in such plants as Lobelia ramosa, Digitalis purpurea, etc., renders the aid of insects almost indispensable for their fertilisation; and bearing in mind the prepotency of pollen from a distinct individual over that from the same individual, such plants will almost certainly have been crossed during many or all previous generations.  So it must be, owing merely to the prepotency of foreign pollen, with cabbages and various other plants, the varieties of which almost invariably intercross when grown together.  The same inference may be drawn still more surely with respect 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 my 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 Ipomoea 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 experiment 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 parallel case.  With various other plants, the wonderful effects of a cross with a fresh stock may be seen in Table 7/C.  Several accounts have also been published of the extraordinary growth of seedlings from a cross between two varieties of the same species, some of which are known never to fertilise themselves; so that here neither self-fertilisation nor relationship even in a remote degree can have come into play. (12/1.  See ‘Variation under Domestication’ chapter 19 2nd edition volume 2 page 159.) We may therefore conclude that the above two propositions are true,—­that cross-fertilisation is generally beneficial and self-fertilisation injurious to the offspring.