Many who would be ready to accept the evolution of physical characteristics find it impossible 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.

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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 walking limbs of amphibia and reptiles, constructed for progression upon land.  Among the mammalia the fore limbs have become structurally adapted so as to be such diverse organs of locomotion as the stilt-like leg of a horse, the flipper of a seal, the whale’s paddle, and the bat’s wing, while among the birds the wing may change into a flipper like that of the penguin, or become reduced to a vestige as in Apteryx.