108 CONDITIONS AFFECTING THE 



The next question, and it is an important one for 

 us, is this : Is there any limit to the amount of varia- 

 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 charac- 

 teristics, I endeavoured to show you, by the skeletons 

 which I had upon the table, and by reference to a 

 great many weU-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 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 struc- 

 tural 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 Tumbler 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 differences 



