We will now consider in some little detail the eighteen cases in which the self-fertilised plants equalled in average height the crossed plants within five per cent; and the eight cases in which the self-fertilised plants exceeded in average height the crossed plants by above five per cent; making altogether twenty-six cases in which the crossed plants were not taller than the self-fertilised plants in any marked degree.
[1. Dianthus caryophyllus (third generation).
This plant was experimented on during four generations, in three of which the crossed plants exceeded in height the self-fertilised generally by much more than five per cent; and we have seen under Table 7/C that the offspring from the plants of the third self-fertilised generation crossed by a fresh stock profited in height and fertility to an extraordinary degree. But in this third generation the crossed plants of the same stock were in height to the self-fertilised only as 100 to 99, that is, they were practically equal. Nevertheless, when the eight crossed and eight self-fertilised plants were cut down and weighed, the former were to the latter in weight as 100 to 49! There can therefore be not the least doubt that the crossed plants of this species are greatly superior in vigour and luxuriance to the self-fertilised; and what was the cause of the self-fertilised plants of the third generation, though so light and thin, growing up so as almost to equal the crossed in height, I cannot explain.
2. Lobelia fulgens (first generation).
The crossed plants of this generation were much inferior in height to the self-fertilised, in the proportion of 100 to 127. Although only two pairs were measured, which is obviously much too few to be trusted, yet from other evidence given under the head of this species, it is certain that the self-fertilised plants were very much more vigorous than the crossed. As I used pollen of unequal maturity for crossing and self-fertilising the parent-plants, it is possible that the great difference in the growth of their offspring may have been due to this cause. In the next generation this source of error was avoided, and many more plants were raised, and now the average height of the twenty-three crossed plants was to that of the twenty-three self-fertilised plants as 100 to 91. We can therefore hardly doubt that a cross is beneficial to this species.
3. Petunia violacea (third generation).
Eight crossed plants were to eight self-fertilised of the third generation in average height as 100 to 131; and at an early age the crossed were inferior even in a still higher degree. But it is a remarkable fact that in one pot in which plants of both lots grew extremely crowded, the crossed were thrice as tall as the self-fertilised. As in the two preceding and two succeeding generations, as well as with plants raised by a crossed with a fresh stock, the crossed greatly exceeded the self-fertilised in height, weight, and fertility (when these two latter points were attended to), the present case must be looked at as an anomaly not affecting the general rule. The most probable explanation is that the seeds from which the crossed plants of the third generation were raised were not well ripened; for I have observed an analogous case with Iberis. Self-fertilised seedlings of this latter plant, which were known to have been produced from seeds not well matured, grew from the first much more quickly than the crossed plants, which were raised from better matured seeds; so that having thus once got a great start they were enabled ever afterwards to retain their advantage. Some of these same seeds of the Iberis were sown on the opposite sides of pots filled with burnt earth and pure sand, not containing any organic matter; and now the young crossed seedlings grew during their short life to double the height of the self-fertilised, in the same manner as occurred with the above two sets of seedlings of Petunia which were much crowded and thus exposed to very unfavourable conditions. We have seen also in the eighth generation of Ipomoea that the self-fertilised seedlings raised from unhealthy parents grew at first very much more quickly than the crossed seedlings, so that they were for a long time much taller, though ultimately beaten by them.