190 THEORY OF EVOLUTION 



descendants that come to inherit this character. 

 It is this increase in the number of individuals 

 possessing a particular character, that might 

 have an influence on the course of evolution. 

 This gives a better chance for improvement by 

 several successive steps; but not because the 

 species is more likely to mutate again in the 

 same direction. An imaginary example will 

 illustrate how this happens: When elephants 

 had trunks less than a foot long, the chance of 

 getting trunks more than one foot long was 

 in proportion to the length of trunks already 

 present and to the number of individuals; but 

 increment in trunk length is no more likely to 

 occur from an animal having a trunk more than 

 one foot long than from an animal with a 

 shorter trunk. 



The case is analogous to tossing pennies. At 

 any stage in the game the chance of accumu- 

 lating a hundred heads is in proportion to the 

 number of heads already obtained, and to the 

 number of throws still to be made. But the 

 number of heads obtained has no influence on 

 the number of heads that will appear in the next 

 throw. 



