54 NATURAL SCIENCE [Juny 
No one acquainted with the facts of animal variation—certainly 
not Professor Pearson himself—will assert that the “ coefficient of 
variation ” is always a measure of the importance of variations; no 
one will believe that in any animal a deviation of ten per cent. in 
excess of the mean of one organ has of necessity the same im- 
portance as a deviation of ten per cent. in excess of the mean in 
another organ. A finger nail of double the normal length, or a hair 
of double the normal thickness, will hardly produce ‘so much incon- 
venience as a leg of double the normal length. It is even certain 
that in closely allied species the same per centage deviation of 
corresponding characters may produce widely different effects. 
There is no doubt that the “coefficient of variation” is for 
certain purposes a valuable measure of variability; and Professor 
Pearson has shown, in some of his more technical papers, that it is 
of great use in establishing important propositions in the theory of 
Chance. At the same time, students of evolution, paying attention 
specially to the functional importance of variation, may need units 
proportional to this importance ; and such units may well be different 
in different cases. The violent assertion that there is only one 
“ scientific ” measure of variability is therefore to be regretted. 
Whether one agrees with Professor Pearson on this single question 
or not, one cannot but be grateful to him for the four essays here 
referred to, as well as for the more elaborate memoirs on which they 
are based. The picture they present of the orderly treatment to 
which animal statistics can be subjected, so that hitherto unwieldy 
and perplexing masses of figures can be made to yield simple and 
intelligible results, should do much to make the study of Probability, 
in its application to the problems of animal evolution, more popular 
than it is, and to enable biologists at last to put before themselves an 
adequate numerical estimate of those phenomena which it is the 
business of their lives to formulate and to explain. 
W. F. R. WELpon. 
