vi PREFACE 



been incorporated of a paper read before the American 

 Philosophical Society at their meeting in honor of the bi- 

 centennary of the birth of their founder, Benjamin Franklin, 

 April, 1906. 



The results of Nilsson have been published only in the 

 Swedish language; those of Burbank have not been de- 

 scribed by himself. My arguments for the theory of mu- 

 tation have been embodied in a German book, " Die Mu- 

 tationstheorie" (2 vols. Leipsic, Veit & Co.), and in lectures 

 given at the University of California in the summer of 1904, 

 published under the title of "Species and Varieties; their 

 Origin by Mutation." A short review of them will be found 

 in the first chapter of these Essays. 



