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ology, and fossil history of the human species, in order to see that there is ample proof of the actual occurrence of evolution, and then, as before, we may look about for the causes which have produced this result by natural methods. While it is necessary to treat the subject directly, namely, by examining the actual evidences relating to the particular case in question, it is worthwhile before doing so to point out that, as the whole includes a part, human evolution has already been proved beyond question. This conclusion must be accepted, unless reasons can be given for excluding mankind from the rest of the living world as an absolutely unique type, supreme and isolated because of some peculiar endowments not shared with the rest of animate nature. If these reasons are lacking, and the unity of organic nature be recognized, human evolution cannot be denied unless some interpretation more reasonable and logical than evolution can be given for the whole mass of facts exemplified and discussed in the foregoing chapters. We may accordingly approach the main questions by asking if there are any reasons for regarding the human species as a unique and isolated type of organism. At the outset, we must recognize that in so far as the human body is material, its movements and mass relations are controlled by physical principles, like all other masses of matter. It is well, indeed, that this is so, for if gravitation and the laws of inertia were not consistent and reliable principles holding true at all times and not intermittently, it would be difficult to order our lives with confidence. In the next place, the general principles of biology hold true for the structure and physiology of the human species as they do for all other living things. A human body is composed of eight systems of organs, whose functions are identical with the eight vital tasks of every other animal. All these organs are made up of cells as ultimate vital units, and the materials of which human cells are composed belong to the class of substances called protoplasm. Human protoplasm, like all other living materials, must replenish itself, and respire and oxidize in obedience to biological laws that have been found to be uniform everywhere. Thus the human organism is no more unique in fundamental organic respects than it is apart from the world of physical processes and laws. How does the matter stand when the general structural plan of a human being
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