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Page:Popular Science Monthly Volume 24.djvu/601

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branches of a tree receive the life-supporting sap from a common trunk, so all living forms have a common origin in protoplasm with which the evolution of their life begins; the entire growth and development of the body consisting in the growth and differentiation of the protoplasm of which its tissues and organs are composed.

Observe how admirably the figure of a tree exhibits the supposed relationship between the various types of animals both extinct and living; indicating, not that each type has been derived directly from one immediately preceding it, either in time or in structural rank, but that various types have had a common ancestor from which, by development in different directions, all have more or less diverged; so that the relationship between man and the existing anthropoid apes, for example, is that of remote cousinship rather than of direct descent. The common stock is represented by the trunk of the tree; from this trunk, which rises higher and higher with each diverging offshoot, has sprung an immense variety of branches; and, at the very pinnacle of this magnificent structure, man appears—the crowning efflorescence of organic evolution.

The permanent types which represent these various phases of embryonic development show a progressively increasing differentiation from their environment. The moner and the amœba are almost as structureless as the water in which they are found, consisting of little more than water with a trace of albumen; in specific gravity, in temperature, in color, etc., the difference between these low organisms and their environment is slight. Compared with the differences—chemical, physical, and structural—between man and the invisible atmosphere in which he is submerged, the contrast in this particular is a striking one. This leads us to other considerations of still greater significance.

The true environment of any organism consists in as much of the external universe as that organism is capable of holding communication with; so that, as the life becomes higher, the environment also becomes more complex.

At the deep-sea bottom, where life is exhibited in its most simple grades, the temperature is unvarying; no light penetrates to those depths; a uniformity of conditions is thus preserved almost unbroken, and the adjustments necessary to the continuance of life under such circumstances are as trifling as the grade of life is simple.

By the greater complexity of the human organism as compared with other animals, man is brought into communication with and under the influence of a vastly increased variety of external conditions, mainly through the organs of the special senses and their intimate relations with a highly developed nervous system.

That without the eye and its connections with the brain we could have no consciousness of light is the merest commonplace of physiology; yet, could we realize the full meaning of this and other similar