122 EVOLUTION 



that there is often a proportion between 

 the frequency of a particular variation 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 measurements 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 numbers 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- 



