82 CAUSES WHICH CHECK Chap. XYI. 



tliis was the case. It lias often been noticed that the 

 dovecot pigeon seems to have an actual aversion towards the 

 several fancy breeds ; ^ yet all have certainly sprung from a 

 common progenitor. The Rev. 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 couj)ling 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, 1 

 know of no well-ascertained case with animals. This fnct, seeing 

 the great difference in structure between some breeds of i^igeons, 

 fowls, pigs, dogs, &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 is not a safe guide 

 for predicting whether or not they will breed together, — some 

 closely allied species when crossed being utterly sterile, and otliers 

 which are extremely unlike being moderately fertile. I have said 

 that no case of sterility in crossed races rests on satisfactory evi- 

 dence ; but here is one which at first seems trustworthy, Mr. 

 Youatt,^ 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,^*' 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 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. 



In the case of plants Gartner states that he fertilised thirteen 

 heads (and subsequently nine others) on a dwarf maize bearing 



* * The Dovecote,* by the Rev. E. S. » * Cattle,' p. 202. 



Dixon, p. 155 ; Bechstein, ' Natur- ^° Mr. J, Wilkinson, in ' Remarks 



gesch. Deutschlands,' Band iv., 1795, addressed to Sir J Sebright," 1820, 



s. 17. p. 38. 



