



104: CAUSES WHICH CHECK Chap. XVI. 



certainly sprung from a common progenitor. The Eev. "W. D. 

 Fox informs me that his flocks of white and common Chinese 

 geese kept distinct. 



These facts and statements, though some of them are incapable 

 of proof, resting only on the opinion of experienced observers, 

 show that some domestic races are led by different habits of life 

 to keep to a certain extent separate, and that others prefer 

 coupling with 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 domestic races, I know of 

 no well- ascertained case with animals. This fact, seeing the great differ- 

 ence in structure between some breeds of pigeons, fowls, pigs, clogs, 

 &c., is extraordinary, in contrast with the sterility of many closely allied 

 natural species when crossed; but we shall hereafter attempt to show 

 that it is not so extraordinary as it at first appears. And it may be well 

 here to recall to mind that the amount of external difference between 

 two species will not safely guide us in foretelling whether or not they will 

 breed together, — some closely allied species when crossed being utterly 

 sterile, and others which are extremely unlike being moderately fertile. 

 I have said that no case of sterility in crossed races rests on satisfactory 

 evidence ; but here is one which at first seems trustworthy. Mr. Youatt, 9 

 and a better authority cannot be quoted, states, 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, 

 was much uncertainty whether the cows would conceive ; and full one-third 

 of the cows among some of these half-breds failed to be in calf." This 

 at first 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 w^ould surely have been noticed. 

 Moreover, supposing that Mr. Youatt had proved his case, it might be 

 argued that the sterility was wholly due 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 suffi- 

 cient evidence. Mr. Sheriff, who has been so successful in the forma- 

 tion of new races of wheat, fertilised the Hopetoun with the Talavera; 

 in the first and second generations the produce w r as intermediate in cha- 

 racter, but in the fourth 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 extinction." 11 Now, considering how little these 



9 'Cattle,' p. 202. 



10 Mr. J. Wilkinson, in 'Bemarks addressed to Sir J. Sebright/ 1820, p. 38. 



11 ' Gardener's Chronicle,' 1858, p. 771. 







* 



* 





