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.  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.