CADETS ARRANGED TO SHOW NORMAL CURVE OF VARIABILITY
Fig. 14.—The above company of students at Connecticut Agricultural College was grouped according to height and photographed by A. F. Blakeslee. The height of each rank, and the number of men of that height, is shown by the figures underneath the photograph. The company constitutes what is technically known as a "population" grouped in "arrays of variates"; the middle rank gives the median height of the population; the tallest array (5 ft., 8 in.) is the mode. If a line be drawn connecting the upper ends of the rows, the resulting geometric figure will be a "scheme of distribution of variates" or more briefly a "variability curve," such as was shown in several preceding figures. The arrangement of homogeneous objects of any kind in such form as this is the first step in the study of variation by modern statistical methods, and on such study much of the progress of genetics depends.
Fig. 15.—Height is one of the stock examples of a continuous character—one of which all grades can be found. As will be seen from the above diagram, every height from considerably under five feet to considerably over six feet can be found in the army, but extreme deviations are relatively rare in proportion to the amount of deviation. The vertical columns represent the total number of individuals of a given height in inches. From Davenport.
If a group of soldiers be measured as the children were measured for arithmetical ability, their height will be distributed in this same curve of probability. Fig. 14 shows the cadets of Connecticut Agricultural College; it is obvious that a line drawn along the tops of the files would again make the step-pyramid shown in Figures 10, 11 and 13. If a larger number were taken, the steps would disappear and give place to a smooth curve; the fact is well shown in a graph for the heights of recruits to the American Army (Fig. 15).
The investigation in this direction need not be pursued any farther. For the purpose of eugenics, it is sufficient to recognize that great differences exist between men, and women, not only in respect of physical traits, but equally in respect of mental ability.