THE COMING OF EVOLUTION 195 



as well as with atoms of other elements, to form 

 very complex molecules. We have seen how Para- 

 celsus and Stahl carried on the old theory of a distinct 

 vital principle in opposition to the equally old idea 

 that in living bodies, as in the outside material world, 

 mechanism would ultimately explain all happenings. 

 It was long thought that the specially complicated 

 substances which are characteristic of animal and 

 vegetable tissue could be prepared only under the 

 influence of vital processes, and the belief in a spiritual 

 interpretation of the phenomenon of life was thought 

 to stand or fall with the truth of this view. But the 

 artificial preparation of urea by Friedrich Wohler in 

 1828 showed that the methods of the laboratory 

 were capable of creating substances hitherto reserved 

 for living processes. Other preparations followed, 

 till in 1887 Fischer and Tafel succeeded in mak- 

 ing sugar from its elements. The secrets of the 

 chemistry of the body seemed in a fair way to become 

 known. 



Simultaneously with the extension of chemistry to 

 cover many vital changes, much advance was made 

 The Physics m a pplyi n g physical principles to the 

 of Physiology, problems of physiology. Harvey ex- 

 plained the motion of the blood as that of a moving 

 fluid pumped through the tubes of arteries and veins 

 by the mechanical action of the heart, and thus 

 gave a naturalistic turn to physiological enquiry. 

 But, in the second half of the eighteenth century, 

 the difficulty of the problem led to the almost 

 universal adoption of the hypothesis of vitalism, the 

 force hyperm&canique of the French school, which 



