88 BIOMETRY 
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 1 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 
