HYBRIDISM 241 



botanists rank as varieties, and found them absolutely sterile, we 

 may doubt whether many species are really so sterile, when inter- 

 crossed, 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 prac- 

 tical purposes it is most difficult to say where perfect fertility ends 

 and sterility begins. I think no better evidence of this can be re- 

 quired 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 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 constitutional and structural differences. 



In regard to the sterility of hybrids in successive generations; 

 though Gartner was enabled to rear some hybrids, carefully guard- 

 ing 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 deviation in structure or constitution is common 

 to both parents, this is often transmitted in an augmented 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, showing on the one hand 

 that an occasional cross with a distinct individual or variety in- 

 creases the vigor and fertility of the offspring, and on the other 

 hand that very close interbreeding lessens their vigor and fertility, 

 that I cannot doubt the correctness of this conclusion. Hybrids are 

 seldom raised by experimentalists 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, if left to themselves, will 



