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Show 454 GENERAL RESULTS. CHAP. XII. generation, differed somewha~ in the successive gonerations. Now, under these cucumstances, tho sexual ·lements of the plants which were. interc1~os~ed. in o~teh generation retained sufficient. dlfferent1atlon ~unng several years for their ~ffspnng_ t? be SUJ)Onor to the self-fertilised, but th1s supenonty gradually and 1nanifostl y decreased, as was shown by the difference in the result between a cross with one of the intercrossed plants and with a ~resh stock. 'fhose intercrossed plants tended also In a few cases to become somewhat more uniform in some of their external characters than they were at first. vVith respect to tho plants which were self-fertilised in each generation, their sexual elements apparently lost, after some years, all differentiation, for a eros. between tho1n did no more good than a eross between the flowers on the same plant. But it is a still more remarkable fact, that although the seedlings of l\linn1lus, lpomma, Dianthus, and Petunia which were first raised, varied excessively in the colour of their flowers, their offspring, after being self-fertilised and grown und · r uniform conditions for some generations, bore flowers almost as uniform in tint as those on a natural 8pecies. In one case also the plants themselves became remarkably unifonn in height. 'rhe conclusion that tho advantages of a cross depend altogether on the differentiation of tho sexual elements, harmonises perfectly with the fact that ~n occasional and slight change in the conditions of hfe is beneficial to all plants and ani1nals.* But the offspring from a cross between organisms w~lich h~ve been exposed to different conditions, profit In an. 1n· comparably higher degree than do young or old beings * I have given sufficient evi- atinn under Domestication,' ch. dence on this head in my' Vari- xviii. vol. ii. 2nd edit. P· 127· CHAP. XII. OENERAL RESULTSc 455 fronl a mere .change in their conditions. In thi .. lat~er case we never see anything like tho ~ ct ~vhi?~ gonora]]y. follows from a cross with another ITnhd ·i vidu· alh, especially from a cross with a f' r s1 t 1... 1 s oc .... Is ~rug t, porha ps, have boon expected, for tho blend~ng to.ge~hor of the sexual olmnents of two differentiated beings will affect tho whole constitution at a. very ear~y period of life, whilst the organisation is highly flexible. We hav<::, moreover, reason to beliov that changed conditions generally act differ ntly on the s_evoral parts or organs of the same individual;·* a~d If w~ n1ay further believe that these now slightly differentiated parts r_eact on one another, the harmony between the beneficial effects on tho individual due to changed conditions, and those duo to the interaction of differentiated sexual elmnents, becomes still closer. That wonderfully accurate observer, Sprongel, who first_ ~ho':ed how i1nportant a part insects play in the fertilisation of flowers, called.. his book ' Tho Secret of Nature Displayed;' yet he only occasionally saw that the object for which so many curious and beautiful adaptations have been acquired, was the cross-fertilisation of distinct plants; and he knew nothing of the b~nefits which the offspring thus receive in growth, VIgour, and fertility. But the veil of secrecy is as yet far from lifted; nor will it be, until we can say why it is beneficial that the sexual elements shon1rl he differentiated to a certain extent, and why, if the differentiation be carried still further, injury follows. It is an extraordinary fact that with many species, flowers fertilised with their own pollen are either absolutely or in son1e degree sterile; if fertilised with * See, for instance, Brackenridge, 'Theory of Diathesis,' Ediuburgh, 1869. |