iv PREFACE 



trons in ceaseless motion, and groups of atoms being con- 

 stantly cast out of the molecule and replaced by new groups 

 split off from outside matter. As soon as a molecule becomes 

 the seat of assimilation and excretion, it is no longer dead; 

 it lives. As a result of assimilation it acquires the property 

 of building up its own structure; then polymerization 

 follows and reproduction in its simplest form begins. The 

 one phenomenon always manifested by living matter, and 

 never exhibited by non-living matter, is metabolism. 



When matter becomes endowed with life it does not 

 cease to be matter; it does not lose its inherent properties; 

 it is not released from the laws that govern its structure, 

 its attractions, and its motions. In studying living things 

 it should be borne in mind that they are material in com- 

 position and subject to the fundamental laws that govern 

 matter, and possessed of those properties essential to 

 matter. 



Matter is alive when it feeds and excretes. The living 

 molecule not only absorbs; it assimilates. It chemically 

 alters what it absorbs, and within limits, it may be altered 

 by what it absorbs. Atomic groups taken into living 

 molecules enter into new combinations. The living mole- 

 cule is not stabile, but is highly labile. Its composition is 

 never constant, and it is never in a condition of equilibrium. 

 It is in constant chemical reaction with outside matter. 

 Apart from other matter it could not exist. There is a 

 constant interchange of atoms between it and external 

 matter. A condition, best designated as latent life, may 

 exist without interchange of atoms. This is exemplified 

 in spores, seeds, and ova. Matter existing in these forms 

 may be awakened into activity by proper stimuli; active 

 life begins with the interchange of atoms. 



