118 NATURAL SCIENCE [August 
as a standard, then the same amount of variation in man and woman 
has more effectiveness in one sex than the other, I must reply, no 
one has yet investigated this point; my own conclusions on skull 
measurements, so far as they have yet gone, seem to show that the 
co-efficient is at least a rough measurement of effectiveness. But it 
must be clear that until we have investigated the relation of effective- 
ness to some clear measure of variation, Darwin’s law of the greater 
variability of the male is entirely unproven. Whether we put 
effectiveness as a function of mean and of standard deviation, or as 
a function of mean and the ratio of standard deviation to mean, is 
not, at first sight, a matter of great importance; it is to be settled 
rather by what the algebraist considers a convenient shape for his 
formulae. It is the biologist who has to determine the form of the 
function. It probably varies widely from species to species and 
organ to organ, but it may reasonably be supposed to vary only 
continuously and gradually with age and sex. If the selective 
death-rate of any species, however, be a function only of the mean 
and standard-deviation of any particular organ, then the theory of 
dimensions shows us at once that the death-rate cannot be a 
function solely of absolute variation, but must be a function of the 
ratio of absolute variation to the mean, 7.e., of the co-efficient of 
variation. 
Lastly, if Professor Weldon thinks I have reviewed my biological 
eritic harshly, I would remark that I submitted my paper a year 
ago in proof to a valued biological friend, I still have in a familiar 
hand-writing “no suggestions to make.” That Professor Weldon 
should find in my paper a “ violent assertion” to be regretted, con- 
firms my view that modern biology is a house divided against itself. 
Kar. PEARSON. 
