with extreme drfficulty, and yet produce fairly fertile hybri~s? Why should tJ:ere often be so great a difference In th~ result of a. remprocal cross between the same two species? Why, It rna~ even be asked, has the production of hybrids been pe:mitted? ~o grant to species the special power of ~rodumn~ hybrids, and then to stop their f~rther propagation by ~I~erent degrees of sterility, not strictly related to the famhty of the first union between their parents, seems to be a strange ar-rangement. . The foregoing rules and facts, on the other hand, appear to me clearly to i~dic~te .that t~e .sterility both of first crosses and of hybnds IS snnply Inmdental or ·dependent on unknown d~fferen?es, chiefly in the repr~ductive systems of the spem.es which .ar~ crossed. The drffe;ences being of so pecuhar and lnnited a nature, that, In reciprocal crosses between two species the male sexual element of the one will often freely act on the female sexual element of the other, but not in a reversed direction. It will be advisable to explain a little more fully by an example what I mean by sterility being incidental on other differences, and not a specially endowed quality. As the capacity of one plant to be grafted or budded on another is so entirely unimportant for its welfare in a state of nature, I presume that no one will suppose that this capacity is a specially endowed quality, but will admit that it is incidental on differences in the laws of growth of the two plants. We can son1etimes see the reason why one tree will not take on another, from differences in their rate of growth, in the hardness of their wood, in the period of the flow or nature of their sap, &c. ; but in a multitude of cases we can assign no reason whatever. Great diversity in the size of two plants, one being woody and the other herbaceous, one being evergreen and the other deciduous, and adaptation to widely different climates, does not always prevent the two grafting together. As in hybridisation, so with grafting, the capacity is limited by systematic affinity, for no one has been able to graft Cu.AP. VIII.] COMPARED WITH GRAFTING. 231 ttees together belonging to quite distinct families · and on the other hand, closely allied species, and varieties of the san1e species, can usually, but not invariably be ~rafted with ease. But this capacity, as in hybridisation, IS by no means abs_ol"';lte]y governed. b~ systematic affinity. Althou?h 1nan;y- distinct gen.era within the sa~e family have been grafted together, In other cases spemes of the same genus will not t~ke on each other. The pear can be grafte~ ~ar more readily on the quince, which is ranked as a distinct genus, than on the apple, which is a member of the .sarn~ senus. Even different varieties of the pear take .with d1fler~n~ degrees of ~acility on the quince; so do different vaneties of the apncot and peach on certain varieties of the plu1n. As Gartner found that there was sometimes an innate ~ifferen?e in different individ~tals of the sa1ne two species 1~ crossing; ~o. Sagaret believes this to be the case with diff~rent Individuals ~f the. same two species in being graft~d togethe~·· As In remprocal crosses, the facility of e;ffectl~g. an uni?n IS often very far from equal, so it sometunes IS In grafting; the common gooseberry, for instance, ca;nnot be grafted on the currant, whereas the currant ·will take, though with difficul~J:' on the g~oseberry. .we have se~n that the ~tenh~y of hybnds, which have thmr r~productive organs In ~n Imperfect ?ondition, is a very. d1ffere~t case from the difficulty of uniting two pure spemes, whiCh .h~ve their reproductive organs perfect; yet these tw~ distinct cases run to a certain extent parallel. Something analo~ous occur~ ~n grafting ; for Thouin found ~hat three spemes of .Robinia, which seeded freely on thm: own roots, and whiCh could be grafted with no great difficulty on another species, when thus grafted were rendered barren. On the other hand, certain species of Sorbus, when grafted on other species yielded twice as much fr~it as when on their own roots.' We are reminded by this latter fact of the extraordinary case of Hippeastrum, I:o.belia, .&c., which seeded much more freely when fertihsed with the pollen of distinct species than 'vhen self-fertilised with their own pollen. ' We thus see, that although there is a clear and funda- |