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.” (8/3.  See the evidence on this head in my ’Variation under Domestication’ chapter 9 volume 1 2nd edition page 397.) On the other hand, most of the varieties of the common pea, which there is no reason to suppose owe their origin to a cross, have had a much shorter existence.  Some also of Mr. Laxton’s varieties produced by artificial crosses have retained their astonishing vigour and luxuriance for a considerable number 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 would not benefit the offspring so much as one between non-related plants.  In support of this conclusion we have some evidence, as Fritz Muller has shown by his valuable experiments on hybrid Abutilons, that the union of brothers and sisters, parents and children, and of other near relations is highly injurious to the fertility of the offspring.  In one case, moreover, seedlings from such near relations possessed very weak constitutions. (8/4.  ‘Jenaische Zeitschrift fur Naturw.’  B. 7 pages 22 and 45 1872 and 1873 pages 441-450.) This same observer also found three plants of a Bignonia growing near together. (8/5.  ‘Botanische Zeitung’ 1868 page 626.) He fertilised twenty-nine flowers on one of them with their own pollen, and they did not set a single capsule.  Thirty flowers were then fertilised with pollen from a distinct plant, one of the three growing together, and they yielded only two capsules.  Lastly, five flowers were fertilised with pollen from a fourth plant growing at a distance, and all five produced capsules.  It seems therefore probable, as Fritz Muller suggests, that the three plants growing near together were seedlings from the same parent, and that from being closely related they had little power of fertilising one another. (8/6.  Some remarkable cases are given in my ‘Variation under Domestication’ chapter 17 2nd edition volume 2 page 121, of hybrids of Gladiolus and Cistus, any one of which could be fertilised by pollen from any other, but not by its own pollen.)

Lastly, the fact of the intercrossed plants in Table 7/A not exceeding in height the self-fertilised plants in a greater and greater degree in the later generations, is probably the result of their having become more and more closely inter-related.