OCR Text |
Show 270 HYBRIDISM. CHAI:'. Vlll. in the sterility of a multitude of species. T~e evidence is also derived from hostile witnesses, who In all other c~ses ~onsider fertility and sterility as saf? criterions of specific distinction. G~rtne~ kept dunng seve~al years a dwarf kind of maize WI~h yellow seeds, and . a tall variety with red seeds, growmg near each other In his garden; and although these plants have separated sexes, they never naturally crossed. He then fertilised thirteen flowers of the one with the pollen of the other ; but only a single head produced any seed, and this one head produced only five grains. Manipulation in this case could not have been injurious, as the plants have separated sexes. No one, I believe, has suspected that these varieties of maize are distinct species ; and it is important to notice that the hybrid plants thus raised were themselves perfectly fertile; so that even Gartner did not venture to consider the two varieties as specifically distinct. Girou de Buzareingues crossed three varieties of gourd, which like the maize has separated sexes, and he asserts that their mutual fertilisation is by so much the less easy as their differences are greater. How far these experiments may be trusted, I know not; but the forms experimentised on, are ranked by Sagaret, who mainly founds his classification by the test of infertility, as varieties. The following case is far more remarkable, and seems at first quite incredible ; but it is the result of an astonishing number of experiments made during many years on nine species of V erbascum, by so good an observer and so hostile a witness, as Gartner: namely, that yellow and white varieties of the same species of V erbascum · when intercrossed produce less seed, than do eitb~r coloured varieties when fertilised with pollen from theu own coloured flowers. Moreover, he asserts that when CHAP. VIII. FERTILITY OF MONGRELS. 271 yellow and white varieties of one species are crossed with yellow and white varieties of a distinet species more seed is produced by the crosses between the sam~ coloured flowers, than between those which are differently coloured. Yet these varieties of V erbascum present no ot.her difference besides the mere colour of the flower ; and one variety can sometimes be raised from the seed of the other. From observations which I have made on certain ~arieties of hollyhock, I am inclined to suspect that they present analogous facts. Kolreuter, whose accuracy has been confirmed by every subsequent observer, has proved the remarkable fact, that one variety of the common tobacco is more fertile, when crossed with a widely distinct species, than are the other varieties. He experimentised on five for~s, which are commonly reputed to be varieties, and whiCh he tested by the severest trial, namely, by reciprocal crosses, and he found their mongrel offspring perfectly fertile. But one of these five varieties, when used either as father or mother, and crossed with the Nicotiana glutinosa, always yielded hybrids not so sterile as those which were produced from the four other varieties when crossed with N. glutinosa. Hence. the r~productive system of this one variety must have been In some manner and in some degree modified. .F~om these facts ; from the great difficulty of ascertairung the infertility of varieties in a state of nature, for a supposed variety if infertile in any degree would generally be ranked as species ; from man selecting only e~ternal characters in the production of the most distinct domestic varieties, and from not wishing or being able to produce recondite and functional differences in th~ reproductive system ; from these several consideratiOns and facts, I do not think that the very general |