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Show 436 GENERAL RESULTS. 0HA1'. XII. CHAPTER XII. GENERAL RES'CLTS. . . . d to be benefici~l, and self-fertilisation in- Uross-ferhhsatwn prov~ d'ffi reatly in the means by which cro::s-jurious- Allied speCie~ 1 ;:~f fertili ation avoided- The benefits fertilisation is fttvoure an d p-end 0~ the deo-ree of diiferclltiation d 'Is of the two processes e o . f an ev1 t Tll evil effects not due to the combinatwn o in thbe'd s etexnudaeln ecl1~emse m~ sth e P"""e i_.,'" D"t -'-N aturo of the conditions to which mor 1 . h n rowing near togc:tll er in a state of nA.turc plants are subJected: ~h g ffects of such conclitions- 'rheoretical or u~der ~ultur~, ha:s ec~ t~ the interaction of differentiated sexual cons1dcratwns Wl.t r ~ -Genesis of the two sexes-Close correelements- Practlcalle s:s t of rross fertilisation and self-fcrtilimspondence betlweeln t.ht~ e t!c a~cl illeo·~imate unions of hctcrostyled tion and of t le egi 1ma . o . pla~ts, in comparison with hybnd umons. THE first and most important of the_ concl~sion s. ·whic~ b drawn from the observations given In this 1nay e . · 11 beneficial olume is that cross-fertilisation IS genera Y - h' va nd sel'f -fertilisati. on I· nJ· ~n·o us. T_l1 I·S _·I s show. n. b.y t de difference in height, weight, constitutional vig o~I; ~:d fertilit of the offspring from crossed and self-fertihs fl.owersy and in the numb r of seeds prodncedl by tthe parent-'p lants. w·I th respec t t o th . .s eco. nd .o f t. lCSe, lWlvO Propositions namely, that self-f rtihsation IS generta J ' . l rrhe struc ure l·nJ·urious we have abundant evic ence. D' 't l's of·, the flo'w rs in such plants as L ob e l ~ .a ramos a ' ttg. tt a 2 1 . _ purpurea, &c., renders t h e a1· a of -I. nsect s <a- .l mos•t Ino• nsd Pensable for their fertilisation ; and beann~ I].n. ~diual I d' t' t Ill(IVI tho prepotency of poll n .fro~. ~ IS Inch )lants will over that from the same In hviCLual, sue. I or a1 l dunn()' many 1nost certainly have been crossec b CHAP. XII. GENERAL RE 'UL'rf:l. 437 all previous generations. ~o it 1nust be, owing merely to tho prepotency of foreign pollen, with cabbage. and vanous other plants, the varieties of which almo 't invariably intercr.oss when grown together. The same inference may be drawn still more surely with resp ct to those plants, such as Reseda and Eschscholtzia which are sterile with their own pollen, but fertile' with that from any other individual. These several plants must therefore have been crossed during a long series of previous generations, and the artificial crosses in n1y experiments cannot have increased the vigour of the offspring beyond that of their progenitors. Therefore the difference between the self-fertilised and crossed plants raised by me cannot be attributed to the superiority of the crossed, but to the inferiority of the self-fertilised seedlings, due to the injurious effects of self-fertilisation. With respect to the first proposition, namely, that cross-fertilisation is generally beneficial, we likewise have excellent evidence. Plants of Ipomcea were intercrossed for nine successive generations; they were then again intercrossed, and at the same time crossed with a plant of a fresh stock, that is, one brought from another garden; and the offspring of this latter cross were to the intercrossed plants in height as 100 to 78, and in fertility as 100 to 51. An analogous experi1nent with Eschscholtzia gave a similar result, as far as fertility was concerned. In neither of these cases were any of the plants the product of self-fertilisation. Plants of Dianthus were self-fertilised for three generations, and this no doubt was ·injurious ; but when these plants were fertilised by a fresh stock and by intercrossed plants of the same stock, there was a great difference in fertility between the two sets of seedlings, and some difference in their height. Petunia offers a nearly |
