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Show 304 TRANSMITTED EFFECTS OF A CROSS. CHAP. VIII. open ground and near to other plants of heartsease; and as both produced an abundance of very fine cap~ sules, the flowers on both were certainly cross-fertilised by insects. Seeds were collect d from both plants, and seedlings raised from the1n. Those fro1n tho crossed plants flowered in all three pots before those from the self-fertilised plants; and wh n fully grown the former were to the latter in height as 100 to 82. As both sets of plants were the product of cross-fertilisation, the difference in their growth and period of flowering was clearly due to their par nts having been of crosseu and self-fertilised parentagA; and it is equally clear that they tr~nsmitted difD rent constitutional powers to their off~pring, the grandchildren of the plants which were originally crossed and self-fertilis d. Thirdly, the Sweet Pea (Lathyrus odoratus) habitually fertilisos itself in this country. As I possessed plants, the parents and grandparents of which had been artificially crossed and other plants descended from the same parents which had been self-fertilised for many previous generations, these two lots of plants were allowed to fertilise the1nselves under a net, and their self-fertilised seeds saved. The seedlings thus raised were grown in co1npetition with each other in the usual manner, and differed in their powers of growth. Those from the self-fertilised ~ plants which had been crossed during the two previous generations were to those from the plants self-fertilised during many previous generations in height as 100 to 90. These two lots of seeds were likewise tried Ly being sown under very unfavourable conditions in pool' exhausted soil, and the plants whose grandparents and great-grandparents had been crossed showed in an unmistakable manner their superior constitutional vigour. In this case, as in that of the heartsease there could be no doubt that ' CHAP. VIII. TRANSMITTED EFFECTS OF A CROSS. 305 the advantage derived from a cross between two plants was not confined to the offspring of the first generation. That constitutional vigour clue to cross-parentage is transmitted for n1any generations may also be inferred as highly probable, fro1n some of Andrew Knight's varieties of the common pea, which were raised by crossing distinct varieties, after which time they no doubt fertilised themselves in each succeeding generation. These varieties lasted for upwards of sixty years, "but their glory is now departed."* On the other hand, most of the varieties of the com1non pea, which there is no reason to suppose owe their origin to a cross, have had a n1uch shorter existence. Some also of Mr. Laxton's varieties produced by artificial crosses have retained their astonishing vigour and luxuriance for a considerable nu1nber of generations; but as Mr. Laxton informs me, his experience does not extend beyond twelve generations, within which period he has never perceived any diminution of vigour in his plants. An allied point may be here noticed. As the force of inheritance is strong with plants (of which abundant evidence could be given), it is almost certain that seedlings from the same capsule or from the same plant would tend to inherit nearly the same constitution; and as the advantage from a cross depends on the plants which are crossed differing somewhat in constitution, it may be inferred as probable that under similar conditions a cross between the nearest relations woulcl not benefit the offspring so much as one between nonrelated plants. In support of this conclusion we hav(' some evidence, as Fritz JHiiller has shown by hi.· * See tho evidence on this head tlcati•m,' chap. ix. vol. i. 2nll e1lit. in my • Variation under Domed- p. 397. X |