SYMMETRICAL FREQUENCY CURVES 101 



limit of variations, so tliat the semicircle DAC constitutes the 

 " variation limit "; AB represents the hne of greatest frequency — 

 that is to say, the greatest number of individuals remain, in their 

 biological constitution, at an equal distance from the two poles 

 C and D. AB may also be described as the " line of constancy " 

 of the species, or as the " line of normality," the constancy of a 

 species being calculated on the constancy of the greater number 

 of its component individuals ; and the normality of a type being 

 that exhibited by the great majority of individuals, the abnormal 

 types being represented by a minority. In the symmetrical 

 curve, where AB represents the line of greater frequency or nor- 

 mality, the variations from this biological average diminish in 



C B D 



Fig. 6. — Stmmetbical Fbeqitency Ctteve. 

 C, Lowest, D, highest limit of variation ; AB, line of greatest frequency. 



number while increasing in amount, in proportion to the distance 

 which separates the hne of normality from the poles C and D, 

 the largest number being near the line AB, and the smallest 

 number at the poles. 



Although symmetrical curves are by far the most frequent, 

 there are also asymmetrical curves, as in Fig. 6. Whereas in the 

 symmetrical curve the hne of constancy falls upon the hne of 

 normality, in the asymmetrical curve the line of constancy 



( ) representing the centre-point of equihbrium between the 



hmits of variation is not coincident with the line of normality 

 or greatest frequency (aB). That is to say, the species is in the 

 paradoxical position of having its line of constancy separated 



