Page:Popular Science Monthly Volume 48.djvu/547

Rh usually attacked and gored to death; elephants not only expel a "rogue" from the herd, but have been known to refuse refuge to one of their number who had escaped into the jungles with the bandages of the capturer still on its legs.

It should be added that the organism itself, and the organic structure, everywhere illustrates the principle of the association of likes and the dissociation of unlikes. All tissue systems in the organism are systems whose units are likes to each other, and the likeness which those units exhibit is a likeness maintained by the assimilative force of the organism acting a whole. So all food material taken into the organic system which is really a community of cells—must undergo assimilation as the very condition of its admission to association, while all material that can not be associated is expelled, either by dissociation without or by dissociation within the system.

In the inorganic world the same law holds good. Its most general evidence is afforded by the fact that matter is always associated with matter, and that where free to move it always constitutes a system of which the parts of like density are associated and the parts of unlike density dissociated. In a general way the bodies which constitute the solar system occupy positions in that system according to the degree of their density, the denser planets lying nearer to, those less dense farther away from, the sun. The ring system of Saturn shows a like collocation, due to differences and likenesses of density. In the earth itself (omitting complications of gravitation produced by internal heat) the denser solids always tend to move nearer to the earth's center than the liquids; the liquids thus displaced take precedence in position over the atmosphere; while even the air shows a gradually diminishing density in a direction away from the earth. That which, moreover, is observable in our own solar system may reasonably be predicated of stellar aggregations generally: most of the planetary nebulæ and star clusters display a gradually increasing density from circumference to center; comets, densest at their nucleus, visibly diminish in mass as their fanlike streamers recede from the sun; in meteor orbits the densest bodies travel first, the remainder, roughly speaking, following in the order of their density. In the star system itself, there are signs of the same kind of segregation, one region of the Milky Way being as remarkable for the multiplicity of its suns and its almost complete absence of nebulae as are the other regions of the heavens for the fewness of their stars and the large number of incipient systems which they contain.

What is true of matter in the varying degrees of its density is also true of the different types of matter; for whether the atom constitutes the molecule, as in the case of a few of the elements,