vi author's treface 



whole the theories which for forty years I have been gradually 

 buikling up on the basis of the legacy of the great workers of 

 the past, and on the results of my own investigations and 

 those of many fellow workers, not because I regard the picture 

 as complete or incapable of improvement, but because I believe 

 its essential features to be correct, and because an eye-trouble 

 which has hindered my work for many years makes it 

 uncertain whether I shall have much more time and strength 

 granted to me for its further elaboration. We are standing 

 in the midst of a flood-tide of investigation, which is ceaselessly 

 heaping up new facts bearing upon the j^roblem of evolution. 

 Every theory formulated at this time must be prepared 

 shortly to find itself face to face with a mass of new facts 

 Avhich may necessitate its more or less complete reconstruction. 

 How much or how Httle of it may remain, in face of the facts 

 of the future, it is impossible to predict. But this will be so 

 for a long time, and it seems to me we must not on that 

 account refrain from following out our convictions to the 

 best of our ability and presenting them sharply and definitely, 

 for it is only well-defined arguments which can be satis- 

 factorily criticized, and can be unproved if they are imperfect, 

 or rejected if they are erroneous. In both these processes 

 progress lies. 



This book consists of ' Lectures ' which Avere given publicly 

 at the university here. In my introductory lecture in 1867 

 I championed the Theory of Descent, which was then the 

 subject of lively controversy, but it was not till seven years 

 later that I gave, by way of experiment, a short summer 

 course with a view to aiding in the dissemination of Dar^vin's 

 views. Then very gradually my own studies and researches 

 and those of others led me to add to the Darwinian edifice, 

 and to attempt a further elaboration of it, and accordingly 

 these ' Lectures,' which were delivered almost regularly every 

 year from 1880 onw^ards, were gradually modified in accordance 

 with the state of my knowledge at the time, so that they 

 have been, I may say, a mirror of the course of my own 

 intellectual evolution. 



