510 THE EVOLUTION OF LIFE. 



a portion of a preceding organism,, and the question is whether, 

 after exposure of the series for a million years to changed in 

 cident 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 mani 

 fest 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 it 

 self 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 undergoing slow variations and compli 

 cations? Clearly the process, ever-advancing towards a tem 

 porary 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 incident 

 forces, will be superposed further changes wrought by a modi 

 fied 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 homo 

 geneous must be recognized as 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 

 the area, be exposed to different sets of incident forces. Still 

 more decided must this difference of exposure be when 

 its members spread into other habitats. Those expansive 

 and repressive energies which set to each species a limit that 



