ch. iv.] THE LAW OF EVOLUTION. 353 



necessitated by the rhythm of motion, and therefore in- 

 directly by the persistence of force. We have now to show 

 how the secondary changes, differentiation and integration, 

 are equally necessitated by the same primordial fact. 



It is a corollary from the persistence of force, "that, in 

 the actions and reactions of force and matter, an unlikenesb 

 in either of the factors necessitates an unlikeness in the 

 effects." When the different portions of any homogeneous 

 aggregate are exposed to the action of unlike forces, or to 

 unequal intensities of the same force, they are of necessity 

 differently affected thereby. Between the unequally exposed 

 parts there arise structural differences, entailing differences 

 of property and function. That which before was homo- 

 geneous has become heterogeneous through the appearance 

 of certain unlikenesses ; and, under the name of differentia- 

 tion, the rise of such unlikenesses has already been described. 

 It remains to be observed that such unlikenesses cannot but 

 arise, that differentiation must needs take place, because it is 

 impossible for all the parts of any aggregate to be similarly 

 conditioned with reference to any incident force. Whether 

 it be the mechanical vibrations caused by a blow, the slow 

 undulations constituting heat, or the more rapid undulations 

 constituting light, that are propagated through any body, it 

 equally follows that the respective vibrations will be com- 

 municated in different degrees to those particles which are 

 situated on the nearer and on the farther side of the body, 

 and to those particles which are laterally near to or remote 

 from the line followed by the incident force. The different 

 parts will be variously moved, heated, or chemically affected, 

 and a series of differentiations will thus have arisen. We 

 need go no farther than the kitchen, to perceive that the 

 crust formed on a loaf of bread or a joint of roasted meat, is 

 due to the necessarily unequal exposure of outside and inside 

 to the incident force coming in the shape of heat from the 

 walls of the oven. In the impossibility of balancing an 



VOL. i. A A 



