102 



HEREDITY AND SELECTION IN SOCIOLOGY 



from its line of normality ; in other words, it is only a minority of 

 individuals who possess the organic characters which, in their 

 totality, represent the biological equilibrium of the species ; the 

 majority have varied in a greater or less degree, and in a direction 

 which disturbs the biological equilibrium by tending to bring the 

 species out of harmony with its actual vital conditions. The 

 result of this state of afEairs must either be a readaptation of the 

 species to new conditions, or the gradual extinction of the species. 

 Asymmetrical curves are realised whenever, in response to a 

 change of external conditions, the germ-plasm of the majority 

 of individuals of a species enters into a period of instabihty and 

 variation, pending the consolidation of readaptation. 



O B D 



Fig. 6. — Asymmetrical Freqihency Cueve. 



The causes of asymmetrical curves, according to Ammon, are 

 three in number : firstly, when the fertility of the species shows 

 a tendency to increase among those groups of individuals which 

 are nearer the limits of variation ; secondly, when germinal 

 selection tends to increase the rate of variation in a given direc- 

 tion towards one or other of the poles ; and thirdly, when natural 

 selection intervenes in unequal proportions at one or the other of 

 the poles. It may be remarked that the first cause is itself an 

 effect of the second one ; and the role of natural selection will be 

 different according as the variation is useful to the species or not. 

 If it be a variation tending to readapt the species to a changed en- 

 vironment (to which the species is biologically adaptable), then 



