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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
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