DEGREES OF STERILITY 301 



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 

 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 experimental- 

 ists in great nurribers ; 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, nothwithstanding the frequent ill effects from manip- 

 ulation, sometimes decidedly increases, and goes on increas- 

 ing. Now, in the process of artificial fertilisation, pollen is 

 as often taken by chance (as I know from my own experi- 

 ence) from the anthers of another flower, as from the anthers 

 of the flower itself which is to be fertilised; so that a cross 

 between two flowers, though probably often on the same 

 plant, would be thus effected. Moreover, whenever compli- 

 cated experiments are in progress, so careful an observer as 

 Gartner would have castrated his hybrids, and this would 

 have ensured in each generation a cross with pollen from 

 a distinct flower, either from the same plant or from another 

 plant of the same hybrid nature. And thus, the strange fact 

 of an increase of fertility in the successive generations of 

 artificially fertilised hybrids, in contrast with those spon- 

 taneously self-fertilised, may, as I believe, be accounted for 

 by too close interbreeding having been avoided. 



Now let us turn to the results arrived at by a third most 

 experienced hybridiscr, namely, the Hon. and Rev. W. Iler- 



