PERPETUATION OF LIVING BEINGS. 103 



tion from the primitive stock which can be produced 

 by this- process of selective breeding? In considering 

 this question, it will be useful to class the character- 

 istics, in respect of which organic beings vary, under 

 two heads : we may consider structural characteristics, 

 and we may consider physiological characteristics. 



In the first place, as regards structural characteris- 

 tics, I endeavoured to show you, by the skeletons which 

 I had upon the table, and by reference to a great many 

 well-ascertained facts, that the different breeds of 

 Pigeons, the Carriers, Pouters, and Tumblers, might 

 vary in any of their internal and important structural 

 characters to a very great degree ; not only might there 

 be changes in the proportions of the skull, and the char- 

 acters of the feet and beaks, and so on ; but that there 

 might be an absolute difference in the number of the 

 vertebrae of the back, as in the sacral vertebrae of the 

 Pouter ; and so great is the extent of the variation in 

 these and similar characters that I pointed out to you, 

 by reference to the skeletons and the diagrams, that 

 these extreme varieties may absolutely differ more from 

 one another in their structural characters than do what 

 naturalists call distinct Species of pigeons; that is to 

 say, that they differ so much in structure that there is 

 a greater difference between the Pouter and the Tum- 

 bler than there is between such wild and distinct forms 

 as the Rock Pigeon or the Ring Pigeon, or the Ring 

 Pigeon and the Stock Dove ; and indeed the differences 

 are of greater value than this, for the structural differ- 

 ences between these domesticated pigeons are such as 

 would be admitted by a naturalist, supposing he knew 

 nothing at all about their origin, to entitle them to 

 constitute even distinct genera. 



