PREFACE 



THE present volume covers a wider area than my last : 

 The Principles of Heredity. No problems can be named 

 of greater importance to the community than those of 

 Heredity. I have tried, therefore, to make the argument as plain 

 as possible to the general reader. And, since facts which we are 

 able to observe and verify for ourselves, are more impressive and 

 valuable intellectually than those we are obliged to accept at 

 second hand, I have used, whenever possible, material that is 

 familiar to every one. 



Some biologists may regard Chapter III., which deals with 

 scientific method, as unnecessary and even as impertinent. But 

 every writer hopes to gain wide assent to his reasoning ; and, 

 there are, apparently, amongst thinkers about living beings, such 

 fundamental differences of opinion as to what constitutes evidence, 

 what logical proof, and what science, that no one can hope to 

 secure wide assent unless he first lay down the principles that 

 have guided his own thinking and obtains assent to them. If 

 the history of biology be considered, I believe it will be seen 

 that many of the controversies, in which its students have en- 

 gaged, arose and persisted merely because the disputants, holding 

 what were really divergent views as to the nature of science, failed 

 to reach a preliminary agreement with respect to their basic 

 assumptions. As far as my powers permitted, I have followed the 

 rules of procedure that have been enunciated by every authority 

 who has both thought and written about the subject. When these 

 rules have been disregarded, the writers, however right they may 

 be able to prove themselves ultimately, have never expressed their 

 dissent in reasoned statements ; they have merely implied it in 

 their methods of work. 



If, then, the reader disagrees with commonly accepted opinions, 

 if, for example, he especially values evidence not solely because 

 it is authentic and relevant but because it has been gathered in 

 some particular way, or if he thinks that we should not use all the 



