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_. We may focus our attention upon the material
likenesses and differences in such a series of locomotory organs, but an
inevitable accompaniment of their physical changes in the transformation
of species has been an evolution in the functional matter of locomotion.
The most complex and differentiated tracts of even the highest animals
have evolved from a simple sac like that of a polyp or jellyfish, as we
know from the independent testimony of comparative anatomy and embryology;
in this case also the evolution of alimentary functions is no less
inseparable from the transformations in structural respects. And again, we
cannot understand the historical development of vision without taking into
account the eyes of various types belonging to lower and higher animals.

So it is with the nervous systems of man and other animals, and with their
functions. The nervous system of the human organism comprises identical
organs with the same arrangements that are found in other primates and in
lower vertebrates as well; the differences in structure are differences in
the degree of the complexity of certain parts, notably of the cerebrum.
Therefore the evolution of human mentality, which depends upon a human
type of brain as a physical basis, is already demonstrated with the proof
that the human brain and nervous system have evolved. It is true that an
invariable and necessary connection between mind and matter is implied in
the foregoing statement, and this is something which demands further
consideration at a later point. But just _how_ the human mind is produced
by or depends upon the brain, is of far less importance for us at this
time than the obvious fact that mental performance requires active nervous
tissues. So far investigation has been unable to discover a valid reason
for a belief in the existence of mental phenomena, as such, apart from
some kind of material basis. And while we may prefer to restrict the use
of the word _mind_ to the series of nervous processes going on in the
human organ of thought, in so far