Geology has revealed to us the existence in prehistoric times of animals allied to those which now exist, but with great variation in organization, and differing very considerably in size. Among the fossils are the skeletons of creatures far exceeding in size any now living, and, on the other hand, bones of a small animal scarcely larger than a dog of one of the breeds of medium size, which geologists assert was the progenitor of the modern horse. But so far as science has been able to discover the human being has ever been of the same average dimensions. Individuals of all races vary in height; the average bulk of the inhabitants of tropical climates is generally less than that of the people who dwell in the regions of temperate climate; and stunted men and women occupy the colder parts of the earth; but so it has been apparently in all ages. The skeletons found in old barrows, representatives of the men of the prehistoric period, the bones found imbedded in strata of great antiquity, are of about the same dimensions as those of the men of to-day. No necessity of existence has lengthened the arms or neck, changed toes into thumbs, or added a finger to the hand.

The general type of all men in all regions, from the equator to the poles, is that they are two-handed, walk erect, have the power of speech more or less developed, and that between even those of lowest organization and the most intelligent of quadrupeds there is a very marked distinction. Human idiots there are, as there are human monstrosities of form; but they are exceptions which prove the permanence of the typical characteristics. While, however, the general agreement in organization appears to be ineradicable, there are certain external differences, in complexion, hair, facial contour, and other minor matters, which seem to indicate separate groups or families of the human race, and have suggested the theories advocated by some ethnologists of distinct centres of creation, in opposition to the more generally accepted belief in the derivation of all human beings from the same stock. In discussing this question, we may fairly take into consideration that, in the respect of the means by which the human race might have spread over the earth, we are not encountered by the difficulties which present themselves when we are examining the history of the movements of other members of animated nature.

The will to travel, inspired by many motives, is added to the power to travel, given by natural adaptability to endure atmospheric and other variations, and by the exercise of the reasoning power; and in cases of accidental drifting to unknown islands or continental coasts, there is a power to make the best of adverse conditions. It is quite possible that the intelligent and active descendants of a small family located in southwestern Asia should in the course of thousands of years have made their way east and west, north and south, making at intervals settlements which became centres of new dispersions. From Asia to Western Europe was a comparatively easy journey, allowing many centuries for its accomplishment. Africa could be peopled not only by passing across the neck of land which divides it from Asia, but by settlements on the coast made by adventurous mariners, or by parties drifted to the shores. The straits which separate north-eastern Asia from northwestern America could be crossed by canoes, visiting the chain of islands on their way.

We know that adventurous Northmen of Europe reached the" North American coast from Greenland centuries before Columbus crossed the Atlantic; and the Chinese have traditions of discoveries and settlements on the western coast of North America, nearly as far south as California. Long residence in hot climates affects the color of the skin, and it becomes hereditary. The necessity of constant physical exertion to maintain existence, and the absence of intellectual training, develop the muscles and bony framework, and induce a dwindling of the brain. The facial angle becomes more acute, the jaw-bone more prominent and the figure more lithe and active. In very hot climates less animal food can be eaten, even by recent settlers, and in the course of ages is dispensed with altogether--sometimes from religious considerations, as among the natives of the Indian peninsula and other parts of Asia - and the resulting difference of physique is very noticeable. Other causes, such as the effects of the chemical constituents of the atmosphere and of water, it may be the effects of terrestrial magnetism, are in continual operation, and the results, aided by hereditary transmission, produce the differences which mark what are popularly called the races of mankind.