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Show 236 HYBRIDISM. [CHAP. VIII. organic beings in some degree sterile; and that grea~er crosses that is crosses between males and females which have b~come widely or specifically different, produce hybrids which are generally st~rile in so~e d~gree. r. cannot persuade myself th~t this parallelism IS an accident or an illusion. Both series of facts seem to be conn~cte.d together by some common.bu_t unkno.wn bond, which IS essentially related to the principle of hfe. Fertility of Varieties w/~;en cr·ossed, and ~~ t!~;eir Mongrel offspring.-It may be urged, as a most forcible arguInent that there must be some essential distinction betwee~ species and varieties, and tha~ there muRt be. so;ne error in all the foregoing remarks, Inasmuch as vanetles, however much they may differ fro1n each other in external appearance, cross with perfect. facility, ~nd. yield perf~ctly fertile offspring. I fully admit that this IS almost Invariably the case. But if we look to varieties produced under nature, we are immediately involved in hopeless difficulties ; for if two hitherto reputed varieties be found in any degree sterile together, they are at once ranked by most naturalists as species. For instance, the blue and red pimpernel, the prhnrose and ~owslip, w~i~h are co~sidered by many of our best botanists as vaneties, are said by Gartner not to be quite fertile when crossed, and he consequently ranks them as undoubted species. If we thus argue in a circle, the fertility of all varieties produced under nature will assuredly have to be granted. If we turn tQ 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 than other dogs with foxes, or that certain South American indigenous domestic dogs do not readily cross with European dogs, the explanation which will occur to every one, and probably the true one, is that these dogs have descended from several aboriginally distinct species. Nevertheless the perfect fertility of so many domestic varieties, differing widely from eaeh other in appearance, for instance of the pigeon or of the cabbage, is a remarkable fact; more OIIAP. VIII.] FERTILITY OF MONGRELS. 237 especially when we reflect how many species there are which, tho"?gh rese~bling each other most closely, ar~ utterly sterile when Intercrossed. Several considerations however, render the fertility of domestic varieties less re: markable than 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 do~ mcstication tends to eliminate sterility in the successive gen~rations ?f hJ:brids, which were at first only slightly stenle; an~ :f this be so, w~ surely ou~ht not to expect to find sterihty 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 do::Uestication by man's methodical and unconscious power of selection for his own use and pleasure: he neither wishes to select' nor could select, slig~t ~iffere~ces in the reproductiv~ system, or other constitutional differences correlated with t~e reproductive systmn. He supplies his several varieties With the same food; treats them in nearly the same ma~ner, and does not w~sh to alter their general habits of ~If e. N a~ure acts uniformly and slowly during vast per!ods of time on the whole organisation, in any way whiCh ?Jay b~ for each creature's own good; and thus she n1ay, m~her dire?tly, or more pro~ably indirectly, through correlation, modify the reproductive system in the several ~escendants 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 ~pecies were invariably fertile when intercrossed. But 1t .seems to me impossible to resist the evidence of the exis.tence of a c~rtain a.mou~t of sterility in the few fol~ OWing cases, whiCh I Will briefly abstract. The evidence Is at least as , good as that from which we believe in the sterility of a multitude of species. The evidence is, also, |