88 VARIATION 



is put into the circulation. In the case of Rose and Nora, pre- 

 viously cited, what did Nora do with the excess of food con- 

 sumed ? Digestion experiments with individuals indicate no such 

 differences in digestion among healthy animals as the differences 

 in milk yield that are known to exist between cows. The only 

 conclusion is that in a case like this the surplus food passed on 

 out of the body, laying excessive labor upon the excreting organs 

 as well as incurring loss upon the one who provided the feed. 1 



From this it will be seen that the excreting organs them- 

 selves act as a kind of safety valve, and that much depends 

 upon their relative ability to discharge their functions well. 

 This they are better adapted to do in some individuals than in 

 others, but that every effort is made to keep up with demands 

 is evidenced by the fact that if one kidney is lost the other 

 acts for both, usually increasing in size. Speaking generally, a 

 cow will give as much milk from three teats as from four. 

 Whether this is from compensation, as with the lost kidney, or 

 whether it is true only because cows are seldom worked up to 

 their limits, the data at hand do not determine. 



The tremendous increase in the activity of the salivary glands 

 on the part of tobacco chewers, the increase in size of muscles 

 through use, and the marvelous development of skill in eye and 

 hand by long-continued practice, as in the playing of musical 

 instruments, reading on the part of the blind, etc., these are 

 all familiar examples of functional development through prac- 

 tice. That this development is or may be greater in some indi- 

 viduals than in others is too well known to need more than 

 passing mention in this connection. 



Resistance to disease and invasions of the animal economy 

 generally differ greatly in different individuals. Some are abso- 

 lutely immune to certain diseases, others peculiarly susceptible. 



It is a matter of common observation that in fields of corn 

 killed by frost an occasional stalk remains green and unaffected, 

 showing unusual powers of resistance, due to some constitu- 

 tional difference. Without a doubt these are the differences 



1 This is conclusive proof of the fact that appetite is not a safe guide to the 

 amount of food that can be profitably consumed. The most that can be said is 

 that it is a good indication of body use among animals whose efficiency is known. 



