The human race, therefore, has passed through all its periods of evolution and development in strict conformity with and submission to this dominant power until it is difficult to conceive any great departure from the narrow limits imposed on the proportions of the human frame.

In the first place, I wish to consider what transformation in our appearance would be produced by a change in the force of gravitation. Let us take extreme cases. Say that the power of gravitation were to be doubled. In that case we should have to exert a vastly increased strength to support ourselves in any other than the prone or dorsal position, it would be hard to rise from the ground, to run, leap, climb, to drag or carry any object. Our muscles would necessarily be more powerful, and the skeleton to which they are attached would need corresponding modification. To work such limbs a more rapid transformation of matter would be required; hence the supply of nutriment must be greater, involving enlarged digestive organs, and a larger respiratory apparatus to allow of the perfect aeration of the increased mass of the blood. To keep up the circulation ,with the necessary force, either the heart would have to be more powerful or the distance through which the blood would require to be impelled must be reduced. The increased amount of nourishment demanded would involve a corresponding increase in the difficulty of its collection, and the struggle for existence would be intensified. More food being required day by day, the jaws would have to be enlarged and the muscles strengthened. The teeth also must be adapted for extra tearing and grinding.

These considerations involve marked changes in the structure of human beings. To accord, with thickened bones, bulging muscles, and larger respiratory and digestive apparatus, the body would be heavier and more massive. The necessity for such alterations in structure would be increased by the liability to fall. The necessity of keeping the center of gravity low, and the great demands made on the system in other respects, must conspire to reduce the size of head and brain. With increase of gravitation the bipedal form would be beset by drawbacks. Assuming that the human race, under the altered circumstances, remained bipedal, it is highly probable that a large increase in the quadruped, hexapod, or octopod structure would prevail in the animal kingdom. The majority of animals ,would be of the saurian class, with very short legs allowing the trunk to rest easily on the ground, and the serpent type would probably be in the ascendant. Winged creatures would suffer severely, and small birds and insects would be dragged to earth by a force hard to resist; although this might be more or less compensated by the increased density of the air. Humming birds, dragon flies, butterflies, and bees, all of which spend a large portion of their time in the air, would, in the struggle for existence, be rare visitants.