ioo8 LIFE : OUTLINES OF GENERAL BIOLOGY 



REGISTRATION OF VARIATIONS 



If a considerable number of organisms of the same age and sex are 

 examined and measured as regards a particular feature, e.g. stature, 

 length of wing, size of leaf, or number of hairs on a particular area of 

 the body, and if the observed differences are indicated by the height 

 of a series of perpendiculars rising from a horizontal scale indicating 

 amount, a joining of the tips of the perpendiculars yields a line which 

 often becomes more and more of a continuous curve as the number 

 of measurements increases. It is called the curve of frequency, and 

 it is illustrated when results depend on a considerable number of 

 independent conditions. Thus in Galton's apparatus the outline of 

 the pellets dropped through hindrances into subjacent compart- 

 ments showed this characteristic curve (Fig. 171). 



Fig. 170. 



An Array of Leaves from the Same Plant, illustrating the observed plus and 

 minus difEerences in size. From De Vries. 



When the lengths of whole organisms of similar age and sex, or 

 the dimensions of particular parts, are thus plotted out, the curve 

 is often symmetrical; there are as many deviations to the right as 

 to the left, but the vast majority are represented by the central 

 perpendicular or by others close to it on either side. 



At the one extreme, for instance, there is a small number of 

 marked giants, at the other extreme there is an equal number of 

 marked dwarfs. The more divergent the variations the fewer there 

 are of them. This biometric registration of observed differences is 

 of great value, and it has been elaborated by Galton and Pearson 

 into an intricate technique. Its value increases with the number of 

 data, for conclusions drawn from thousands of observations are 

 obviously much more reliable than those based on hundreds. Much 

 depends also on the homogeneity of the material, thus a quantitative 

 registration, on one curve, of, say, the sizes of organisms of different 



