vi INTRODUCTION 



skeptical toward every advance. Our usefulness will, 

 in the long run, be proven by whether or not we have 

 been discriminating and sympathetic in our attitude 

 toward the important discoveries of our time. While 

 every one will probably admit such generalities, some of 

 us may call those who accept less than ourselves con- 

 servatives ; others of us who accept more will be called 

 rash or intemperate. To maintain the right balance 

 is the hardest task we have to meet. In attempting to 

 bring together, and to interpret, work that is still in the 

 making I cannot hope to have always made the right 

 choice, but I may hope at least for some indulgence 

 from those who realize the difficulties, and who think 

 with me that it may be worth while to make the 

 attempt to point out to those who are not specialists 

 what specialists are thinking about and doing. 



What I most fear is that in thus attempting to for- 

 mulate some of the difficult problems of present-day 

 interest to zoologists I may appear to make at times 

 unqualified statements in a dogmatic spirit. I beg to 

 remind the reader and possible critic that the writer 

 holds all conclusions in science relative, and subject 

 to change, for change in science does not mean so much 

 that what has gone before was wrong as the discovery 

 of a better strategic position than the one last held. 



