IMITATION AMONG ATOMS AND ORGANISMS. 505 



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 com- 

 munity 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 disso- 

 ciation 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 asso- 

 ciated and the parts of unlike density dissociated. In a general 

 way the bodies which constitute the solar system occupy posi- 

 tions 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 (omit- 

 ting 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 gradu- 

 ally 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 nebulse and star clusters display a gradually in- 

 creasing 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 com- 

 plete 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, 



VOL. XLvm. 3G 



