THE PROTEIN MOLECULE 99 



duction. However, changes in the chemical 

 structure may and do occur and in these lies the 

 basis of variation. 



The keystone or archon of the protein mole- 

 cule is the protein poison. It is common to all 

 protein molecules. Physiologically it is the 

 same in all molecules ; i. e., when set free it is a 

 poison and it is a poison on account of its in- 

 tense chemism which enables it to tear off 

 groups from other proteins. One protein dif- 

 fers from another in its secondary and tertiary 

 groups. Most native proteins are not poison- 

 ous because in them the chemism of the pri- 

 mary group is satisfied by combination with 

 secondary groups. Strip off the secondary 

 groups and the primary becomes poisonous on 

 account of the avidity with which they combine 

 with the secondary groups of other molecules. 

 Biological relationship between proteins de- 

 pends upon the secondary groups. In this way 

 varieties and species have developed. 



The living molecule is never in a state of 

 equilibrium. There is a constant exchange of 

 atoms between it and the outside world. It ab- 

 sorbs, assimilates and eliminates. It is con- 

 stantly trading in energy. It takes in oxygen 

 and gives off carbonic acid ; it takes in nitrog- 

 enous material, and, having utilized it, the waste 



