i2 PEOGKESS: ITS LAW AUB CAUSE. 



deal, the causes are more or less compound ; as indeed are 

 nearly all causes with which we are acquainted. Scarcely 

 any change can with logical accuracy be wholly ascribed to 

 one agency, to the neglect of the permanent or temporary 

 conditions under which only this agency produces the 

 change. But as it does not materially affect our argument, 

 we prefer, for simplicity's sake, to use throughout the popu- 

 lai mode of expression. 



Perhaps it will be further objected, that to assign loss 

 of heat as the cause of any changes, is to attribute these 

 changes not to a force, but to the absence of a force. And 

 this is true. Strictly speaking, the changes should be at- 

 tributed to those forces which come into action when the 

 antagonist force is withdrawn. But though there is an in- 

 accuracy in saying that the freezing of water is due to the 

 loss of its heat, no practical error arises from it ; nor will 

 a parallel laxity of expression vitiate our statements respect- 

 ing the multipHcation of effects. Indeed, the objection 

 serves but to draw attention to the fact, that not only does 

 the exertion of a force produce more than one change, but 

 the withdrawal of a force produces more than one change. 

 And this suggests that perhaps the most correct statement 

 of our general principle would be its most abstract state- 

 ment — every change is followed by more than one othei 

 change. 



Returning to the thread of our exposition, w^e have next 

 to trace out, in organic progress, this same all-pervading 

 principle. And here, where the evolution of the homoge- 

 neous into the heterogeneous was first observed, the produc- 

 tion of many changes by one cause is least easy to demon- 

 strate. The development of a seed into a plant, or an 

 ovum into an animal, is so gradual, while the forces which 

 determine it are so involved, and at the same time so unob- 

 trusive, that it is difiicult to detect the multiplication of effects 

 which is elsewhere so obvious. Nevertheless, guided by 



