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.

Table 7/B.

A few words must be added on the weights of the crossed plants of the same stock, in comparison with the self-fertilised.  Eleven cases are given in Table 7/B, relating to eight species.  The number of plants which were weighed is shown in the two left columns, and their relative weights in the right column, that of the crossed plants being taken as 100.  A few other cases have already been recorded in Table 7/C in reference to plants crossed by a fresh stock.  I regret that more trials of this kind were not made, as the evidence of the superiority of the crossed over the self-fertilised plants is thus shown in a more conclusive manner than by their relative heights.  But this plan was not thought of until a rather late period, and there were difficulties either way, as the seeds had to be collected when ripe, by which time the plants had often begun to wither.  In only one out of the eleven cases in Table 7/B, that of Eschscholtzia, do the self-fertilised plants exceed the crossed in weight; and we have already seen they are likewise superior to them in height, though inferior in fertility, the whole advantage of a cross being here confined to the reproductive system.  With Vandellia the crossed plants were a little heavier, as they were also a little taller than the self-fertilised; but as a greater number of more productive capsules were produced by the cleistogene