162 HYBRIDISM. CuAP. XIX. 



diminishing, generally adds to tlie fertility of tlie first 

 union and of the mongrel offspring. Whether all the more 

 widely distinct domestic varieties are invariably quite fertile 

 when crossed, we do not positively know ; much time and 

 trouble would be requisite for the necessary experiments, and 

 many difficulties occur, such as the descent of the various 

 races from aboriginally distinct species, and the doubts 

 whether certain forms ought to be ranked as species or 

 varieties. Nevertheless, the wide experience of practical 

 breeders proves that the great majority of varieties, even if 

 some should hereafter prove not to be indefinitely fertile 

 inter se, are far more fertile when crossed, than the vast 

 majority of closely allied natural species. A few remarkable 

 cases have, however, been given on the authority of excellent 

 observers, showing that with plants certain forms, which un- 

 doubtedly must be ranked as varieties, yield fewer seeds when 

 crossed than is natural to the parent-species. Other varieties 

 have had their reproductive powers so far modified that they 

 are either more or less fertile than their parents, when crossed 

 with a distinct species. 



Kevertheless, the fact remains indisputable that domesti- 

 cated varieties, of animals and of plants, which differ greatly 

 from one another in structure, but which are certainly 

 descended from the same aboriginal species, such as the races 

 of the fowl, pigeon, many vegetables, and a host of other 

 productions, are extremely fertile when crossed ; and this 

 seems to make a broad and impassable barrier between 

 domestic varieties and natural species. But, as I will now 

 attempt to show, the distinction is not so great and over- 

 whelmingly important as it at first aji^^ears. 



On the Difference in Fertility between Varieties and Species wJien 



crossed. 



This work is not the proper place for fully treating the 

 subject of hybridism, and I have already given in my 'Origin 

 of Species ' a moderately full abstract. I will here merely 

 enumerate the general conclusions w^hich may be relied on, 

 and which bear on our present point. 



Firstly, the laws governing the production of hybrids are 





