Page:Popular Science Monthly Volume 84.djvu/142

138 series of seeds which survive greater than that of the original distribution, while in 56 cases it is less. This is a deviation from equality of 17 ± 2.98, or over five times its probable error. One might have to toss coins a long time to get 22 heads and 56 tails in a series of 78 throws!

Furthermore, the individual constants indicating a lowering of variability by selection are statistically much more trustworthy than those suggesting a decrease. To carry much weight a difference between two constants should be at least two and one half times its probable error. But only 5 of the broken lines reach a straight edge laid 2.5 above (to the right of) the zero bar, while 22 extend beyond the same limit on the other side.

The coefficients of variation are already in relative terms—variability expressed in percentages of the mean—hence they may be represented directly by the direction and length of the lines in the figure to the right. For this graph, one unit on the base scale means one tenth of one per cent. The results amply confirm those secured by the preceding method.

These graphs are deduced from experiments involving tens of thousands of individually weighed seeds. Their evidence for a selective mortality can not, therefore, be lightly set aside. That the average weight remains unchanged while the variability is decreased can only mean that there is an elimination from both the upper and lower extremes of variation, that is, of both large and small seeds.

Nevertheless, too great caution can not be used in the interpretation of the result. Purposely, the materials selected for study were most varied, and while the validity of the general average result can not be seriously questioned, there remains the problem of determining whether, and if so to what extent, the selective mortality may not fluctuate widely with different varieties and conditions.

The whole problem of the underlying physical and chemical causes of the differential mortality remains to be investigated. Finally, the possible relationship of the selective mortality to organic evolution can not be discussed until we have further evidences along several different lines.

Potential Characteristics and Seed Mortality.—When we turn to the question of the possible influence of the characteristics of the adult plant innate but invisible in the seed upon the chances of survival, the data are very scarce indeed.

Montgomery's studies of the rise in productiveness in corn as a result of increased competition may perhaps be of interest here. The