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Show 282 SUMMARY OF MEASUREMENTS. CHAP. VII. plants. This having been done in each case in which more than one generation was raised, it is easy to calculate the average of the average heio-hts of the crossed and self-fertilised plants of all ~he species included in Table A. It should however be observed that . as only a few plants of so1ne species, whilst a considerable number of others, were . measured, the value of the mean or average heights of the several species is very different. Subject to this source of error, it may be worth while to give the mean of the mean heights of the fifty-four species in Table A· and the result is, calling the mean of the mean h~ights of the crossed plants 100, that of the self-fertilised plants is 87. But it is a better plan to divide the fifty-four species into three groups, as was done with the previously given eighty-three cases. The first group consists of species of which the mean heio·hts of the self-fertilised plants are within five per c~nt. of 100; so. that the crossed and self-fertilised plants are approximately equal; and of such species there are twelve about which nothing need be said, the mean of the mean heights of the self-fertilised being of course very nearly 100, or exactly 99 ·58. The second group consists of the species, thirty-seven in nurnber, of which the mean heights of the crossed plants exceed that of tho self-fertilised plants by more than five per cent.; and the mean of their n1ean heights is to that of the self-fertilised plants as 100 to ~8. The third group consists of the species, only five In number, of which the 1nean heights of the self-fertilised plants exceed that of the crossed by more than five per cent.; and here the n1ean of the mean h~i.ghts of the crossed plants is to that of the s If-fertilised as 100 to 109. Therefore if we exclude the species which are approxin1ately equal, there are CHAP. VII. TABLE B. 283 thirty-seven species in which the mean of the mean heights of the crossed plants exceeds that of the selffertilised by twenty-two per cent. ; whereas there are only :five species in which the mean of the mean heights of the self-fertilised plants exceeds that of the crossed, and this only by nine per cent. The truth of the conclusion--that the good effects of a cross depend on the plants having been subjected to different conditions or to their belonging to different varieties, in both of which cases they would almost certainly differ soinewhat in constitution-is supported. by a comparison of the Tables A and C. The latte1· table gi yes the results of crossing plants with a fresh stock or with a distinct variety; and the superiority of the crossed offspring over the self-fertilised is here 1nuch more general and much more strongly marked than in Table A, in which plants of the sa1ne stock were crossed. We have just seen that the 1nean of the mean heights of the crossed plants of the whole fiftyfour species in Table A is to that of the self-fertilised plants as 100 to 87; whereas the mean of the mean heights of the plants crossed by a fresh stock is to that of the self-fertilised in Table 0 as 100 to 74. So that the crossed plants beat the self-fertilised plants by thirteen per cent. in Table A, and by twenty-six per cent., or double as much, in Table 0, which includes the results of a cross by a fresh stock. TABLE B. A few words mti.st be added on the weights of the crossed plants of the same stock, in comparison with the self-fertilised. Eleven cases are given in Table B, relating to eight species. The nun1ber of plants which were weighed is shown in the two left columns, and their relative weights in the ~-ight |