XXXvi INTRODUCTION. 



opium, hashish, nitrous oxide, etc., when absorbed into the blood. 

 Within the limits of their peculiar action upon the nervous centres, 

 the effect of each is strictly proportionate to the quantity taken. 

 There is a constant ratio between the antecedents and consequents. 



" How this metamorphosis takes place how a force existing 

 as motion, heat, or light, can become a mode of consciousness 

 how it is possible for aerial vibrations to generate the sensation 

 we call sound, or for the forces liberated by chemical changes in the 

 brain, to give rise to emotion, these are mysteries which it is im- 

 possible to fathom. But they are not profounder mysteries than 

 the transformation of the physical forces into each other. They 

 are not more completely beyond our comprehension than the 

 natures of mind and matter. They have simply the same insolu- 

 bility as all other ultimate questions. "We can learn nothing more 

 than that here is one of the uniformities in the order of phe- 

 nomena." 



The law of correlation being thus applicable to human energy 

 as well as to the powers of nature, it must also apply to society, 

 where we constantly witness the conversion of forces on a compre- 

 hensive scale. The powers of nature are transformed into the activ- 

 ities of society; water-power, wind-power, steam-power, and electri- 

 cal-power are pressed into the social service, reducing human labor, 

 multiplying resources, and carrying on numberless industrial pro- 

 cesses : indeed, the conversion of these forces into social activities 

 is one of the chief triumphs of civilization. The universal forces 

 of heat and light are transformed by the vegetable kingdom into 

 the vital energy of organic compounds, and then, as food, are again 

 converted into human beings and human power. The very exist- 

 ence as well as the activity of society are obviously dependent upon 

 the operations of vegetable growth. "When that is abundant, popu- 

 lation may become dense, and social activities multifarious and 

 complicated, while a scanty vegetation entails sparse population 

 and enfeebled social action. Any universal disturbance of the 

 physical forces, as excessive rains or drouth, by reducing the har- 



