DEGREES OF STERILITY. 268 



a distinct individual or variety increases the vigor and fer- 

 tility of the offspring, and on the other hand that very close 

 interbreeding lessens their vigor and fertility, that I cannot 

 doubt the correctness of this conclusion. Hybrids are sel- 

 dom 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 pre- 

 vented during the flowering season : hence hybrids, if left to 

 themselves, will generally be fertilized during each genera- 

 tion by pollen from the same flower ; and this would prob- 

 ably 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 fertilized 

 with hybrid pollen of the same kind, their fertility, notwith- 

 standing the frequent ill effects from manipulation, some- 

 times decidedly increases, and goes on increasing. Now. in 

 the process of artificial fertilization, pollen is as often taken 

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

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

 itself which is to be fertilized; so that a cross between two 

 flowers, though probably often on the same plant, would be 

 thus effected. Moreover, whenever complicated experiments 

 are in progress, so careful an observer as Gartner would have 

 castrated his hybrids, and this would have insured 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 fertilized 

 hybrids, in contrast with those spontaneously self-fertilized, 

 may, as I believe, be accounted for by too close interbreed- 

 ing having been avoided. 



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

 experienced hybridizer, namely, the Hon. and Rev. W. 

 Herbert. He is as emphatic in his conclusion that some 

 hybrids are perfectly fertile — as fertile as the pure parent- 

 species — as are Kolreuter and Gartner tha'. some degree 

 of sterility between distinct species is a universal law of 

 nature. He experimented on some of the very same species 

 as did Gartner. The difference in their results may, I 

 think, be in part accounted for by Herbert's great horticul- 

 tural skill, and by his having hot-houses at his command. 

 Of his many important statements I will here give only 

 a single one as an example, namely, that K every ovule in a 



