Page:Popular Science Monthly Volume 18.djvu/803

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AN stands as a connecting link between two worlds—the world of matter and that of mind. He forms the apex of the development of matter, the loftiest effort of evolution in substance. Mind, it is true, has its foundation in the regions of life below him, but all its superstructure—the towering arches and lofty pinnacles of the ideal—rests upon the human intellect. Man thus forms the gateway which Nature has placed between her two vast kingdoms of substance and thought, and in the human brain these two realms meet and merge, energy flowering into intellect, substance into soul.

But is the human form the true culmination of the development of matter? Has Nature really reached in man her acme in this direction? A deductive philosopher would perhaps answer this question in the affirmative, on the theory that Nature would not stop short of the most completely developed physical form, as the starting-point of mental evolution. He might claim that a perfect soul could only arise in a perfect body, and that, as Nature is striving toward perfection, she must lay all her foundations at the highest possible point.

But inductive science starts with no theories. It builds its theories out of facts, not its facts out of theories, and follows Nature upward from her roots, not downward from her branches. What, then, do the facts of Nature say as to the question of animal evolution? Is man truly the paragon of animals?

Unfortunately, this question opens before us a field of investigation too broad for consideration in a single article. We have already seen that Nature has exposed organic forms to an almost unlimited variety of conditions, during the long geologic ages, and has probably tried every line of development of which organic life is susceptible. By a close review of the various animal types, their advantages and deficiencies have been traced, and we think it has been shown that the vertebrate type is the one suited to the highest evolution, and the one toward which all the lower forms tend in their highest representatives.

For a complete review of organic form development the plant types should also be considered, but we must confine ourselves in this article to a consideration of the vertebrate type of animals alone.

And first, What are the causes, what the modes, what the laws, of evolution? What features in one animal constitute superiority to another animal? These questions we shall but briefly answer. There are certain requirements absolutely necessary to the continuance of