EVOLUTION OF METAZOA 117 



ters arising from the heredity-chromatin are changed by long- 

 continued selection through a great number of generations in 

 the form of slow gradations which would not be revealed by 

 imperfect selection for a few generations. This is doubtless 

 the way in which nature works. In the protozoan known as 

 Diffiugia the inherited changes produced by selection seem as 

 gradual as could well be observed. Large steps do occur, but 

 much more frequent is the slow alteration of the stock with 

 the passage of generations. The question is asked whether 

 even such slight and seemingly gradual hereditary changes 

 may not really be little jumps or mutations, since all chemical 

 change is discontinuous. In reply, Jennings observes that it is 

 highly probable that every inherited variation does involve a 

 chemical change, for there is no character change so slight that 

 it may not be chemical in nature. In the relatively immense 

 organic molecule, with its thousands of groups, the simple trans- 

 fer of one atom, one ion, perhaps one electron, is a chemical 

 change and, in this sense, discontinuous even though its effect 

 is below our powers of perception with the most refined instru- 

 ments. 



Through this modern chemical interpretation of the pro- 

 tozoan life cycle we may conceive how the laws of thermody- 

 namics may be apphed to single-celled organisms, and espe- 

 cially our fundamental biologic law of action, reaction, and inter- 

 action. By far the most difficult problem in biologic evolution 

 is the mode of working of this law among the many-celled or- 

 ganisms (Metazoa) including both invertebrates and vertebrates. 



Evolution of Many- Celled Animals or Metazoa 



It is possible that during the long period of pre-Cambrian 

 time, which, from the actual thickness of the Canadian pre- 

 Cambrian rocks, is estimated at not less than thirty million 



