CHAP. IX.] DEGREES OF STERILITY. 221 



from other plants. Nearly all the plants experimented on by 

 Gartner were potted, and were kept in a chamber in his house. 

 That these processes are often injurious to the fertility of a plant 

 cannot be doubted ; for Gartner gives in his table about a score of 

 cases of plants which he castrated, and artificially fertilised with 

 their own pollen, and (excluding all cases such as the Leguminosai, 

 in which there is an acknowledged difficulty in the manipulation) 

 half of these twenty plants had their fertility in some degree 

 impaired. Moreover, as Gartner repeatedly crossed some forms, 

 such as the common red and blue pimpernels (Anagallis arvensis 

 and coerulea), which the best botanists rank as varieties, and 

 found them absolutely sterile, we may doubt whether many 

 species are really so sterile, when intercrossed, as he believed. 



It is certain, on the one hand, that the sterility of various 

 species when crossed is so different in degree and graduates away 

 so insensibly, 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 Kolreuter 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 into details the evidence advanced by 

 our best botanists on the question whether certain doubtful forms 

 should be ranked as species or varieties, with the evidence from 

 fertility adduced by different hybridisers, or by the same observer 

 from experiments made during different years. It can thus be 

 shown that neither sterility nor fertility affords any certain dis- 

 tinction 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 

 guarding 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 

 ind 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 aug- 

 mented 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. 



