to treat the history of human mentality
as a subject for dispassionate consideration, because above all else the
intellectual powers of mankind seem to be truly distinctive. It is only
after constant use of the methods of science that we can bring ourselves
to see how closely we resemble lower forms in physical make-up; still
greater reluctance must be overcome before we can view our mental
processes as counterparts of those of inferior animals, so essential to
our very humanity do they seem. But our duty to undertake the task is
plain, and its discharge will be greatly facilitated by a clear
realization that mental evolution is but a part of human transformation in
times past, as the latter is only a small fraction of the universal
process of organic evolution in general. While our own nature and
inquisitiveness give us so intense an interest in the teachings of science
that relate to the constitution and history of human faculty, wherefore
these matters gain an undue prominence in perspective, it must never be
forgotten that these teachings do not stand by themselves, for they are
built upon the sure foundations already laid in physical evolution; and
these foundations cannot be disturbed by our failure to use them as a
basis when we construct our own conceptions of human intellect and its
history.

* * * * *

Before passing to the systematic review of the facts and principles of
comparative psychology which demonstrate evolution, there are certain
general aspects of the subject to be considered so as to clear the ground,
as it were, for further progress. When the several organic systems of the
human body were compared with those of the apes and of lower animals,
their evolution was proved as far as the purely physical and material
characteristics were concerned. But we know that there is no part of any
one of these systems which has not its own particular function, even
though this may be a relatively passive one; while furthermore, science
does not know of any physiological activity without some organ or tissue
or cell as its material basis. Therefore the evolution of an organic
system in material respects involves its functional or dynamic evolution
as an inseparable correlate; the two proceed in unity, and they cannot be
regarded as entirely distinct without violating common-sense.

The fin of a fish is used as an organ of locomotion in water; from some
such organ have evolved the