248 HYBRIDISM. Chap. VIII. 



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 experi- 

 enced observers who have ever lived, namely, Kolreuter 

 and Gartner, should have arrived at diametrically oppo- 

 site conclusions in regard to the very same species. 

 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 author, from experiments 

 made during different years. It can thus be shown that 

 neither sterility nor fertility affords any clear distinction 

 between species and varieties ; but that 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 

 ten generations, yet he asserts positively that their fer- 

 tility never increased, but generally greatly decreased. 

 I do not doubt that tin's is usually the case, and that 

 the fertility often suddenly decreases in the first few 

 generations. Nevertheless I believe that in all these 

 experiments the fertility has been diminished by an 

 independent cause, namely, from close interbreeding. 

 I have collected so large a body of facts, showing 



