10 THE ORIGIN OF SPECIE^ 



It is certain, on the one hand, that the sterility of 

 various species when crossed is so different in degree and 

 graduates away so insensibly, and, on the other hand, 

 that the fertility of pure species is so easily affected by 

 various circumstances, that for all practical purposes it 

 is most difficult to say where perfect fertility ends and 

 sterility begins. I think no better evidence of this can 

 be required than that the two most experienced observ- 

 ers who have ever lived, namely Kdlreuter and Gartner, 

 arrived at diametrically opposite conclusions in regard to 

 some of the very same forms. It is also most instructive 

 to compare — but I have not space here to enter on de- 

 tails — the evidence advanced by our best botanists on the 

 question whether certain doubtful forms should be ranked 

 as species or varieties, with the evidence from fertility 

 adduced by different hybridizers, or by the same observer 

 from experiments made during different years. It can 

 thus be shown that neither sterility nor fertility affords 

 any certain distinction between species and varieties. 

 The evidence from this source graduates away, and is 

 doubtful in the same degree as is the evidence derived 

 from other constitutional and structural differences. 



In regard to the sterility of hybrids in successive 

 generations; though Gai-tner was enabled to rear some 

 hybrids, carefully guarding them from a cross with either 

 pure parent, for six or seven, and in one case for ten 

 generations, yet he asserts positively that their fertility 

 never increases, but generally decreases greatly and sud- 

 denly. With respect to this decrease, it may first be no- 

 ticed that when any deviation in structure or constitution 

 is common to both parents, this is often transmitted in 

 an augmented degree to the offspring; and both sexual 



i 



n 



