PREFACE 



A PREFACE should be long, like one of Mr. 

 Shaw's, or short. I propose the latter. 

 The essays here collected were written on 

 very various occasions. This must excuse the con- 

 siderable overlap that will be found among them. 

 I have not thought it worth while to attempt to get 

 rid of this, since, though facts may be repeated, the 

 point of view and general context are on each oc- 

 casion different. 



Contrary to all custom, I have put the meat courses 

 at the two ends of my menu. If an author may pre- 

 sume to advise his readers, I would suggest that, 

 after finishing the first essay, they should (if they 

 retain a stomach for more) proceed at once to the 

 last. This done, they will find the others all in a 

 sense lesser variations (if I may change my meta- 

 phor) upon the same themes. 



In spite, however, of the diversity of their occa- 

 sions, there is a common thread running through 

 them, a common background of ideas. I do not 

 know whether I am justified in calling those ideas 

 especially biological, but they are certainly ideas 

 which must present themselves to any biologist who 



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