PREFACE ix 



That is, perhaps, in some respects unfortunate, 

 because the impression which must be pro- 

 duced is unlike that of the f acts t hemselves, 

 and the argument is certainly very different 

 from the mental process through which I have 

 myself passed in reaching the conclusion. But 

 it would be very difficult indeed in any^ other 

 manner to set forth all the necessary consid- 

 erations, so that they should be intelligible to 

 any one but the physico-chemical biologist, 

 and on the whole there seems to be no choice 

 but to follow a logical rather than a descrip- 

 tive method. The reader will find, however, 

 that in the main Chapters III, IV, V, and 

 VI are concerned with evidence alone. This 

 taken together, as a whole, is the only true 

 ground for a conclusion, and it is upon the 

 general character of the evidence rather than 

 upon an argument which only serves as a 

 means to the end that I should wish to rest 

 my case. To many, on the other hand, as to 

 any biologist who may not care to examine 

 the difficult facts of physics and chemistry, 

 Chapters I, II, VII, and VIII will be sufficient 

 to explain both the purpose of the book and 

 the outcome of the investigation. 



There has been a constant effort to restrict all 

 discussions to the utmost, because at the pres- 

 ent time encyclopaedic handbooks covering all 



