ii6 EXPERIMENTAL EVOLUTION LECT. 



equal magnitude, they admit of being detected 

 with real accuracy by appropriate methods. While 

 calling this variation physiological, I understand it 

 to be, in fact, chemical ; the difference is in chemical 

 constitution most probably but it displays itself 

 mostly in physiological differences. I have collected 

 some cases of this variability, endeavouring specially 

 to obtain widely different instances, in order to show 

 the extent to which this kind of variation occurs. 



Of positive chemical variability among animals, I 

 meet with a good instance provided by two well- 

 known savants Ch. Robin, the histologist, and 

 Sainte-Claire Deville, the chemist who, at Sanson's 

 request, examined comparatively the structure and 

 composition of the bony structures of the common 

 breed, and of a perfected breed, of sheep. While the 

 microscope detected no difference at all between the 

 two breeds, chemical analysis showed considerable 

 variations in the respective percentages of organic 

 and inorganic matters, as follows : 



Organic Substances. Inorganic Substances. 



Perfected breed 32*3 per cent. 677 per cent. 



Common breed 38*6 61*4 



Similarly, considerable variations obtain in the 

 chemical constitution of the integumentary append- 

 ages of different varieties of animals. Here follows, 



