Seite - 300 - in The Origin of Species

300 ORIGIN OF SPECIES Gartner gives in his table about a score of cases of plants which he castrated, and artificially fertilised with their own pollen, and (excluding all cases such as the Leguminosae, in which there is an acknowledged difficulty in the manipula- tion) 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 (Anagallis arvensis and cceulea), which the best 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 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 Kolreuter and Gartner, arrived at diametrically opposite conclusions in regard to some of the very same forms. It is also most in- structive to compare—but I have not space here to enter into 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 observer from ex- periments made during different years. It can thus be shown that neither sterility nor fertility affords any certain distinc- tion 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 struc- tural differences. In regard to the sterility of hybrids in successive genera- tions ; though Gartnerwas enabled to rear some hybrids, care- fully 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 gen- erally 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