Preface vii 



tary characters cannot contribute at all or only to a 

 limited extent to the formation of new species. Such 

 an idea is supported by the work on immunity, which 

 shows that genus- and probably species-specificity are 

 due to specific proteins, while the Mendelian characters 

 may be determined by hormones which need neither be 

 proteins nor specific or by enzymes which also need 

 not be specific for the species or genus. Such a con- 

 ception would remove the difficulties which the work on 

 Mendelian heredity has seemingly created not only for 

 the problem of evolution but also for the problem of 

 the harmonious character of the organism as a whole. 



Since the book is intended as a companion volume 

 to the writer's former treatise on The Comparative 

 Physiology of the Brain a discussion of the functions of 

 the central nervous system is omitted. 



Completeness in regard to quotation of literature 

 was out of the question, but the writer notices with 

 regret, that he has failed to refer in the text to so 

 important a contribution to the subject as Sir E. A. 

 Schafer's masterly presidential address on 'Life ' or 

 the addresses of Correns and Goldschmidt on the de- 

 termination of sex. Credit should also have been given 

 to Professor Raymond Pearl for the discrimination be- 

 tween species and individual inheritance. 



The writer wishes to acknowledge his indebtedness 

 to his friends Professor E. G. Conklin of Princeton, 

 Professor Richard Goldschmidt of the Kaiser Wilhelm 



