100 THE PERPETUATION OF LIVING BEINGS, 



breeds of pigeons ; that is to say, a notification of the 

 various points in which their organization 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 inde- 

 pendent variation of its own. Then the amount of 

 naked skin round the eyes, and at the base of the beak, 

 may vary enormously ; so may the length of the eye- 

 lids, the shape of the nostrils, and the length of the 

 neck. I have already noticed the habit of blowing out 

 the gullet, so remarkable in the Pouter, and compara- 

 tively so in the others. There are great differences, 

 too, in the size of the female and the male, the shape 

 of the body, the number and width of the processes of 

 the ribs, the development of the ribs, and the size, 

 shape, and development of the breastbone. "We may 

 notice, too, — and I mention the fact because it has 

 been disputed by what is assumed to be high author- 

 ity, — the variation in number of the sacral vertebrae. 

 The number of these varies from eleven to fourteen, 

 and that without any diminution in the number of the 

 vertebrae of the back or of the tail. Then the number 

 and position of the tail-feathers may vary enormously, 

 and so may the number of the primary and secondary 

 feathers of the wings. Again, the length of the feet 

 and of the beak, — although they have no relation to 

 each other, yet appear to go- together, — that is, you 

 have a long beak wherever you have long feet. There 

 are differences also in the periods of the acquirement of 

 the perfect plumage, — the size and shape of the eggs, — 



