THE LAW OF EVOLUTION 



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 communicated 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 re- 

 mote 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 

 necessary unequal exposure of outside and in- 

 side to the incident force coming in the shape 

 of heat from the walls of the oven. In the im- 

 possibility of balancing an accurately made pair 

 of scales, in the equal impossibility of keeping 

 a tank of water free from currents, in the rust- 

 ing of iron, and in the uneven cooling of a 

 heated metal, is exemplified the principle that 

 the state of homogeneity is an unstable state. 



H5 



