THE PHYSICAL BASIS OF LIFE 17 



and energy taken from its environment, constructs more 

 protoplasm and, if the available materials are adequate, the 

 specifically organized living substance tends to increase 

 indefinitely. Thus it is not only the method of growth 

 which is diagnostic of animals and plants, but also the fact 

 that when the individual body has reached a certain phys- 

 iological balance, or maturity, in which it ceases to increase 

 in size, under normal conditions it expresses the inherent 

 growth power of living matter by setting free certain living 

 units, which go through a cycle of growth phenomena that 

 result in re-productions of the parent individual. 



4. Reproduction 



So far as is known, living matter never arises except under 

 the direct influence of preexisting living matter. We have 

 seen that this transformation is continually going on in the 

 constructive phase of metabolism in the animal or plant, 

 and brings about repair and growth of the individual; 

 but it is in reproduction that what may be termed the over- . 

 growth of the individual results in the production of a new 

 one. A larger or smaller part of the parent generation is 

 detached and becomes the new generation, so that in ultimate 

 analysis reproduction is division. This is a highly unique 

 characteristic of living things which provides for the con- 

 tinuation of the race. 



5. Adaptation 



The discussion of metabolism has emphasized the close 

 interrelationship between the living complex and its sur- 

 roundings, and the dependence of life upon the interplay 

 and interchange between protoplasm and its environment. 

 As a matter of fact the plant or animal retains its individual- 

 ity lives solely by its powers of developing and main- 



