PREFACE 



This book gives the substance of a series of lectures delivered 

 in Yale University, where I had the privilege of holding the office 

 of Silliman Lecturer in 1907. 



The delay in publication was brought about by a variety of 

 causes. 



Inasmuch as the purpose of the lectures is to discuss some of 

 the wider problems of biology in the light of knowledge acquired 

 by Mendelian methods of analysis, it was essential that a fairly 

 full account of the conclusions established by them should first 

 be undertaken and I therefore postponed the present work till 

 a book on Mendel's Principles had been completed. 



On attempting a more general discussion of the bearing of 

 the phenomena on the theory of Evolution, I found myself 

 continually hindered by the consciousness that such treatment 

 is premature, and by doubt whether it were not better that the 

 debate should for the present stand indefinitely adjourned. 

 That species have come into existence by an evolutionary 

 process no one seriously doubts; but few who are familiar with 

 the facts that genetic research has revealed are now inclined to 

 speculate as to the manner by which the process has been ac- 

 complished. Our knowledge of the nature and properties of 

 living things is far too meagre to justify any such attempts. 

 Suggestions of course can be made: though, however, these 

 ideas may have a stimulating value in the lecture room, they 

 look weak and thin when set out in print. The work which may 

 one day give them a body has yet to be done. 



The development of negations is always an ungrateful task 

 apt to be postponed for the positive business of experiment. 

 Such work is happily now going forward in most of the centers 

 of scientific life. Of many of the subjects here treated we already 

 know more than we did in 1907. The delay in production has 

 made it possible to incorporate these new contributions. 



The book makes no pretence at being a treatise and the 



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