We have seen that man incessantly presents individual differences in
all parts of his body and in his mental faculties. These differences or
variations seem to be induced by the same general causes, and to obey
the same laws as with the lower animals. In both cases similar laws of
inheritance prevail. Man tends to increase at a greater rate than his
means of subsistence; consequently he is occasionally subjected to a
severe struggle for existence, and natural selection will have effected
whatever lies within its scope. A succession of strongly-marked
variations of a similar nature is by no means requisite; slight
fluctuating differences in the individual suffice for the work of
natural selection; not that we have any reason to suppose that in the
same species, all parts of the organisation tend to vary to the same
degree. We may feel assured that the inherited effects of the
long-continued use or disuse of parts will have done much in the same
direction with natural selection. Modifications formerly of importance,
though no longer of any special use, are long-inherited. When one part
is modified, other parts change through the principle of correlation,
of which we have instances in many curious cases of correlated
monstrosities. Something may be attributed to the direct and definite
action of the surrounding conditions of life, such as abundant food,
heat or moisture; and lastly, many characters of slight physiological
importance, some indeed of considerable importance, have been gained
through sexual selection.
No doubt man, as well as every other animal, presents structures, which
seem to our limited knowledge, not to be now of any service to him, nor
to have been so formerly, either for the general conditions of life, or
in the relations of one sex to the other. Such structures cannot be
accounted for by any form of selection, or by the inherited effects of
the use and disuse of parts. We know, however, that many strange and
strongly-marked peculiarities of structure occasionally appear in our
domesticated productions, and if their unknown causes were to act more
uniformly, they would probably become common to all the individuals of
the species. We may hope hereafter to understand something about the
causes of such occasional modifications, especially through the study
of monstrosities: hence the labours of experimentalists, such as those
of M. Camille Dareste, are full of promise for the future. In general
we can only say that the cause of each slight variation and of each
monstrosity lies much more in the constitution of the organism, than in
the nature of the surrounding conditions; though new and changed
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