PREFACE 



IT is generally admitted that the individual physio- 

 logical processes, such as digestion, metabolism, the 

 production of heat or of electricity, are of a purely 

 physicochemical character; and it is also conceded that 

 the functions of individual organs, such as the eye or 

 the ear, are to be analysed from the viewpoint of the 

 physicist. When, however, the biologist is confronted 

 with the fact that in the organism the parts are so 

 adapted to each other as to give rise to a harmonious 

 whole; and that the organisms are endowed with 

 structures and instincts calculated to prolong their life 

 and perpetuate their race, doubts as to the adequacy of 

 a purely physicochemical viewpoint in biology may 

 arise. The difficulties besetting the biologist in this 

 problem have been rather increased than diminished 

 by the discovery of Mendelian heredity, according to 

 which each character is transmitted independently of 

 any other character. Since the number of Mendelian 

 characters in each organism is large, the possibility 

 must be faced that the organism is_'merely a mosaic of 

 independent hereditary characters. If this be the case 



