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Show 104 CAUSES WHICH CITECK CHAP. XVI. certainly sprung from a common progenitor. The Rev. vV. D. Fox informs me that his flocks of ·white and common Chinese geese kept di tinct. These facts and statement.-,, though some of them are incapable of 1 roof, re;:;tiug only on the opinion of experienced observers, show that some dome tic races are led by different habits of life to keep to a certain extant separate, and that others prefer coupling "Vvith their own kind, in the same manner as species in a state of nature, though in a much less degree. With respect to sterility from the crossing of domc.·tic races, I know of no well-ascertained case ·with animals. This fact, ::ecing the great difference in structure between some breeds of pigeons, fowls, pigR, clogs, &c., is extraordinary, in contrast with the sterility of many closely allied natural species when crossed; but we .·hall h rcaftcr attempt to show that it is not so extraordinary as it at first appears. And it 111ay be well here to recall to mind that the amount of external difference between two species will not sn.fcly guide us in foretelling whether or not they will breed togcthcr,-somc closely allied specie.· when crossed being utterly sterile, and others which are extremely unlike being moderately f0rtile. I have said that no c.'t e of sterility in crossed races rests on sati.'faetory evidence; but here is one which at fir. t seems trustworthy. lVIr. Youatt/ and a better authority cannot be quoted, . tateR, that formerly in Lancashire crosses were frequently made between longhorn and shorthorn cattle; the first cross was excellent, but the produce was uncertain; in the third or fourth generation the cows were bad milkers; "in addition to which, there wa much lmccrtainty whether the cows would conceive; and full one-third of the cows among some of these half-brcds failed to be in calf." This at fir t seems a good case; but Mr. Wilkinson states,10 that a breed derived from this same cross was actually established in another part of England; and if it had failed in fertility, the fact would surely have been noticud. Moreover, supposing that Mr. Youatt had proved his case, it might be argued that the sterility was wholly clue to the two parent-breeds being descended from primordially distinct species. · I will give a case with plants, to show how difficult it is to get sufficient evidence. Mr. Sheriff, who has boon so successful in the formation of new races of wheat, fertilised the Hopctolm with the Talavera; in the first and second generations the produce was intermediate in character, but in the foillth generation "it was found to consist of many varieties; nine-tenths of the florets proved barren, and many of the seeds seemed shrivelled abortions, void of vitality, and the whole race was evidently verging to extinetion." 11 Now, considering how little these 9 ' Cattle,' p. 202. 10 Mr. J. Wilkinson, in • Remarks addressed to Sir J. Sebright,' 1820, p. 38. 11 ' Gardenc·r's Chronicle,' 1858, p. 771. CHAP. XVI. THE CROSSING OF VARIETIES. 105 varieties of wheat differ in any important character, it seems to me very improbable that the sterility Tesultecl, as Mr. Sheriff thought, from the cross, but from some quite distinct cause. Until such experiments are many times repeated, it would be rash to trust them; but unfortunately they have been rarely tried even once with sufficient care. Gartner has recorded a more remarkable and trustworthy case : he fertili,.cd thirteen panicles (and subsequently nine others) on a dwarf maize bearing yellow scec1 12 with pollen of a tall maize having red seed ; and one head alone produced good seed, only five in number. Though these plants arc moncecious, and therefore do not require castration, yet I should have suspected some accident in the manipulation had not Giirtncr expressly stated that he had during many years grown these two varieties together, and they did not spontaneously cross; and this, considering that the plants arc moncecious and abound with pollen, and arc well known generally to cross freely, seems explicable only on the belief that these two varieties are in some degree mutually infertile. The hyurid plant.- raised from the above five seed wore intermediate in structure, extremely variable, and perfectly fcrtile.13 ~ o one, I believe, has hitherto suspected that these varieties of maize are distinct species ; but bad the hybrids been in the least stetile, no doubt Gartner would at once have so classed them. I may here remark, that with undoubted species there is not necessarily any close relation between the sterility of a first cross and that of the hybrid offspring. Some species can be crossed with facility, but produce utterly sterile hybrids; others can be eros. eel with extreme difficulty, but the hybrids when produced arc moderately fertile. I am not aware, however, of any instance quite like this of the maize with natural species, namely, of a first cross made with difficulty, but yielding perfectly fertile hybrids. The following case is much more remarkable, and evidently perplexed Gartner, whose strong wish it was to draw a broad line of distinction between species and varieties. In the genus Verbascum, he made, during eighteen years, a vast number of experiments, and crossed no less than 108G flowers and counted their seeds. Many of these experiments consisted in cr·ossing white and yellow varieties of both V. lychnitis and V. bta,/t,~riu with nine other species and their hybrids. That the white and yellow flowered plants of these two species are really varieties, no one has doubted; and Gartner actually raised in the case of both species one variety from the seed of the other. Now in two of his works 14 he distinctly asserts that crosses between similarly-coloured flowers yield more seed than between dissimilarly-coloured; so that the yellow-flowered variety of either species (and conversely with the white-flowered variety), when crossed with pollen of its own kind, yields more seed than when crossed with that of the white variety; and so it is when differently coloured species are crossed. The general results may be seen in the Table at the 12 ' Basta.rut'rzcugung,' s. 87, 169. See ulso the 1\~ble at the end of volume. 13 • Bastarderzeugung,' s. 87, 577. H 'Kcnntniss dcr Bcfruchtung,' s. 137 ; • Bastarderzeugung,' s. 92, 181. On raising the two variutics from seed sees. 307. |