84 CAUSES WHICH CHECK Chap. XVI 



crossed with that of the white variety; and so it is when differently 

 coloured species are crossed. The general results may be seen in the 

 Table at the end of his volume. In one instance he gives 16 the 

 following details ; but I must premise that Gartner, to avoid ex- 

 aggerating the degree of sterility in his crosses, always compares the 

 maximum number obtained from a cross with the average number 

 naturally given by the pure mother-plant. The white variety of 

 V. hjchnitis, naturally fertilised by its own pollen, gave from an 

 average of twelve capsules ninety-six good seeds in each; whilst 

 twenty flowers fertilised with pollen from the yellow variety of this 

 same species, gave as the maximum only eighty-nine good seeds ; so 

 that we have the proportion of 1000 to 90S, according to Gartner's 

 usual scale. I should have thought it possible that so small a difference 

 in fertility might have been accounted for by the evil effects of the 

 necessary castration ; but Gartner shows that the white variety of V. 

 lychn it is. when fertilised first by the white variety of V. blattaria, and 

 then by the yellow variety of this species, yielded seed in the propor- 

 tion of 622 to 438 ; and in both these cases castration was performed. 

 Now the sterility which results from the crossing of the differently 

 coloured varieties of the same species, is fully as great as that which 

 occurs in many cases when distinct species are crossed. Unfortu- 

 nately Gartner compared the results of the first unions alone, and 

 not the sterility of the two sets of hybrids produced, from the white 

 variety of V. lychnitis when fertilised by the white and yellow 

 varieties of V. blattaria, for it is probable that they would have 

 differed in this respect. 



Mr. J. Scott has given me the results of a series of experiments on 

 Yerbascum, made by him in the Botanic Gardens of Edinburgh. 17 He 

 repeated some of 'Gartner's experiments on distinct species, but 

 obtained only fluctuating results, some confirmatory, the greater 

 number contradictory ; nevertheless these seem hardly sufficient to 

 overthrow the conclusion arrived at by Gartner from experiments 

 tried on a larger scale. Mr. Scott also experimented on the relative 

 fertility of unions between similarly and dissimilarly- coloured 

 varieties of the same species. Thus he fertilised six flowers of the 

 yellow variety of V. hjchnitis by its own pollen, and obtained six 

 capsules; and calling, for the sake of comparison, the average 

 number of good seed in each of their capsules one hundred, he found 

 that this same yellow variety, when fertilised by the white variety, 

 yielded from seven capsules an average of ninety-four seed. On the 

 same principle, the white variety of V. lychnitis by its own pollen 

 (from six capsules), and by the pollen of the yellow variety (eight 

 c ipsules), yielded seed in the proportion of 100 to 82. The yellow 

 variety of V. thapsus by its own pollen (eight capsules), and by 

 that of the white variety (only two capsules), yielded seed in the 

 proportion of 100 to 91. Lastly, the white variety of V. blattaria 



19 * Bastarderzeugung,' s. 216. published in ' Journ. Asiatic Sec. of 



17 The results have since been Bengal,' 1867, p. 145. 



