PREFACE IX 



sible, the criticisms which are only hinted, at here. At the 

 same time I cannot hope to build. That is the task of the 

 great band of workers in many departments of Biology, who, 

 undeterred by failure and urged on by the fire, enthusiasm, 

 and generous aspirations of youth, return time after time, 

 generation after generation, to the assault of the fortresses 

 of nature well knowing that their material reward will be 

 small, that defeat means the world's neglect and that success, 

 except the greatest, brings but a pittance of its esteem. To 

 them I inscribe this book in the hope that it may serve if 

 only to a small extent to smooth over the difficulties of part 

 of the road which at first they have to traverse. 



A. SEDGWICK. 

 Trinity College, Cambridge, 

 February, 1905. 



