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Show 270 SUMMARY OF MEASUREMENTS. CHAP. VII. <luring several generations profit only to a small extent by a cross with intercross d plants of tho smne stock (as in the case of Dianthus), in c01nparison with the effects of a cross by a fresh stock. Plants of tho same stock intercrossed during s veral g n rations (as with Petunia) were inferior in a marked n1annor in fertility to those derived from the corresponding solf-fortilise~l plants crossed by a fresh stock. Lastly, certain plants which are regularly intercross d by insects in a state of natur , and which wer artificially crossed in each succeeding generation, in the course of Iny oxperiInents, so that they can n ver or most rarely have suffered any evil from self-fertilisation (as with Eschscholtzia and Ipo1nroa), nevertheless profited greatly by a cross with a fresh stock. These several cases taken togeth r show us in the clearest manner that it is not the mere crossing of any two individuals which is beneficial to the offspring. The b nofit thus derived depends on the plants which are united differing in some manne·:~;, and ther can harJly be a douLt that it is in the constitution or nature of tho sexual clements. 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 prog nitors have been exposed during several generations to different conditions. Bnt to this latter subject I shall have to recur in a futuro chapter. TABLE A. We will now turn to our first table, which relates to crossed and s If-fertilised plants of tho same stock. These consist of fifty-four species belonging to thirty natural orders. The total number of crossed plants of which measure1nents are given is 796, and CIIAP. VII. TABLE A. 271 of self-fertilised plants 809 ; that is altogether 1,605 plants. Some of the species were experimented on during several successive generations; and it should be borne in 1nind that in such cases the cro:::;sed plants in each generation were crossed with pollen from another crossed plant, and the flowers on the selffertilised plants were almost always fertilised with their own pollen, though sometimes with pollen fro1n other flowers on the same plant. rrhe crossed plants thus became more or less closely inter-related in the later generations ; and both lots were · sub)' ected in each ae- • b neration 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 in tercrossed plants of each generation with pollen from a non-related plant, grown under different conditions, as was done with the plants in Table 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 generatioll$ in Table 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 al wa.ys followed the plan in Table 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 1night |