VARIATION AND HEREDITY 121 



clearly, especially from the pioneering work 

 of Gallon, that there is often a proportion 

 between the frequency of a particular varia- 

 tion and the amount of its deviation from 

 the mean of the character in question. Let 

 us take Wallace's illustration, which makes 

 this point clear at a glance : Among measure- 

 ments of 2,600 men, taken at random, there 

 is 1 of 4 ft. 8 in. and 1 of 6 ft. 8 in.; 12 of 5 ft. 

 and about 12 of 6 ft. 4 in.; i.e. equal num- 

 bers at equal distances from the mean of 5 ft. 

 8 in. In other words, when the frequency 

 and the magnitude of the variations are 

 registered, they often show what is called the 

 Normal Curve of Frequency. 



This tedious task of registering the varia- 

 tions that occur may seem far from life-lore, 

 but a little consideration and a little actual 

 registration — of buttercup petals, of length 

 of bird wings, of brittle-star arms, of jelly-fish 

 canals, or the like — will convince the student 

 that biometrics may lead him into the very 

 heart of the matter. If the registration of 

 the dimensions of a particular character be 

 carried on year after year in similar material, 

 and show a consistent increase in the asym- 

 metry or skewness of the curve, this must 

 mean that the species is moving in a definite 

 direction as regards the particular character 

 measured. Similarly, the persistent occur- 



