1897] THE SCIENTIFIC MEASURE OF VARIABILITY 117 
What is it then that I have asserted? Simply this, that if it 
be necessary to compare the variability of the same organ in two 
sexes which have on the average different sizes, it is absurd to use 
absolute variations. This conclusion is nothing new; it has long 
been familiar to craniologists and anthropologists. They have, for 
instance, compared brain-weight relative to body-weight or to 
stature. I contend that the proper measure is the percentage 
variation on the mean. My words are, “I hold that the only 
useful sense in which we can study relative variability is by 
endeavouring to answer the problem. Is one sex closer to its 
mean, more conservative to its type than the other? and that 
the only scientific answer to this les in the magnitudes of the 
percentage variations of the two sexes for corresponding organs.” 
It will be seen at once that this is not, as Professor Weldon appears 
to misinterpret it, an assertion of a single scientific measure of 
variability but a statement of opinion as to the only useful way 
in which we can compare in the two sexes the relative variability 
in the same organ. Professor Weldon, indeed, seems to confuse 
two things, the scientific measure of variability and the effectiveness 
of this variability for different organs in the struggle for existence. 
Because the variability of one organ is said to be twice that of 
another organ, it does not follow that the functional importance is 
doubled. The scientific measure of variability is one thing, the 
effectiveness of this amount of variability in the struggle for exist- 
ence is another and different thing. But even here I am prepared 
to assert, although I have not done so in my paper, that the co- 
efficient of variation, without being proportional to the “ effective- 
ness,’ is far more reasonable as a measure of effectiveness, when we 
are dealing with the same organ in different sexes, or in individuals 
of the same sex at different ages, than absolute variation. It seems to 
me that the non-regard of this point has led to the nugatory character 
—not of the splendid system of measurements on crabs made by Pro- 
fessor Weldon—but of several of the conclusions he has endeavoured 
to base upon those measurements. I cannot get over the fact that 
the variation of an inch in the leg of a pony is not the same thing 
as a variation of an inch in the leg of a horse. Out of the 155 
cases dealt with in my paper, woman is in 62 or 63, I think, 
absolutely more variable than man, and man absolutely more 
variable in some 85; in the remainder the sexes are sensibly equal. 
But since woman is smaller than man in the weight and size of nearly 
all organs, absolute variability can only be adopted with the same 
_ justification as we should say that an inch is the same variation in 
the leg of a pony or a horse, or a cubic centimeter the same varia- 
tion in the capacity of the brain of a man or a new-born infant. 
If Professor Weldon asserts that taking the co-efficient of variation 
