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Show 438 GENERAL . RESULT H. CHAP. XII. parallel case .. With various other pl~nts, the wonclerful effects of a cross with a> fresh stqck 1nay be seen in Table C. Several accounts have also been published.* of the extraordinary growth of seedlings fro1n a cross between two varieties of the same species, some of which are known never to fertilise thomsel ves; so that here neither self-fertilisation nor relationship evon in a I remote degree can have come into play. vVe Jnay therefore conclude that the above two propositions· are true,-that cross-fertilisatio:t;l is generally beneficial and self-fertilisation injurious to the offspring. That certain plants, for instance, · Viola tricolor, Digitalis purpurea, Sarotha1nnus scoparius, Cyclamen persicum, &c., 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 kind bas been observed in our domestic : ~nimals; but then we 1nust rem·ember that the closest possible intol'brooding with such animals, that is, b tween brothers ancl 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 fro1n my experiments that the increase if any is far fro1n rapid. 1\..ft r plants have been propagated by self-fertilisation for several gener~tions, a single cross with a fresh stock restores then pristine vigour; and we have a strictly analogous result with our domestic animals. t Tho good effects of cross-fertilisation are transmitted by ph1:nts to the next g 'neration ; and judging fro1n the varieties of * Sec 'Variation under Do:. mestiration,' ch. xix. 2nd edit. vol. ii. p. 159. ' t Ibid. ch. xix. 2nd edit. vol. ii. p. 159. CHAP. XTI. GENERAL RESULTS. 439 tb~ COn1mon pea, to 1nany succeedi11g generations. But th1s n1~y 1nerely be that crossed plants of the first generation are extremely vigorous, an<l transmit their vigour, like any other character, to their successors. Notwithstanding the evil which 1na.ny plants suffer fro1n self-fertilisation, they can be thus propa0·ated under favourable conditions for many generatio~s, as shown by so.me of ~y experiments, and more especially by the s~rv~ val dunng at least half a century of the same vanetie.s of tho common pea and sweet-pea. The Saine conclusion probably holds good with several other exo~i? pl~nts,. which are never or most rarely crossfertilised In th1s country. But all these plants, as far as they have been tried, profit greatly by a cross with a fresh stock. Soine few plants, for instance, Ophrys apift3ra, 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, &c.* (their flowers never producing seeds), should make us doubt that seminal generation must have some great advantage, as it is the common plan followed Ly nature. vVhether any species has been reproduced asexually from a very ren1ote 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. Andrew Knight formerly maintained that under these circumstances they always become weakly, but thi:s conc]usion * I have given several rases in tion,' ch. xviii. 2nd edit. vol. ii. my ' Variation under Domestica- p. 152. |