CHAPTER V.
LAWS OF VARIATION.
Effects of changed conditions -- Use and disuse, combined with natural
selection; organs of flight and of vision -- Acclimatisation -- Correlated
variation -- Compensation and economy of growth -- False correlations --
Multiple, rudimentary, and lowly organised structures variable -- Parts
developed in an unusual manner are highly variable: specific characters
more variable than generic: secondary sexual characters variable --
Species of the same genus vary in an analogous manner -- Reversions to
long-lost characters -- Summary.
I have hitherto sometimes spoken as if the variations--so common and
multiform with organic beings under domestication, and in a lesser degree
with those under nature--were due to chance. This, of course is a wholly
incorrect expression, but it serves to acknowledge plainly our ignorance of
the cause of each particular variation. Some authors believe it to be as
much the function of the reproductive system to produce individual
differences, or slight deviations of structure, as to make the child like
its parents. But the fact of variations and monstrosities occurring much
more frequently under domestication than under nature, and the greater
variability of species having wide ranges than of those with restricted
ranges, lead to the conclusion that variability is generally related to the
conditions of life to which each species has been exposed during several
successive generations.
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