THE PHYSICAL BASIS OF LIFE 19 



occurs in non-living things when a salt is dissolved in water; 

 but such formal objections only emphasize the unique condi- 

 tions which obtain in life. 



The reader may be surprised to note that the power of 

 movement has not been mentioned as a characteristic of life, 

 but a moment's thought will make it apparent that visible 

 movement is not confined to living matter. Though this is 

 so, movement is one of the most obvious manifestations of 

 life and depends, of course, in every instance, upon molar 

 changes resulting from tumultuous ultramicroscopic chemical 

 changes of protoplasm itself. 



And it is to these changes that, in the last analysis, we must 

 turn for the energy which brings about the visible movements 

 in animals and plants, such as the contraction of the muscles 

 of animals, the streaming movement (amoeboid movement) 

 of the simple animals known as Amoebae, the rotation and 

 circulation of the protoplasm in certain of the living units of 

 plants and, finally, the lashing of threads of cytoplasm (cilia) 

 which not only enables many a tiny plant and animal to swim, 

 but also aids in numerous ways in certain parts of the 

 bodies of higher organisms. The phenomena of life are quite 

 generally expressed in visible movements, but the latter are 

 not peculiar to living things. 



In our discussion thus far we have endeavored to describe 

 the characteristics of matter in the living state on the basis 

 of the fundamental vehicle of life manifestations proto- 

 plasm. We have not attempted formally to define 'life' or 

 ' protoplasm ' because they are so unique that it is impossible 

 to resort to the lexicographer's trick of comparing them with 

 something else; and because the expressions 'protoplasm' 

 and 'life' are abstractions; one indicating that all individual 

 animals and plants have to a large extent a common organi- 

 zational foundation, and the other that they exhibit certain 



