Chap. IX.] DEGREES OF STERILITY. 237 



sensibly, 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 observers who have ever lived, 

 namely KSlreuter 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 

 details — the evidence advanced by our best botanists on the question 

 whether certain doubtful forms should be ranked as species 01 

 varieties, with the evidence from fertility adduced by different 

 bybridisers, 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 Gartner was enabled to rear some hybrids, carefully guard- 

 ing 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 noticed 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 elements in hybrid plants are already 

 affected in some degree. But I believe that their fertility has been 

 diminished in nearly all these cases by an independent cause, 

 namely, by too close interbreeding. I have made so many experi- 

 ments and collected so many facts, showing on the one hand that 

 an occasional cross with a distinct individual or variety increases 

 the vigour and fertility of the offspring, and on the other hand that 

 very close interbreeding lessens their vigour and fertility, that I 

 cannot doubt the correctness of this conclusion. Hybrids are seldom 

 raised by experimentalists in great numbers ; and as the parent- 

 species, or other allied hybrids, generally grow in the same garden, 

 the visits of insects muat be carefully prevented during the flowering 

 season: hence hybrids, if left to themselves, will generally be 

 fertilised during each generation by pollen from the same flower ; 

 and this would probably be injurious to their fertility, already 

 lessened by their hybrid origin. I am strengthened in this con- 

 viction by a remarkable statement repeatedly made by Gartner, 

 namely, that if even the less fertile hybrids be artificially fertilised 



