PREFACE ix 



very, recent and has been little tried. Harvey 

 discovered the circulation of the blood in 1645. 

 Obviously biological conceptions of life previous to 

 that time must have been too crude to have any 

 significance for human conduct. Lavoisier in 1799 

 showed that combustion consists in the union of 

 oxygen with the objects burned. This was a dis- 

 covery destined to have the most significant influ- 

 ence on conceptions of human physiology, but 

 imagine the state of biological thought previous to 

 the development of this influence ! Only fifty years 

 ago Darwin published the " Origin of Species "and 

 gave the first compelling hypothesis of the nature 

 of human evolution from lower forms. How was it 

 possible to form reasonable biological views of human 

 life previous to that tune? The last half century 

 has witnessed a wonderful extension of biological 

 conceptions in general, and people have barely begun 

 to realize that the discoveries of this period have 

 made it necessary to wholly revise our ideas as to 

 life processes. But if this revision of ideas as to life 

 in general is necessary, is it not also necessary to 

 extend it to the special and important case of human 

 life? We are in truth in the midst of a quiet but 

 irrepressible and progressive revolution in thought, 

 which cannot fail profoundly to alter men's point 

 of view, not only of life in general, but of the nature 

 and judgment of human conduct. Is it not rea- 

 sonable to welcome any aid that biological thought 

 may extend to us hi relation to human actions ; 



