HYBRIDISM 223 



metrically opposite 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 evi- 

 dence 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 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 fer- 

 tility never increased, but generally greatly decreased. 

 I do not doubt that this 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 

 that close interbreeding lessens fertility, and, on the 

 other hand, that an occasional cross with a distinct in- 

 dividual or variety increases fertility, that I cannot doubt 

 the correctness of this almost universal belief amongst 

 breeders. Hybrids are seldom raised by experimen- 

 talists 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 prevented during 

 the flowering season : hence hybrids will generally be 

 fertilised during each generation by their own indi- 

 vidual pollen ; and I am convinced that this would 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 



