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, 



