HISTORIC GROWTH OF 3IAN.
THE FORCES WHICH UNDERLIE human evolution are broader in their sweep than even the wide range of
history. They have governed the growth of the earth itself. They caused the earth to pass through many steps of preparation for the noble advent of man. From the geologic age of Fishes, up to that in which man appeared, we may trace a
succession of animals with higher and higher types of brain. At last man came to crown the organic series with a brain so complex in its parts that it ranks him as lord paramount of the earth.
EVOLUTION REQUIRES CONDITIONS. But mere ex
ternal conditions are not all that is required. A man needs ground on which to build a house. But the house is not generated and produced by the ground. The latter is only one out of several factors. At the end of each geologic age the conditions had become such as. to favor a higher kind of life. By passing through the form of organized bodies, matter becomes more and more vitalized; it acquires a more permanent tendency to vibrate in unison with the living forces, and it thus becomes more capable of being molded into new kinds of plants or animals. At every step of this progress the internal or vital forces have acted in concert with the external conditons to produce the new results.
Evolution describes the great methods of growth which rule in the world of living forms. It deals with the past, but it also foretells the future. And this latter work gives its greatest value to man
EVOLUTION OF THE BRAIN. 11
SEVEN GREAT FORCES were concerned in the vast movements of early creation. Gravity marked elliptic orbits for the path of worlds. Electricity and magnetism polarized and thus rotated these worlds on their axes. Chemic force, heat and light built up the solid rocks and arranged their wide-spread layers. And the vital force crowded the sea and land with the myriad tribes of animal and plant life.
These forces then held the sane relations to each other that they sustain at present.
COMPARE THE BRAIN of a
fish with that of man. That is, take the lowest and the highest in the great scale of vertebrate animals. This comparison will bring the law of progress before us in a striking form. As we see in this. engraving at the side, the brain of the fish is only about one-third greater in diameter than the spinal cord or spinalis. The balance of nerve-force, of brain power, is only slightly in favor of the head. A large part of the nervous force is in the spinalis, that great bundle whose branching nerves spread to the various parts of the body.
Now look at the brain of man. Its diameters are from six to ten times greater than that of the spinalis
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