CHAPTER VI. 



A BRIEF HISTORY OF OPINIONS ON ISOLATION 

 AS A FACTOR OF ORGANIC EVOLUTION. 



THIS historical sketch must begin with a considera- 

 tion of Darwin's opinions on the subject ; but as these 

 were considerably modified from time to time during 

 a period of thirty years by the publications of other 

 naturalists, it will be impossible to avoid cross- 

 references as between his writings and theirs. It 

 may also be observed that the Life and Letters of 

 Charles Darwin was not published until the year 

 1887, so that the various opinions which I shall 

 quote from the letters, and which show some con- 

 siderable approximation in his later years to the 

 views which have been put forward by Mr. Gulick 

 and myself, were not before us at the time when our 

 papers were read. 



The earliest allusion that I can find to geographical 

 isolation in the writings of Darwin occurs in a 

 correspondence with Sir Joseph Hooker, as far back 

 as 1844. He there says : 



I cannot give my reasons in detail ; but the most general 

 conclusion which the geographical distribution of all organic 



