434 THE EVOLUTION OF LIFE. 



from without, a supply of force to replace the force 

 which it expends; but this continual giving to its part3 a 

 new momentum, to make up for the momentum continually 

 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, which we call 

 organs, having their definitely-established actions and re- 

 actions, which we call functions. These rhythmical actions 

 or functions, and the various compound rhythms resulting 

 from their combinations, are in such adjustment as to balance 

 the actions to which the organism is subject : there is a con- 

 stant or periodic genesis of forces, which, in their kinds, 

 amounts, and directions, suffice to antagonize the forces 

 which the organism has constantly or periodically to bear. 

 If then there exists this state of moving equilibrium among 

 a definite set of internal actions, exposed to a definite set of ex- 

 ternal actions ; what must result if any of the external actions 

 are changed ? Of course there is no longer an equilibrium. 

 Some force which the organism habitually generates, is too 

 great or too small to balance some incident force ; and there 

 arises a residuary force exerted by the environment on the 

 organism, or by the organism on the environment. This 

 residuary force — this unbalanced force, of necessity expends 

 itself in producing some change of state in the organism. 

 Acting directly on some organ and modifying its function, 

 it indirectly modifies dependent functions, and remotely 

 influences all the functions. As we have already seen 

 (§§ 68, 69), if this new force is permanent, its effects must 

 be gradually diffused throughout the entire system ; until it 

 has come to be equilibrated in working those structural re- 

 arrangements which produce an exactly counterbalancing 

 force. 



The bearing of this general truth on the question we are 

 now dealing with, is obvious. Those modifications upon 

 modifications, which the unceasing mutations of their en- 

 vironments have been all along generating in organisms > 



