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Show 268 HYBRIDISM. CHAP. VIII. ment, that there must be some essential distinction b et ween speci.e s and varieties' and that the. re must be some error I.n a ll the foregoing remark.s , Inasmuch as var1.e t•1 es, h owev er much they may. differ from .e a. ch ot he r I.n ext ern al appearance' cross With perfect fa.C ility; and yield perfectly fertile offspring. I ful~y adm1t that this is almost invariably the case. But 1~ we lo_ok to va r1·e t•1 es pro duced unde.r natu. re, we ar. e Immed. iately involved in hopeless difficulties; for 1f two h~therto reputed varieties be found in any degree stenle .together, they are' at once ranked by most n~turahsts as species. For instance, the blue and red ~1mpernel, the primrose and cowslip, which _ar~ cons1der~d by many of our best botanists as vanetws, are sa1d by Gartner not to be quite fertile when crossed, and he consequently ranks them as u~d_oubted speci~s .. If we thus argue in a circle, the fert1hty of all varieties produced under nature will assuredly have to be granted. If we turn to varieties, produced, or supposed to have been produced, under domestication, we are . still involved in doubt. For when it is stated, for Instance, that the German Spitz dog unites more easily ~han other dogs with foxes, or that certa~n South ~mencan indigenous domestic dogs do not read1ly cross w1th European dogs, the explanation which will occur to every one, and probably the true one, is that t~1e~e dogs h~ve descended from several aboriginally d1st1nct speme~. Nevertheless the perfect fertility of so man~ domestic varieties, differing widely from each other 1n appea~ance, for instance of the pigeon or of the cabbage, 18 a remarkable fact; more especially when we reflect how many species there are, which, though resembling each other most closely, are utterly sterile when intercrossed. Several cons1· a erat .I ons, h ow ever ' render the fertility of domestic varieties less remarkable than CHAP. VIII. FERTILITY OF MONGRELS. 269 at first appears. It can, in the first place, be clearly shown that mere external dissimilarity between two species does not determine their greater or lesser degree of sterility when crossed; and we may apply the same rule to domestic varieties. In the second place, some eminent naturalists believe that a long course of domestication tends to eliminate sterility in the successive generations of hybrids, which were at first only slightly sterile ; and if this be so, we surely ought not to expect to find sterility both appearing and disappearing under nearly the same conditions of life. Lastly, and this seems to me by far the most important consideration, new races of animals and plants are produced under domestication by man's methodical and unconscious power of selection, for his own use and pleasure : he neither wishes to select, nor could select, slight differences in the reproductive system, or other constitutional differences correlated with the reproductive system. He supplies his several varieties with the same food ; treats them in nearly the same manner, and does not wish to alter their general habits of life. Nature acts uniformly and slowly during vast periods of time on the whole organisation, in any way which may be for each creature's own good; and thus she may, either directly; or more probably indirectly, through correlation, modify the reproductive system in the several descendants from any one species. Seeing this difference in the process of selection, as carried on by man and nature, we need not be surprised at some difference in the result. I have as yet spoken as if the varieties of the same species were invariably fertile when intercrossed. But it seems to me impossible to resist the evidence of the ~xistence of a certain amount of sterility in the few following cases, which I will briefly abstract. The evidence is at least as good as that from which we believe .. |