viii PREFACE 



unverifiable speculation bids fair to dull our appreciation for hypoth- 

 eses whose chief value lies in the possibility of their verification; 

 but those who have spent their time and their imagination in such 

 speculations cannot hope for long to hold their own against the 

 slow but certain advance of a scientific spirit of investigation of 

 organic phenomena. The historical questions with which so many 

 problems seem to be connected, and for which there is no rigorous 

 experimental test, are perhaps responsible for the loose way in which 

 many problems in biology are treated, where fancy too often supplies 

 the place of demonstration. If, then, I have tried to use my mate- 

 rial in such a way as to turn the evidence against some of the 

 uncritical hypotheses of biology, I trust that the book may have 

 a wider bearing than simply as a treatment of the problems of 

 regeneration. 



I wish to acknowledge my many obligations to Professor H. F. 

 Osborn and to Professor E. B. Wilson for friendly criticism and 

 advice ; and in connection with the revision of the text I am greatly 

 indebted to Professor J. W. Warren, to Professor W. M. Wheeler, 

 to Professor G. H. Parker, and to Professor Leo Loeb. 



BRYN MAWR COLLEGE, PENNSYLVANIA, 

 June ii, 1901. 



