That certain plants, for instance, Viola tricolor, Digitalis purpurea, Sarothamnus scoparius, Cyclamen persicum, etc., which have been naturally cross-fertilised for many or all previous generations, should suffer to an extreme degree from a single act of self-fertilisation is a most surprising fact.  Nothing of the kkind has been observed in our domestic animals; but then we must remember that the closest possible interbreeding with such animals, that is, between brothers and sisters, cannot be considered as nearly so close a union as that between the pollen and ovules of the same flower.  Whether the evil from self-fertilisation goes on increasing during successive generations is not as yet known; but we may infer from my experiments that the increase if any is far from rapid.  After plants have been propagated by self-fertilisation for several generations, a single cross with a fresh stock restores their pristine vigour; and we have a strictly analogous result with our domestic animals. (12/2.  Ibid chapter 19 2nd edition volume 2 page 159.) The good effects of cross-fertilisation are transmitted by plants to the next generation; and judging from the varieties of the common pea, to many succeeding generations.  But this may merely be that crossed plants of the first generation are extremely vigorous, and transmit their vigour, like any other character, to their successors.

Notwithstanding the evil which many plants suffer from self-fertilisation, they can be thus propagated under favourable conditions for many generations, as shown by some of my experiments, and more especially by the survival during at least half a century of the same varieties of the common pea and sweet-pea.  The same conclusion probably holds good with several other exotic plants, which are never or most rarely cross-fertilised in this country.  But all these plants, as far as they have been tried, profit greatly by a cross with a fresh stock.  Some few plants, for instance, Ophrys apifera, have almost certainly been propagated in a state of nature for thousands of generations without having been once intercrossed; and whether they would profit by a cross with a fresh stock is not known.  But such cases ought not to make us doubt that as a general rule crossing is beneficial, any more than the existence of plants which, in a state of nature, are propagated exclusively by rhizomes, stolons, etc. (their flowers never producing seeds), (12/3.  I have given several cases in my ‘Variation under Domestication’ chapter 18 2nd edition volume 2 page 152.) (their flowers never producing seeds), should make us doubt that seminal generation must have some great advantage, as it is the common plan followed by nature.  Whether any species has been reproduced asexually from a very remote period cannot, of course, be ascertained.  Our sole means for forming any judgment on this head is the duration of the varieties of our fruit trees which have been long propagated by grafts or buds.