direct action of changed conditions, whether of
a definite or indefinite nature, as with the fleeces of sheep in hot
countries, with maize grown in cold countries, with inherited gout, &c.,
the tissues of the body, according to the doctrine of pangenesis, are
directly affected by the new conditions, and consequently throw off
modified gemmules, which are transmitted with their newly acquired
peculiarities to the offspring. On any ordinary view it is unintelligible
how changed {395} conditions, whether acting on the embryo, the young or
adult animal, can cause inherited modifications. It is equally or even more
unintelligible on any ordinary view, how the effects of the long-continued
use or disuse of any part, or of changed habits of body or mind, can be
inherited. A more perplexing problem can hardly be proposed; but on our
view we have only to suppose that certain cells become at last not only
functionally but structurally modified; and that these throw off similarly
modified gemmules. This may occur at any period of development, and the
modification will be inherited at a corresponding period; for the modified
gemmules will unite in all ordinary cases with the proper preceding cells,
and they will consequently be developed at the same period at which the
modification first arose. With respect to mental habits or instincts, we
are so profoundly ignorant on the relation between the brain and the power
of thought that we do not know whether an inveterate habit or trick induces
any change in the nervous system; but when any habit or other mental
attribute, or insanity, is inherited, we must believe that some actual
modification is transmitted;[925] and this implies, according to our
hypothesis, that gemmules derived from modified nerve-cells are transmitted
to the offspring.

It is generally, perhaps always, necessary that an organism should be
exposed during several generations to changed conditions or habits, in
order that any modification in the structure of the offspring should ensue.
This may be partly due to the changes not being at first marked enough to
catch the attention, but this explanation is insufficient; and I can
account for the fact, only by the assumption, which we shall see under the
head of reversion is strongly supported, that gemmules derived from each
cell before it had undergone the least modification are transmitted in
large numbers to successive generations, but that the gemmules derived from
th