Effects of external conditions. Use and disuse, combined with natural
selection; organs of flight and of vision. 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. The male
and female sexual elements seem to be affected before that union takes
place which is to form a new being. In the case of sporting plants,
the bud, which in its earliest condition does not apparently differ
essentially from an ovule, is alone affected. But why, because the
reproductive system is disturbed, this or that part should vary more or
less, we are profoundly ignorant. Nevertheless, we can here and there
dimly catch a faint ray of light, and we may feel sure that there must
be some cause for each deviation of structure, however slight.
How much direct effect difference of climate, food, etc., produces on
any being is extremely doubtful. My impression is, that the effect is
104