As the case stands, the self-fertilised plants of the successive generations in Table 7/A were put into competition with and compared with intercrossed plants, which were probably deteriorated in some degree by being more or less inter-related and grown under similar conditions. Nevertheless, had I always followed the plan in Table 7/C, I should not have discovered the important fact that, although a cross between plants which are rather closely related and which had been subjected to closely similar conditions, gives during several generations some advantage to the offspring, yet that after a time they may be intercrossed with no advantage whatever to the offspring. Nor should I have learnt that the self-fertilised plants of the later generations might be crossed with intercrossed plants of the same stock with little or no advantage, although they profited to an extraordinary degree by a cross with a fresh stock.
With respect to the greater number of the plants in Table 7/A, nothing special need here be said; full particulars may be found under the head of each species by the aid of the Index. The figures in the right-hand column show the mean height of the self-fertilised plants, that of the crossed plants with which they competed being represented by 100. No notice is here taken of the few cases in which crossed and self-fertilised plants were grown in the open ground, so as not to compete together. The table includes, as we have seen, plants belonging to fifty-four species, but as some of these were measured during several successive generations, there are eighty-three cases in which crossed and self-fertilised plants were compared. As in each generation the number of plants which were measured (given in the table) was never very large and sometimes small, whenever in the right hand column the mean height of the crossed and self-fertilised plants is the same within five per cent, their heights may be considered as practically equal. Of such cases, that is, of self-fertilised plants of which the mean height is expressed by figures between 95 and 105, there are eighteen, either in some one or all the generations. There are eight cases in which the self-fertilised plants exceed the crossed by above five per cent, as shown by the figures in the right hand column being above 105. Lastly, there are fifty-seven cases in which the crossed plants exceed the self-fertilised in a ratio of at least 100 to 95, and generally in a much higher degree.
If the relative heights of the crossed and self-fertilised plants had been due to mere chance, there would have been about as many cases of self-fertilised plants exceeding the crossed in height by above five per cent as of the crossed thus exceeding the self-fertilised; but we see that of the latter there are fifty-seven cases, and of the former only eight cases; so that the cases in which the crossed plants exceed in height the self-fertilised in the above proportion are more than seven times as numerous as those in which the self-fertilised exceed the crossed in the same proportion. For our special purpose of comparing the powers of growth of crossed and self-fertilised plants, it may be said that in fifty-seven cases the crossed plants exceeded the self-fertilised by more than five per cent, and that in twenty-six cases (18 + 8) they did not thus exceed them. But we shall now show that in several of these twenty-six cases the crossed plants had a decided advantage over the self-fertilised in other respects, though not in height; that in other cases the mean heights are not trustworthy, owing to too few plants having been measured, or to their having grown unequally from being unhealthy, or to both causes combined. Nevertheless, as these cases are opposed to my general conclusion I have felt bound to give them. Lastly, the cause of the crossed plants having no advantage over the self-fertilised can be explained in some other cases. Thus a very small residue is left in which the self-fertilised plants appear, as far as my experiments serve, to be really equal or superior to the crossed plants.