414 THE CAUSES OF THE pees ae 
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, | 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, 
