520 THE EVOLUTION OF LIFE. 



and now decrease, is here presented to ns. Suppose a new 

 factor were brought to bear on this moving equihbrium — say 

 by the arrival of some wandering mass, or by an additional 

 momentum given to one of the existing masses — what would 

 be the result? If the strange body or the extra energy were 

 very large, it might so derange the entire system as to cause 

 its collapse. But what if the incident energy, falling on the 

 system from without, proved insufficient to overthrow it? 

 There would then arise a set of perturbations which would, 

 in the course of an enormous period, slowly work round into 

 a modified moving equilibrium. The effects primarily im- 

 pressed on the adjacent masses, and in a smaller degree on 

 the remoter masses, would presently become complicated 

 with the secondary effects impressed by the disturbed masses 

 on one another; and these again with tertiary effects. Waves 

 of perturbation would continue to be propagated throughout 

 the entire system; until, around a new centre of gravity, 

 there had been established a set of planetary motions different 

 from the preceding ones. The new energy must gradually be 

 used up in overcoming the energies resisting the divergence 

 it generates; which antagonizing energies, when no longer 

 opposed, set up a counter-action, ending in a compensating 

 divergence in the opposite direction, followed by a re-com- 

 pensating divergence, and so on. Now though instead 

 of being, like the Solar System, in a state of independent 

 moving equilibrium, an organism is in a state of dependent 

 moving equilibrium {First Principles, § 170) ; y^i this does 

 not prevent the manifestation of the same law. Every 

 animal daily obtains from without, a supply of energy to 

 replace the energy it expends ; but this continual giving to its 

 parts a new momentum, to make up for the momentum con- 

 tinually lost, does not interfere with the carrying on of 

 actions and reactions like those just described. Here, as 

 before, we have a definitely-arranged aggregate of parts, 

 called organs, having their definitely-established actions and 

 reactions, called functions. These rhythmical actions or 



