Chap. IX.] DEGREES OF STERILITY. 5 



ten generations, yet he asserts positively that their fer- 

 tility 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 constitu- 

 tion 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 di- 

 minished in nearly all these cases by an independent 

 cause, namely, by too close interbreeding. I have made 

 so many experiments and collected so many facts, show- 

 ing on the one hand that an occasional cross with a dis- 

 tinct individual or variety increases the vigour and fer- 

 tility 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 this would probably 

 be injurious to their fertility, already lessened by their 

 hybrid origin. I am strengthened 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. N^ow, in the process of artificial fertilisa- 

 tion, pollen is as often taken by chance (as I know from 

 my own experience) from the anthers of another flower. 



