INTRODUCTION. IX 



brates has been to me a stimulus and a guide. What appears to be an approxi- 

 mate solution of it has been tested and tested again, and elaborated in itself the 

 severest test of all by many methods and from many points of view, for it has 

 seemed to me the one great problem that must be solved before the biologist can 

 approach the problems of creative evolution on a reasonably secure footing. To 

 gain this end, I have given the best I had; whether that is much or little is of no 

 consequence, except in so far as it is a guarantee of serious endeavor and of good 

 faith. That I am conscious of many difficulties and imperfections need not be 

 emphasized. I would gladly make them less. But to be overconscious of the 

 one, unsteadies the hand and draws the eye away from the open waters, and too 

 long a delay over the inevitable defects means to be surprised by the night, and 

 still unprepared. 



