tendency to render
the reproductive organs more or less impotent, as shown in the chapter
devoted to this subject; and these organs consequently often fail to
transmit faithfully the parental characters. Changed conditions also act
directly and definitely on the organisation, so that all or nearly all the
individuals of the same species thus exposed become modified in the same
manner; but why this or that part is especially affected we can seldom or
never say. In most cases, however, of the direct action of changed
conditions, independently of the indirect variability caused by the
reproductive organs being affected, indefinite modifications are the
result; in nearly the same manner as exposure to cold or the absorption of
the same poison affects different individuals in various ways. We have
reason to suspect that an habitual excess of highly nutritious food, or an
excess relatively to the wear and tear of the organisation from exercise,
is a powerful exciting cause of variability. When we see the symmetrical
and complex outgrowths, caused by a minute atom of the poison of a
gall-insect, we may believe that slight changes in the chemical nature of
the sap or blood would lead to extraordinary modifications of structure.

The increased use of a muscle with its various attached parts, and the
increased activity of a gland or other organ, lead to their increased
development. Disuse has a contrary effect. With domesticated productions
organs sometimes become rudimentary through abortion; but we have no reason
to suppose that this has ever followed from mere disuse. With natural
species, on the contrary, many organs appear to have been rendered
rudimentary through disuse, aided by the principle of the economy of
growth, and by the hypothetical principle discussed in the last chapter,
namely, the final destruction of the germs or gemmules of such useless
parts. This difference may be partly {419} accounted for by disuse having
acted on domestic forms for an insufficient length of time, and partly from
their exemption from any severe struggle for existence, entailing rigid
economy in the development of each part, to which all species under nature
are subjected. Nevertheless the law of compensation or balancement
apparently affects, to a certain extent, our domesticated productions.

We must not exaggerate the importance of the definite action of changed
conditions in modifying all the individuals of the same species in the sam