418 THE CAUSES OF THE X I 



variety which had arisen in this manner the 

 variation might be perpetuated, as far as we can 

 see, indefinitely. 



The next question, and it is an important one 

 for us, is this : Is there any limit to the amount 

 of variation 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 characteristics, 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 charac- 

 teristics, 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 characters 

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

 might be an absolute difference in the number of 

 the vertebra of the back, as in the sacral vertebras 

 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 



