THE LIVING SUBSTANCE 7 



The dead body is the seat of rapid change ; it falls 

 apart, loses its identity. This is precisely what the 

 living substance does not ; it has the marvelous power 

 of retaining its identity as a system, though all its 

 actual atoms may be lost and replaced by others. We 

 are reminded of a river, which looks the same from day 

 to day, though the water passes by. This ability to 

 retain its character in the midst of change applies not 

 merely to protoplasm in the broad sense, but also to 

 all the myriads of particular kinds, of which, as we shall 

 see, many coexist in the same individual. Thus this 

 ever changing substance (in one sense) is so stable 

 (in another sense) that it may continue almost un- 

 modified for millions of years, while mountain ranges 

 are raised up or worn away. This we know by com- 

 paring the fossil remains of animals and plants with 

 their modern representatives. The ancient protoplasm 

 itself has not been preserved, but the exact forms of 

 creatures of bygone ages are often clearly represented 

 in the rock, and we can rest assured in many cases 

 that their living substance was similar to that of 

 existing types. 



4. Students of chemistry recognize inorganic and carbon 

 organic chemistry. So-called organic chemistry is the com P ound8 

 chemistry of those carbon compounds which were 

 formerly supposed to stand in a class by themselves, 

 being produced by living beings, or derived from the 

 products of such. As carbon is only one of the numer- 

 ous elements, the study of its more intricate com- 

 pounds might seem to be a very small branch of the 

 science ; but as a matter of fact it is a very large part 

 of chemistry, and the most complex and difficult part. 

 The carbon atom has quite unique properties, and its 

 power of combining with other atoms is such that it 



