422 THE EVOLUTION OF LIFE. 



tinue through successive individuals. Given a series of oi> 

 ganisms, each of which is developed from a portion of a 

 preceding organism, and the question is, whether, after 

 exposure of the series for a million years to changed incident 

 forces, one of its members will be the same as though the 

 incident forces had only just changed. To say that it will, 

 is implicitly to deny the persistence of force. In relation to 

 any cause of divergence, the whole series of such organisms 

 may be considered as fused together into a continuously- 

 existing organism ; and when so considered, it becomes 

 manifest that a continuously- acting cause will go on working 

 a continuously-increasing effect, until some counteracting 

 cause prevents any further effect. 



But now if any primordial organic aggregate, must, in itself 

 and through its descendants, gravitate from uniformity to 

 multiformity, in obedience to the more or less multiform 

 forces acting on it ; what must happen if these multiform 

 forces are themselves ever undergoing slow variations and 

 complications ? Clearly the process, ever- advancing towards 

 a temporary limit but ever having its limit removed, must 

 go on unceasingly. On those structural changes wrought 

 in the once homogeneous aggregate by an original set of in- 

 cident forces, will be superposed further changes wrought 

 by a modified set of incident forces ; and so on throughout 

 all time. Omitting for the present those circumstances 

 which check and qualify its consequences, the instability of 

 the homogeneous must be recognized an ever-acting cause of 

 organic evolution, as of all other evolution. 



While it follows that every organism, considered as an in- 

 dividual and as one of a series, tends thus to pass into a more 

 heterogeneous state ; it also follows that every species, con- 

 sidered as an aggregate of individuals, tends to do the like. 

 Throughout the area it inhabits, the conditions can never 

 be absolutely uniform : its members must, in different parts 

 of its area, be exposed to different sets of incident forces. 

 Still more decided must this difference of exposure be when 



