This table shows that on the average the male possesses 0'3 of a
whole vertebrae less than the female. It has already been shown that
the position of the pelvic girdle in the male is on the average 0'82
metamere more rostral than in the female. The interesting question
of the correlation between the number of whole vertebrae and the
position of the pelvic girdle, will be examined in more detail later on
in this paper.
In bringing these statistics to a close, the question suggests itself which of the two sexes is the more variable 1 To attempt to answer this, I have calculated the standard deviation (o-) and also the prob- ability error (E) for the two sexes on the following points :
Table VII.
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No. of cases in each sex.
ff.
c?.
E.
cr.
?.
E.
100
0-99
0-066
0-96
0-064
100
0-8(5
0-058
0-96
-064
50
0'98
0-093
1-29
0-122
Serial number of 1st girdle-
piercitig nerve
100
0-99
0-066
1-09
0-073
From this table it will be seen that the variability of the female is
greater on three of the above points, viz., with regard to (1) the serial
number of the first girdle-piercing nerve, (2) the number of post-girdle
nerves, and (3) the number of the whole vertebrae ; whilst on the
remaining point the number of the collector nerves, the male shows
a slightly greater variability, though here the two sexes are almost on
an equality. Of course it must be borne in mind that the numbers
dealt with are small for applying this method. Still the differences in
the two sexes are fairly well marked, particularly in the case of the
whole vertebras, where the standard deviation in the female pre-
ponderates even when the maximum allowance is made in each sex for
the probable error.
Having now concluded these statements on the amount of variation involved on the various points enumerated, we shall next enquire into the possibility of establishing any correlations between the position of the girdle and other points of variation.
A. Between the First Girdle-piercing Nerve and the Number of Collector
Nerves.
As the nerves which take part in the formation of the collector are only concerned in the innervation of muscles connected with the most
