SUPPLEMENTS—NOTES SUPPLEMENTARY TO CHAPTER VI. 363
Table A shows results for four observers as illustrations, the
frequencies being reduced for comparability to a total of 1000.
Column A is based on measures by myself, on drawings, to the
nearest tenth of a millimetre. It is recognised, of course, that
measures cannot really be made to such a degree of precision ;
but I believed that I was making them carefully, and as they
were made with a Zeiss scale, in which the divisions are ruled
on the under side of a piece of plate-glass, readings are unaffected
by parallax. Nevertheless it will be seen that I heavily over-
emphasised the zeros, and also 2, 8 and 9—an odd selection of
preferences! On the whole, the centre of the millimetre was
neglected and measures piled up at the two ends.
The data for columns B, C, and D were all drawn from the
same published report, and refer to sundry head measurements
taken on the living snbject. Guided by a statement in the intro-
duction, it was possible to compile the data separately for the
three assistants (B, C, D) who had done the actual measuring.
It will be seen that B was rather good : there is a relatively slight
excess at 0 and 5, but otherwise his measurements are fairly
uniformly distributed. C was decidedly not good, rounding off
nearly one measurement in two to the nearest centimetre or
half-centimetre. D was simply outrageously bad—so bad that
it might have been better not to publish his measurements.
Nearly 57 per cent. of his measurements are made only to the
nearest centimetre or half centimetre—a quite inadequate degree
of precision for head measurements often only a few centimetres
in magnitude.
Compilation of data in the form of Table A is recommended as
some control of their value, and as a check on assistants.
15. The Extremely Asymmetrical or J-shaped Distribution.—
Dr J. C. Willis has shown that any number of illustrations of
this form of distribution may be obtained by compiling the
frequency distribution for numbers of genera with 1, 2, 3 . . .
species in any biological group. Table B shows the distribution
for the Chrysomelid beetles.
[TaBLE
