The difference between the two sets of plants in the table is generally much greater in fertility than in height or weight. On the other hand, with some of the species, as with Nicotiana, there was no difference in fertility between the two sets, although a great difference in height and weight. Considering all the cases in this table, there can be no doubt that plants profit immensely, though in different ways, by a cross with a fresh stock or with a distinct sub-variety. It cannot be maintained that the benefit thus derived is due merely to the plants of the fresh stock being perfectly healthy, whilst those which had been long intercrossed or self-fertilised had become unhealthy; for in most cases there was no appearance of such unhealthiness, and we shall see under Table 7/A that the intercrossed plants of the same stock are generally superior to a certain extent to the self-fertilised,--both lots having been subjected to exactly the same conditions and being equally healthy or unhealthy.
We further learn from Table 7/C, that a cross between plants that have been self-fertilised during several successive generations and kept all the time under nearly uniform conditions, does not benefit the offspring in the least or only in a very slight degree. Mimulus and the descendants of Ipomoea named Hero offer instances of this rule. Again, plants self-fertilised during several generations profit only to a small extent by a cross with intercrossed plants of the same stock (as in the case of Dianthus), in comparison with the effects of a cross by a fresh stock. Plants of the same stock intercrossed during several generations (as with Petunia) were inferior in a marked manner in fertility to those derived from the corresponding self-fertilised plants crossed by a fresh stock. Lastly, certain plants which are regularly intercrossed by insects in a state of nature, and which were artificially crossed in each succeeding generation in the course of my experiments, so that they can never or most rarely have suffered any evil from self-fertilisation (as with Eschscholtzia and Ipomoea), nevertheless profited greatly by a cross with a fresh stock. These several cases taken together show us in the clearest manner that it is not the mere crossing of any two individuals which is beneficial to the offspring. The benefit thus derived depends on the plants which are united differing in some manner, and there can hardly be a doubt that it is in the constitution or nature of the sexual elements. Anyhow, it is certain that the differences are not of an external nature, for two plants which resemble each other as closely as the individuals of the same species ever do, profit in the plainest manner when intercrossed, if their progenitors have been exposed during several generations to different conditions. But to this latter subject I shall have to recur in a future chapter.
TABLE 7/A.
We will now turn to our first table, which relates to crossed and self-fertilised plants of the same stock. These consist of fifty-four species belonging to thirty natural orders. The total number of crossed plants of which measurements are given is 796, and of self-fertilised 809; that is altogether 1,605 plants. Some of the species were experimented on during several successive generations; and it should be borne in mind that in such cases the crossed plants in each generation were crossed with pollen from another crossed plant, and the flowers on the self-fertilised plants were almost always fertilised with their own pollen, though sometimes with pollen from other flowers on the same plant. The crossed plants thus became more or less closely inter-related in the later generations; and both lots were subjected in each generation to almost absolutely the same conditions, and to nearly the same conditions in the successive generations. It would have been a better plan in some respects if I had always crossed some flowers either on the self-fertilised or intercrossed plants of each generation with pollen from a non-related plant, grown under different conditions, as was done with the plants in Table 7/C; for by this procedure I should have learnt how much the offspring became deteriorated through continued self-fertilisation in the successive generations.