4 HYBRIDISM. [Chap. IX. 



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 be- 

 lieved. 



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 ex- 

 perienced observers who have ever lived, namely Kol- 

 reuter 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 

 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 

 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 con- 

 stitutional 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 



