Page:Popular Science Monthly Volume 84.djvu/141

Rh whole 78 series the mean difference is less than five tenths of one per cent.

Thus if we confine our attention to the mean, there is apparently no selective elimination whatever, for within the limits of experimental error there is no certain change in the mean value of the character considered. But an entirely new and different light is thrown upon the whole question when the variabilities are examined. These are distinctly less on the average for the series which develop to maturity.

This is brought out with great clearness by Diagram 3.

 The figure to the left shows the ratio of differences in standard deviation to their probable errors (1 space on scale =.2). The figure to the right shows differences in coefficients of variation (1 space on scale =.11 per cent.). The vertical lines give the points of zero difference.

In general form these figures are similar to the one representing the means, but an additional point is brought out by the one for the standard deviations, to the left. In this case the length of the bars indicates not absolute nor relative values of the variabilities, but the trustworthiness of the constants. Here the base scale is in terms of the ratio

each unit being equivalent to .20.

Instead of the light and dark line areas being approximately equal they are widely different. In only 22 cases is the variability of the