Chap. XV.] RECAPITULATION. 269 * 



united, they yield few or no seed, and their offspring are 

 more or less sterile ; and these forms belong to the same 

 undoubted species, and differ from each other in no respect 

 except in their reproductive organs and functions. 



Although the fertility of varieties when intercrossed 

 and of their mongrel offspring has been asserted by so 

 many authors to be universal, this cannot be considered 

 as quite correct after the facts given on the high 

 authority of Gartner and Kolreuter. Most of the varieties 

 which have been experimented on have been produced 

 under domestication; and as domestication (I do not 

 mean mere confinement) almost certainly tends to elimi- 

 nate that sterility which, judging from analogy, would 

 have affected the parent-species if intercrossed, we ought 

 not to expect that domestication would likewise induce 

 sterility in their modified descendants when crossed. 

 This elimination of sterility apparently follows from 

 the same cause which allows our domestic animals to 

 breed freely under diversified circumstances ; and this 

 again apparently follows from their having been gradu- 

 ally accustomed to frequent changes in their conditions 

 of life. 



A double and parallel series of facts seems to throw 

 much light on the sterility of species, when first crossed, 

 and of their hybrid offspring. On the one side, there is 

 good reason to believe that slight changes in the con- 

 ditions of life give vigour and fertility to all organic 

 beings. We know also that a cross between the distinct 

 individuals of the same variety, and between distinct 

 varieties, increases the number of their offspring, and 

 certainly gives to them increased size and vigour. This 

 is chiefly owing to the forms which are crossed having 

 been exposed to somewhat different conditions of life ; 



