AUTHOR'S PREFACE 



When a life of pleasant labour is drawing towards a close, 

 the wish naturally asserts itself to gather together the main 

 results, and to combine them in a well-defined and harmonious 

 picture which may be left as a legacy to succeeding generations. 



This wish has been my main motive in the pubHcation 

 of these lectures, which I delivered in the University of 

 Freiburg in Breisgau. But there has been an additional 

 motive in the fact that the theory of heredity published 

 by me a decade ago has given rise not only to many in- 

 vestigations prompted by it, but also to a whole literature 

 of ' refutations,' and, what is much better, has brought to 

 light a mass of new facts which, at first sight at least, seem 

 to contradict my main theory. As I remain as convinced 

 that the essential part of my theory is well grounded as 

 I was when I first sketched it, I naturally wish to show how 

 the new facts may be brought into harmony with it. 



It is by no means only with the theory of heredity by 

 itself that I am concerned, for that has served, so to speak, 

 as a means to a higher end, as a groundwork on which to 

 base an interpretation of the transformations of life through 

 the course of the ages. For the phenomena of heredity, like 

 all the functions of individual life, stand in the closest 

 association with the whole evolution of life upon our earth ; 

 indeed, they form its roots, the nutritive basis from which all 

 its innumerable branches and twigs are, in the long run, 

 derived. Thus the phenomena of the individual life, and 

 especially those of reproduction and inheritance, must be 

 considered in connexion with the Theory of Descent, that 

 the latter may be illumined by them, and so brought nearer 

 our understanding. 



I make this attempt to sum up and present as a harmonious 



