414 THE CAUSES OF THE xi 



external appearance ; but these differences are by 

 no means the whole or even the most important of 

 the differences which obtain between these birds. 

 There is hardly a single point of their structure 

 which has not become more or less altered ; and to 

 give you an idea of how extensive these alterations 

 are, I have here some very good skeletons, for which 

 I am indebted to my friend, Mr. Tegetmeier, a 

 great authority in these matters ; by means of 

 which, if you examine them by and by, you will 

 be able to see the enormous difference in their 

 bony structures. 



I had the privilege, some time ago, of access to 

 some important MSS. of Mr. Darwin, who, I may 

 tell you, has taken very great pains and spent 

 much valuable time and attention on the investi- 

 gation of these variations, and getting together all 

 the facts that bear upon them. I obtained from 

 these MSS. the following summary of the differ- 

 ences between the domestic breeds of pigeons ; 

 that is to say, a notification of the various points 

 in which their organisation differs. In the first 

 place, the back of the skull may differ a good deal, 

 and the development of the bones of the face may 

 vary a great deal ; the back varies a good deal ; 

 the shape of the lower jaw varies ; the tongue 

 varies very greatly, not only in correlation to the 

 length and size of the beak, but it seems also to 

 have a kind of independent variation of its own. 

 Then the amount of naked skin round the eyes, 



