The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom · Charles Darwin
TABLE 7/A.
The first group consists of species of which the mean heights of the self-fertilised plants are within five per cent of 100; so that the crossed and self-fertilised plants are approximately equal; and of such species there are twelve about which nothing need be said, the mean of the mean heights of the self-fertilised being of course very nearly 100, or exactly 99.58. The second group consists of the species, thirty-seven in number, of which the mean heights of the crossed plants exceed that of the self-fertilised plants by more than five per cent; and the mean of their mean heights is to that of the self-fertilised plants as 100 to 78. The third group consists of the species, only five in number, of which the mean heights of the self-fertilised plants exceed that of the crossed by more than five per cent; and here the mean of the mean heights of the crossed plants is to that of the self-fertilised as 100 to 109. Therefore if we exclude the species which are approximately equal, there are thirty-seven species in which the mean of the mean heights of the crossed plants exceeds that of the self-fertilised by twenty-two per cent; whereas there are only five species in which the mean of the mean heights of the self-fertilised plants exceeds that of the crossed, and this only by nine per cent.
The truth of the conclusion--that the good effects of a cross depend on the plants having been subjected to different conditions or to their belonging to different varieties, in both of which cases they would almost certainly differ somewhat in constitution--is supported by a comparison of the Tables 7/A and 7/C. The latter table gives the results of crossing plants with a fresh stock or with a distinct variety; and the superiority of the crossed offspring over the self-fertilised is here much more general and much more strongly marked than in Table 7/A, in which plants of the same stock were crossed. We have just seen that the mean of the mean heights of the crossed plants of the whole fifty-four species in Table 7/A is to that of the self-fertilised plants as 100 to 87; whereas the mean of the mean heights of the plants crossed by a fresh stock is to that of the self-fertilised in Table 7/C as 100 to 74. So that the crossed plants beat the self-fertilised plants by thirteen per cent in Table 7/A, and by twenty-six per cent, or double as much, in Table 7/C, which includes the results of the cross by a fresh stock.
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