278 THE ORIGIN OF SPECIES 



conclusion, for when the forms are illegitimately united, 

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

 or less sterile; and these forms belong to the same un- 

 doubted 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 au- 

 thority of Gartner and Kdlreuter. 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 elim- 

 inate 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 gradually 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 condi- 

 tions of life give vigor and fertility to all organic beings. 

 We know also that a cross between the distinct individ- 

 uals of the same variety, and between distinct varieties, 

 increases the number of their offspring, and certainly 

 gives to them increased size and vdgor. This is chiefly 

 owing to the forms which are crossed having been ex- 



