1872.] ISOLATION. 339 



on my father's views on the action of isolation as regards the 

 origin of new species :] 



C. Darwin to K. Semper. 



Down, November 26, 1878. 



My dear Professor Semper, — -When I published the 

 sixth edition of the ' Origin,' I thought a good deal on the 

 subject to which you refer, and the opinion therein expressed 

 was my deliberate conviction. I went as far as I could, per- 

 haps too far in agreement with Wagner ; since that time I 

 have seen no reason to change my mind, but then I must add 

 that my attention has been absorbed on other subjects. 

 There are two different classes of cases, as it appears to me, 

 viz. those in which a species becomes slowly modified in the 

 same country (of which I cannot doubt there are innumerable 

 instances) and those cases in which a species splits into two 

 or three or more new species, and in the latter case, I should 

 think nearly perfect separation would greatly aid in their 

 " specification," to coin a new word. 



I am very glad that you are taking up this subject, for you 

 will be sure to throw much light on it. I remember well, 

 long ago, oscillating much ; when I thought of the Fauna and 

 Flora of the Galapagos Islands I was all for isolation, when I 

 thought of S. America I doubted much. Pray believe me, 



Yours very sincerely, 



Ch. Darwin. 



P.S. — I hope that this letter will not be quite illegible, 

 but I have no amanuensis at present. 



C. -Darwin to K. Semper. 



Down, November 30, 1878. 

 Dear Professor Semper, — Since writing I have recalled 

 some of the thoughts and conclusions which have passed 

 through my mind of late years. In North America, in going 

 from north to south or from east to west, it is clear that the 

 changed conditions of life have modified the organisms in the 



