58. The second group consists of the species, thirty-seven
in number, of which the mean heights of the crossed plants exceed that
of the self-fertilised plants by more than five per cent; and the mean
of their mean heights is to that of the self-fertilised plants as 100 to
78. The third group consists of the species, only five in number, of
which the mean heights of the self-fertilised plants exceed that of the
crossed by more than five per cent; and here the mean of the mean
heights of the crossed plants is to that of the self-fertilised as 100
to 109. Therefore if we exclude the species which are approximately
equal, there are thirty-seven species in which the mean of the mean
heights of the crossed plants exceeds that of the self-fertilised 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 somewhat in constitution--is supported by a
comparison of the Tables 7/A and 7/C. The latter table gives 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 much more general and much more strongly marked than in Table 7/A,
in which plants of the same stock were crossed.
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