Acclimatisation.
Correlation of growth.
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 in organic beings under domestication, and in a lesser
degree in those in a state of nature--had been 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 very slight deviations of
structure, as to make the child like its parents. But the much greater
variability, as well as the greater frequency of monstrosities, under
domestication or cultivation, than under nature, leads me to believe
that deviations of structure are in some way due to the nature of the
conditions of life, to which the parents and their more remote
ancestors have been exposed during several generations. I have
remarked in the first chapter--but a long catalogue of facts which
cannot be here given would be necessary to show the truth of the
remark--that the reproductive system is eminently susceptible to
changes in the conditions of life; and to this system being
functionally disturbed in the parents, I chiefly attribute the varying
or plastic condition of the offspring.
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