METABOLISM 7S 



fact that a dog fed on mutton breaks 

 down the sheep protein into simpler sub- 

 stances which are in part built up into 

 the proteins characteristic of the dog. 

 Each species of animal or plant appar- 

 ently has its special kind of protein. 

 This can be demonstrated by injecting 

 into an animal the protein of another 

 species. This is always more or less 

 toxic. 



We are inclined to assume that this 

 power of transformation is indispensable 

 to life but it would be interesting to 

 know what would happen if it were 

 taken away. Doubtless the organism 

 would change, but whether all organisms, 

 including the simplest, would die is an 

 open question. 



We may sum up by saying that metab- 

 olism, once regarded as too mysterious 

 to be understood, now appears to be only 

 a complex chemical process which has 

 been imitated to a remarkable degree. 

 If we can continue the rapid progress 

 of recent years we have reason to hope 



