Chap. VIII. DEGREES OF STERILITY. 235 



ropcalcdly cnissed some forms, such as the common red and 

 blue pimpernels (Anagallis arvensis and ccerulea), wliicli the 

 best botanists rank as varieties, and found them absolutely 

 sterile, -sve may doubt whether many species are really so ster- 

 ile, when intercrossed, as he believed. 



It is certain, on the one hand, that the sterility of various 

 species "when crossed is so dilferent in deg'ree and graduates 

 away so insensibly, and, on the other hand, that the fertility 

 of pure sjiecies is so easily aflected 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, Kolrcuter 

 and Gartner, should luivc arrived at diametrically opposite con- 

 clusions 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 hybridizers, or by the same author, from ex])eri- 

 ments made during different years. It can thus be shown that 

 neither sterility nor fertility alfords any clear distinction ];c- 

 tween species and varieties; but tliat 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 Giirtner 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 posi- 

 tively that their fertility never increased, but generally de- 

 creased 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 oiTspring; and both sexual ele- 

 ments in hyljritl ])lants are already allected in some degree. 

 15ut I believe in nearly all these cases, that the fertility has 

 been diminished by an independent cause, namely, by too close 

 interbreeding. I have collected so large a body of facts, show- 

 ing on the one liand that an occasional cross with a distinct 

 individual or variety increases the vigor and fertility of the 

 offspring, and on the other hand that very clc>8e interbreeding 

 lessons their vigor and fcrtihty, that I must admit the correct- 



