Chap. IX.] DEGREES OF STERILITY. 5 



ten generations, yet he asserts positively that their 

 fertility never increases, but generally decreases greatly 

 and suddenly. 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 inter- 

 breeding. I have made so many experiments 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 must 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 tliis would probably be injurious to their fertility, 

 already lessened by their hybrid origin. I am strength- 

 ened in this conviction by a remarkable statement 

 repeatedly made by Gartner, namely, that if even the 

 less fertile hybrids be artificially fertilised with hybrid 

 pollen of the same kind, their fertility, notwithstanding 

 the frequent ill effects from manipulation, sometimes 

 decidedly increases, and goes on increasing. Now, in 

 the process of artificial fertilisation, pollen is as often 

 taken by chance (as I know from my own experience) 



