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Show 444 GENERAL RESULTS. CHAP. XII. offers another instructive case, showing that tho benefit of a cross depends on the previous treatment of th0 progenitors: plants which h~d been solf-fortiliseu for the eight previous generations wore crossed with plants which had been inte1:crossed for tho same numb r of generations, all hav1ng been kept under the sa1ne conditions as far aR possible; . seedlings from this cross were grown in competition with others ueri ved from the same self-fertilised mother-plant crossed by a fresh stock ; and the latter seedlings wore to the fornler in height a~ 100 to 52, anu in fertility as 100 to 4. An exactly parallel expori1nont was tried on Dianthus, with this difference, that tho plants had been s If-fertilised only for the three previous generations, and the result was similar though not so strongly marked. 'fho foregoing two cases of tho offspring of IpomCBa and Eschscholtzia, derived from a cross with a fresh stock, being as 1nuch superior to the intorcrossed plants of the old stock, as those latter were to tho selffertilised offspring, strongly supports tho san1e conclusion. A cross with a fresh stock or with another variety seems to be always highly b n ficial, whether or not the mother-plants have been intercrossod or selffertilised for s veral pr vious g norations. The fact that a cross b twe n two flowers on the same plant does no good or v .ry little good, is likewise a strong corroborati n of our conelusion; for the sexual elements in the flow rs on tho same plant can rarely have been differentiated, though this is possible, as flower-buds are in one ·son e d.istinct individuals, sometimes varying and differing from one a:l?ther in structure or constitution. rrhus tho proposltlOn that the 1onc:fit fron1 cross-f rtilisation depends on the plants which are crossed having b en subj ected ~u.ring previous generations to son1ewhat different condltwns, CHAP. XII. GENERAL RESULTS. 445 or to their having varied from if they had been thus subiect . dso~o unknown cause as all sides. J 0 ' Is securol Y fortified on Before proceedino- any further th . . b . . b · , e VIew wh1eh has ee.n ma1n ta1ned by several Ph ys1.o 1o o-.1 sts must b noticed, namely that all th -1 £? e . l ' e evi s rom breodino-animas too closely, and no doubt th 0 f h ' as ey would say • · rom t e self-fertilisation of p-lants , I.s th e resu 1t of the' InC·I ·ease of some morbid tendenc Y or wea1 r ness of. constl. _ tution common to the closely r.e 1a t ed parents or to ~h~ two sexes of hermaphrodite plants. Undo~btedl InJury has often thus resulted '. b u t I. t I.s a .v ai.n y at. t ern p. t to ,e xtend this view to the n u1nerous oases given In Iny rables. It should be remembered that the same mo~her-plant was both self-fertilised and crossed, so that. 1f she had been unhealthy sh e wou lcl h ave trffa: nsm. itted half her morbid tendenci·es to h er crossed o spnng. But plants appearing perfectly health some of them growing wild, or the immedi.ate o.{!{' . y, f 'ld 1 . uspnng o 1W I p ants, or vigorous common o-arden-plants w t ·a .i! • o , ere se ec .e lOr expenment. Considerino- the b f . • b nun1 er o species which we~·e tried, it is nothing less than absurd to suppose that In. all_ these cases tho mother-plants, though not a~peanng In any way diseased, were wealc or ~~healthy In so peculiar a Inanner that their selffertilised ~eedl~ngs, many hundreds in number, were r~ndered Infenor in height, weight, constitutional VIgour, _and f~rtility ~o their crossed offspring. Moreove~, this behef canno~ be . extended to the strongly marked a~vantages which Invariably follow, as far as my expenence serves, froin intercrossing the indivi: als of the same variety or of distinct varieties, if . ese have been subjected during some generations to different conditions. -· |
