120 THE SOCIAL LIFE OF ANIMALS 



successfully it must possess at all times a store of 

 concealed potential variability. 



I may interject parenthetically that at times this 

 appears to call for the presence of a considerable 

 number of individuals as a necessary condition to 

 provide the needed variations. A part of this reserve 

 of variability may be of no use under any circum- 

 stances; some characters may be useful, some may 

 never meet with the circumstances under which they 

 would have survival value; while others, though of 

 no use or even harmful when they appear, may later 

 enable the species to live under newly changed con- 

 ditions. 



Hereditary changes tend to be eliminated as soon 

 as they run counter to decided environmental selec- 

 tion. In large populations the results of mutations 

 tend to stabilize about some average gene frequency, 

 which represents the interaction between the rate of 

 mutation and the degree of selection. Frequently 

 mutation pressure pushes in one direction and selec- 

 tion in another and the resulting gene frequency in 

 the population represents a point or zone of equi- 

 librium between these forces. In small populations 

 which are not too small, selection between genes 

 becomes relatively ineffective, and the gene fre- 

 quencies drift at random over a wide range about a 

 certain mean position. In very small breeding popu- 



