262 HYBRIDISM. 



which there is an acknowledged difficulty in the manipula- 

 tion) 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 on 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 hybridizers, or by the 

 same observer from experiments made during different 

 years. It can thus be shown that neither sterility nor fer- 

 tility affords any certain distinction 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 genera- 

 tions ; 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 and suddenly. With respect 

 to this decrease, it may first be noticed that when any devia- 

 tion in structure or constitution is common to both parents, 

 this is often transmitted in an augmented degree to the off- 

 spring; 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 



