262 Evolution and Adaptation 



If the line is more carefully examined, it will be found that 

 somewhere near the middle the men are much more nearly 

 of the same height, or rather there are more men having 

 about the same height than there are near the ends of the 

 line. Another arrangement will bring this out better. If we 

 stand in a line all the men from 60 to 61.9 inches, and in an- 

 other parallel line all those between 62 and 63.9, then those 

 between 64 and 65.9, then between 66 and 67.9 inches in 

 height, etc., it will be found that there are more men in some 

 of these lines than in others. The longest line will be that 

 containing the men of about 65 inches ; the two lines formed 

 out of men on each side of this one will contain somewhat 

 fewer men, and the next ones fewer still, and so on. If we 

 looked at our new group of men from above, we should have 

 a figure triangular in outline, the so-called frequency polygon, 

 Figure 3 B. With a larger amount of data of this sort it is 

 possible to construct a curve, the curve of frequency, Figure 

 3 A. In order to obtain this curve of frequency, it is of 

 course not necessary to actually put the individuals in line, 

 but the curve can be drawn on paper from the measurements. 

 We sort out the measurements into classes as in the case 

 given above. The classes are laid off at regular intervals 

 along a base-line by placing points at definite intervals. 

 Perpendiculars are then erected at each point, the height of 

 each being proportional to the frequency with which each 

 class occurs. If now we join the tops of these perpendiculars, 

 the curve of frequency is the result. 



" In arranging the individuals it will be found, as has been 

 said, that certain groups contain more individuals. They 

 will form the longest line. This value that occurs with the 

 greatest frequency is called the mode. The position of this 

 modal class in the polygon is one of the points of importance, 

 and the spread of the polygon at its base is another. A 

 polygon with a low mode and a broad range means great 

 variability. The range may, however, be much affected by a 



