i 4 PHYSICAL BASIS OF CIVILIZATION 



coming from that other. Two substances different 

 from each other, when mixed, produce a mixture 

 which differs from both. Sexual offspring being 

 the result of such a mixture, therefore the proba- 

 bility is infinitely great that no such offspring can 

 be exactly like either of its parents. And inductive 

 experience confirms the conclusion thus deductively 

 reached. Each generation slightly varies from the 

 parent type. This is "variation" — a result of 

 heredity. 



Another kind of variation, so far as known only 

 appertaining to forms of life lower than sexual, 

 must now be briefly outlined, because it bears on 

 future arguments. 



The unit of life is the cell. It consists of a speck 

 of the life substance, protoplasm, which has become 

 segregated, or, so to speak, individualized, by the 

 formation of a cell wall. The cell wall is formed 

 by the differentiation of the external portion of the 

 speck of protoplasm, through its greater proximity 

 and therefore more energetic interactions with en- 

 vironing materials and conditions. 



The efficiency of any force is in inverse ratio to 

 the square of the distance from its source. No 

 matter how infinitesimally thin a cell wall may be, 

 yet is its thickness mathematically divisible into 

 an outer half and an inner half. The forces and 

 influences of the environment, therefore, react at 

 the outer surface of the cell wall with four times the 

 energy displayed at the inner surface. That is on 



