The importance of physical agents and physical laws in the social as well as in the individual economy, is variously illustrated by Professor Draper, who points out the essential part they play in several departments of nature. To the merely mechanical inclination of the earth's axis of rotation toward the plane of her orbit of revolution around the sun, we owe the changing seasons and the method of life which is dependent on these. The alteration of that physical arrangement would involve a corresponding alteration in the whole life of the globe. So, again, the possibility of existence upon the earth, in any way, depends upon conditions altogether of a material kind. It is necessary that our planet should be at a definite mean distance from the source of light and heat, the sun; and that the form of her orbit should be almost a circle, since it is only within a narrow range of temperature, secured by these conditions, that life can be maintained. It is through natural agents also that the means of regulation are secured in the present economy of the globe. Through heat, the distribution and arrangement of the vegetable tribes are accomplished; through their mutual relations with the atmospheric air, plants and animals are interbalanced, and neither permitted to obtain a superiority. The condensation of carbon from the air and its inclusion in the strata constitute the chief epoch in the organic life of the earth giving a possibility for the appearance of the hot-blooded and more intellectual animal tribes. That event was due to the influence of the rays of the sun. Passing from inorganic to organic forms, our author remarks that their permanence is altogether dependent 'on the invariability of the material conditions under which they live. Any variation therein, no matter how insignificant it might be, would be forthwith followed by a corresponding variation in the form.' At this point we are brought to the far-famed 'development theory,' which, since the publication of the 'Vestiges of Creation,' has been the scientific battle field of the naturalists of the world. Professor Draper is, of course, a firm adherent of this theory. He continues: