nce of the embryo of man to that,
for instance, of a dog--the construction of his skull, limbs and whole
frame on the same plan with that of other mammals--the occasional
appearance of various structures, for instance, of several distinct
muscles, which man does not normally possess, but which are common to
the Quadrumana--and a crowd of analogous facts--all point in the
plainest manner to the conclusion that man is the co-descendant of other
mammals of a common progenitor.

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 in the work of natural
selection. 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,
as far as we can judge with our little knowledge, are not now of any
service to him, nor to have been so during any former period of his
existence, either in relation to his general conditions of life, or 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