BIOMETRICAL METHODS 81 



numbers which represent the character of each indi- 

 vidual, whilst measurements are artificially limited 

 through the fact that they have to be made in units 

 of some kind e.g., to the nearest inch or some other 

 value. Such groups, characterized by equality of 

 range each, that is to say, covering an equal number 

 of units are technically known as classes. 



Thus if we are dealing with human stature, and if our 

 measurements were made only to the nearest inch, all 

 the individuals of 6 feet in height would fall into one 

 class, those of 6 feet i inch into another class, and so 

 on. If, on the other hand, we were engaged in count- 

 ing the number of ray florets in the heads of daisies, 

 a class would include all those heads which possessed 

 a particular number of rays. 



Without division into classes, however, a survey of 

 a comparatively small number of measurements may 

 be facilitated by ranging the values in some kind of 

 order. This is done, for example, in the accompanying 

 figure for the measurements to hundredths of an inch 

 of the lengths of the body, wing, and tail of thirty-one 

 specimens of a North American bird. The diagram 

 is taken from A. R. Wallace's ' Darwinism.' 



Even with this small number of measurements the 

 diagram brings out two points very clearly. In the 

 first place, there is no close correspondence between the 

 variations in length of body, wing, and tail. Secondly, 

 in the case of body-length, in respect of which the 

 specimens are ranged in order, the number of indi- 

 viduals of a medium size is seen to be greater than the 

 number of those which show extreme values. This 



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