DEGREES OF STERILITY. 270 



were potted, and were kept in a chamber in his house. 

 That these processes are often injurious to the fertility of 

 a plant cannot be doubted; for Gartner gives in his table 

 about a score of cases of plants which he castrated, and 

 artificially fertilized with their own pollen, and (excluding 

 all cases such as the Leguminosae, in which there is an 

 acknowledged difficulty in the manipulation) half of these 

 twenty plants had their fertility in some degree impaired. 

 Moreover, as Gartner repeatedly crossed some forms, such 

 as the common red and blue pimpernels (Anagalhs 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 

 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 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 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 dif- 

 ferent hybridizers, or by the same observer from experi- 

 ments made during different years. It can thus be shown 

 that neither sterility nor fertility affords any certain dis- 

 tinction 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 consti- 

 tutional and structural differences. 



In regard to the sterility of hybrids in successive genera- 

 tions; though Gartner was enabled to rear some iiybrids, 

 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 fertility never increases, 

 but generally decreases greatly and suddonlv. With re^ 



