Hence
modifications of structure, viewed by systematists as of high value, may be
wholly due to the laws of variation and correlation, without being, as far
as we can judge, of the slightest service to the species.
We may often falsely attribute to correlated variation structures which are
common to whole groups of species, and which in truth are simply due to
inheritance; for an ancient progenitor may have acquired through natural
selection some one modification in structure, and, after thousands of
generations, some other and independent modification; and these two
modifications, having been transmitted to a whole group of descendants with
diverse habits, would naturally be thought to be in some necessary manner
correlated. Some other correlations are apparently due to the manner in
which natural selection can alone act. For instance, Alph. De Candolle has
remarked that winged seeds are never found in fruits which do not open; I
should explain this rule by the impossibility of seeds gradually becoming
winged through natural selection, unless the capsules were open; for in
this case alone could the seeds, which were a little better adapted to be
wafted by the wind, gain an advantage over others less well fitted for wide
dispersal.
COMPENSATION AND ECONOMY OF GROWTH.
The elder Geoffroy and Goethe propounded, at about the same time, their law
of compensation or balancement of growth; or, as Goethe expressed it, "in
order to spend on one side, nature is forced to economise on the other
side.
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