INTRODUCTION TO SOCIOLOGY 85
two factors aggregates and limits and show how, in reality, in a constant and necessary fashion, every mass is differentiated from the surrounding environment only by the condition of being limited. That which we have done from the point of view of mathematics, mechanics, astronomy, physiology, chemistry, biology, and psychology, we shall now attempt to do from the point of view of concrete phenomena, considered as masses, with which sciences equally concrete with the former are occupied : terrestrial astronomy, geology, mineralogy, botany, and zoology. We shall consider these phenomena then as masses delimited by zones from the ensemble.
In the chapter devoted to astronomical limits we have shown sufficiently that it is there that the best proof is found that the equilibrium of all bodies results from their peculiar composition in correspondence with the surrounding environment. This law of the general mechanics presided even at the formation of our planet. It is useless to dwell on this longer, if it is only to recall that the history of this formation is in relation to that of all beings and their distribution on the surface of the earth by a derivative factor climate.
The earth, at first in a molten state, has been gradually cool- ing; at the same time, its crust has thickened, and the atmos- phere, becoming less vaporous, preserves at its surface a lower temperature. We know that there have been successive phases of temperature, and that each of these phases has been marked by special modes of existence. Vegetable and animal forms appear only when the temperature is compatible with their life and the atmosphere contains more or less carbon and oxygen.
The distribution of fossils proves that this was comparatively late, i. <?., at the beginning of the Tertiary age. These were pro- duced by a differentiation of zones and that of corresponding organisms. Thus mechanical and physical laws have determined the formation and consecutive differentiation of our planet :
1. Differences in the crust produced by cooling, prominences and depressions, mountains and valleys.
2. Shortening of the terrestrial diameter by cooling.
3. Appearance of water as a consequence of the lowering of
