In plants the down on the
fruit and the colour of the flesh are considered by botanists as
characters of the most trifling importance: yet we hear from an
excellent horticulturist, Downing, that in the United States
smooth-skinned fruits suffer far more from a beetle, a curculio, than
those with down; that purple plums suffer far more from a certain
disease than yellow plums; whereas another disease attacks
yellow-fleshed peaches far more than those with other coloured flesh.
If, with all the aids of art, these slight differences make a great
difference in cultivating the several varieties, assuredly, in a state
of nature, where the trees would have to struggle with other trees and
with a host of enemies, such differences would effectually settle
which variety, whether a smooth or downy, a yellow or purple fleshed
fruit, should succeed.
In looking at many small points of difference between species, which,
as far as our ignorance permits us to judge, seem to be quite
unimportant, we must not forget that climate, food, etc., probably
produce some slight and direct effect. It is, however, far more
necessary to bear in mind that there are many unknown laws of
correlation of growth, which, when one part of the organisation is
modified through variation, and the modifications are accumulated by
natural selection for the good of the being, will cause other
modifications, often of the most unexpected nature.
As we see that those variations which under domestication appear at
any particular period of life, tend to reappear in the offspring at
the same period;--for instance, in the seeds of the many varieties of
our culinary and agricultural plants; in the caterpillar and cocoon
stages of the varieties of the silkworm; in the eggs of poultry, and
in the colour of the down of their chickens; in the horns of our sheep
and cattle when nearly adult;--so in a state of nature, natural
selection will be enabled to act on and modify organic beings at any
age, by the accumulation of profitable variations at that age, and by
their inheritance at a corresponding age.
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