334 THE LAW OF EVOLUTION. 



tion of his own, that the cataracts of the Orinoco are heard 

 at a greater distance by night than by day when he notes 

 the essential parallelism existing between these facts and 

 the fact that the unusual visibility of remote objects is 

 also an indication of coming rain and when he points 

 out that the common cause of these variations is the smaller 

 hindrance offered to the passage of both light and sound, 

 by media which are comparatively homogeneous, either 

 in temperature or hygrometric state; he helps in bringing 

 under one generalization the phenomena of light and those 

 of sound. Experiment having shown that these conform 

 to like laws of reflection and refraction, the conclusion 

 that they are 'both produced by undulations gains proba- 

 bility: there is an incipient integration of two great orders 

 of phenomena, between which no connexion was suspected 

 in times past. A still more decided integration has been of 

 late taking place between the once independent sub-sci- 

 ences of Electricity, Magnetism, and Light. 



The process will manifestly be carried much further. 

 Such propositions as those set forth in preceding chapters, 

 on " The Persistence of Force," " The Transformation and 

 Equivalence of Forces," " The Direction of Motion," and 

 " The Ehythm of Motion," unite within single bonds phe- 

 nomena belonging to all orders of existences. And if there 

 is such a thing as that which we here understand by Phi- 

 losophy, there must eventually be reached a universal 

 integration. 



114. Nor do the industrial and aesthetic Arts fail to 

 supply us with equally conclusive evidence. The progress 

 from rude, small, and simple tools, to perfect, complex, and 

 large machines, is a progress in integration. Among what 

 are classed as the mechanical powers, the advance from the 

 lever to the wheel-and-axle is an advance from a simple 

 agent to an agent made up of several simple ones. On com- 

 paring the wheel-and-axle, or any of the machines used in 



