x [ v PREFACE. 



I endeavored to describe their warfare. This work has passed through 

 a great many editions in America and England. It has been translated 

 into French, Spanish, German, Russian, Italian, Polish, Servian, etc. It- 

 finds very many readers in Eastern Europe. And of some of these 

 translations several editions have been issued. 



When I thus look back on the objects that have occupied my atten- 

 tion, I recognize how they have been interconnected, each preparing the 

 way for its successor. Is it not true that for every person the course of 

 life is along the line of least resistance, and that in this the movement 

 of humanity is like the movement of material bodies ? 



To my American reader I need say nothing for the purpose of secur- 

 ing his kind appreciation of this work. I know that he, recognizing the 

 difficulties encountered in such a long series of experiments, will exten- 

 uate its imperfections, and regard it as a contribution from this side of 

 the Atlantic to the common fund of human knowledge, and especially 

 to one of its most important departments, at a period when the main 

 subject on which it treats had scarcely attracted scientific attention. 



NEW YORK, January, 1878. 



