x PREFACE 



line of work, have again and again abandoned 

 the most dearly loved hypothesis when a new 

 interpretation was seen to be more consistent with 

 an ever-growing store of facts. And I submit 

 that it is even more difficult to keep an open 

 mind in the pursuit of a special line of research 

 than in the consideration of the broadest and 

 most far-reaching problems which confront the 

 human intellect. 



Although the splendidly thorough work of the 

 present day must rightly compel the warmest 

 admiration, there are valid reasons why we should 

 direct a searching and critical gaze upon the pro- 

 clamation of each enthusiastic specialist that the 

 foundations of organic evolution are wholly sur- 

 rounded by the boundaries of his own field of 

 inquiry. Organic evolution, to be understood, 

 must be studied not in the light of one special 

 line of work, but of all. This was the great secret 

 of Darwin's unique power in dealing with it. He 

 could see the subject from all sides. And an 

 ample measure of Darwin's strength was possessed 

 by his great comrades of half a century ago. How 

 we long for a little of the sure insight and com- 

 prehensive vision of Asa Gray as we read the 

 address of his distinguished living representative, 

 Professor J. M. Coulter, who considers that an 

 adaptive response to environment is destructive 

 of Natural Selection, and finds it hard to imagine 



