PREFATORY NOTE 



IN the present volume the writers have tried to give a lucid 

 elementary account, in limited space, of the processes of evo- 

 lution as they are so far understood. We have turned to ani- 

 mals for illustrative purposes, nearly to the exclusion of refer- 

 ences to plants, simply because both authors are zoologists 

 and have made use of the facts most familiar to them. 



The book is composed primarily of the substance of a univer- 

 sity course of elementary lectures delivered jointly by the 

 authors each year to students representing all lines of college 

 work. This fact, and the desirable limiting of the book to a 

 convenient size for the general reader and student, account for 

 the extremely laconic treatment of various important moot 

 points concerning the evolution mechanism, and for the omission 

 of certain discussions which otherwise might well have been 

 included. But on the whole the authors feel that the interested 

 general reader will find this small volume a fairly comprehensive 

 introduction to our present-day knowledge of the factors and 

 phenomena of organic evolution. 



To the general reader we may perhaps with propriety ad- 

 dress the following words, used to the students in the opening 

 lecture of the course: 



We cannot talk long without saying something others do 

 not believe. Others cannot talk long without saying something 

 we do not believe. We wish you to accept no view of ours 

 unless you reach it through your own investigation. What we 

 hope for is to have you think of these things and find out for 



yourselves. 



D. S. J. 

 V. L. K. 



LELAND STANFORD JUNIOR UNIVERSITY, 

 March 30, 1907. 



i. 





