Preface. vii 



expressed ; but I should like to take this opportunity 

 of also expressing my obligations to the students who 

 attended the lectures in the University of Edinburgh. 

 For alike in respect of their large numbers, their 

 keen intelligence, and their generous sympathy, the 

 members of that voluntary class yielded a degree of 

 stimulating encouragement, without which the labour 

 of preparing the original lectures could not have been 

 attended with the interest and the satisfaction that I 

 found in it. My thanks are also due to Mr. R. E. 

 Holding for the painstaking manner in which he has 

 assisted me in executing most of the original drawings 

 with which this volume is illustrated ; and likewise to 

 Messrs. Macmillan and Co. for kindly allowing me 

 to reprint without special acknowledgment in every 

 case certain passages from an essay which they 

 published for me many years ago, under the title 

 "Scientific Evidences of Organic Evolution. Lastly, 

 I must mention that I am indebted to the same firm 

 for permission to reproduce an excellent portrait of 

 Mr. Darwin, which constitutes the frontispiece. 



G. J. R. 



CHRIST CHURCH, OXFORD, 

 April \()th, 1892. 



