d-capsule.
At long intervals of time, out of millions of individuals reared in the
same country and fed on nearly the same food, deviations of structure so
strongly pronounced as to deserve to be called monstrosities arise; but
monstrosities cannot be separated by any distinct line from slighter
variations. All such changes of structure, whether extremely slight or
strongly marked, which appear among many individuals living together,
may be considered as the in definite effects of the conditions of life
on each individual organism, in nearly the same manner as the chill
effects different men in an in definite manner, according to their
state of body or constitution, causing coughs or colds, rheumatism, or
inflammation of various organs.

With respect to what I have called the in direct action of changed
conditions, namely, through the reproductive system of being affected,
we may infer that variability is thus induced, partly from the fact of
this system being extremely sensitive to any change in the conditions,
and partly from the similarity, as Kolreuter and others have remarked,
between the variability which follows from the crossing of distinct
species, and that which may be observed with plants and animals when
reared under new or unnatural conditions. Many facts clearly show how
eminently susceptible the reproductive system is to very slight changes
in the surrounding conditions. Nothing is more easy than to tame an
animal, and few things more difficult than to get it to breed freely
under confinement, even when the male and female unite. How many animals
there are which will not breed, though kept in an almost free state in
their native country! This is generally, but erroneously attributed to
vitiated instincts. Many cultivated plants display the utmost vigour,
and yet rarely or never seed! In some few cases it has been discovered
that a very trifling change, such as a little more or less water at some
particular period of growth, will determine whether or not a plant will
produce seeds. I cannot here give the details which I have collected and
elsewhere published on this curious subject; but to show how singular
the laws are which determine the reproduction of animals under
confinement, I may mention that carnivorous animals, even from the
tropics, breed in this country pretty freely under confinement, with
the exception of the plantigrades or bear family, which seldom produce
young; whereas, carnivorous birds,