Chap. XV.] RECAPITULATION. 269 



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

 more or less sterile; and these formsbelong to the same 

 .undoubted species, and differ from each otherjn no re- 

 spect except in their reproducfive"organs"and functiojis. 



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 varie- 

 ties which have been experimented on have been pro- 

 duced under domestication; and as domestication (I do 

 not mean mere confinement) almost certainly tends to 

 eliminate 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; 



