PREFACE 



MY chief object in writing this volume was to discuss 

 the relations of modern discoveries concerning 

 hormones or internal secretions to the question of 

 the evolution of adaptations, and on the other hand 

 -to the results of recent investigations of Mendelian 

 heredity and mutations. I have frequently found, 

 from verbal or written references to my opinions, 

 that the evidence on these questions and my own 

 conclusions from that evidence were either im- 

 perfectly known or misunderstood. This is not 

 surprising in view of the fact that hitherto my only 

 publications on the hormone theory have been a 

 paper in a German periodical and a chapter in an 

 elementary text-book. The present publication is 

 by no means a thorough or complete exposition of 

 the subject, it is merely an attempt to state the 

 fundamental facts and conclusions, the importance 

 of which it seems to me are not generally appreciated 

 by biologists. 



I have reviewed some of the chief of the recent 

 discoveries concerning mutations, Mendelism, chromo- 

 somes, etc., but have not thought it necessary 

 to repeat the illustrations which are contained in 

 many of the volumes to which I have referred. I 

 have made some Mendelian experiments myself, 

 not always with results in agreement with the strict 



