R. C. PUNNETT 
337 
+ 458 and +• 461 for (/s and $s respectively fall much below the observed values 
+ -879 and + 700. It is therefore evident that the hypothesis of excalation will not 
serve to account for the magnitude of the observed variability. 
On Intercalation. 
By some morphologists (cf. Baur, '97, p. 52) it has been held that intercalation 
of segments as well as excalation may go on in a linear meristic series. To take the 
case of a portion of a vertebral column between two specialized and homologous 
segments, there are often to be found individuals exhibiting either a less or a 
greater number of segments than the normal. The former condition we are called 
upon to regard as the result of excalation, the latter as being due to intercalation ; 
and to account for variations any such demarcated region of the segmented series 
must be held capable of undergoing intercalation in one specimen and excalation 
in another. In order to test this possibility we may consider the case of that 
portion of the vertebral column which lies between two highly specialized segments, 
TABLE 7. 
Correlation of Segments lying between First Oirdle-piercing Nerve and 
Posterior Spine with Segments giving off Post-girdle Nerves to Fin. 
b. 
10 
11 12 
IS 
u 
8 
2 3 
5 
9 
2 
8 25 
8 
1 
44 
10 
1 
1 16 
10 
28 
11 
— 1 
5 
6 
3 
11 45 
23 
1 
83 
j-=-343±063 
10 
11 12 
13 
U 
6 
1 2 
3 
7 
8 14 
1 
23 
8 
1 
11 31 
17 
3 
63 
9 
2 3 
8 
4 
17 
10 
— 1 
1 
2 
1 
22 51 
26 
8 
108 
r= -370 ±-056 
viz. the segment whose nerve is the most rostral of those piercing the girdle, and 
the segment which carries the posterior spine. This division of the vertebral 
column consists of some 10 — 14 segments, of which the greater number (nearly 
three-quarters) are characterized by the fact that their spinal nerves send off 
branches to the pelvic fin. Now if the variations in the number of these 10 — 14 
segments are due to intercalation and excalation we should expect the variations 
of those of them which supply the fin muscles to be in sympathy. In other words 
we should look for a high correlation between the segments bounded by the 
1st g. p. nerve and the posterior spine and those which give off the post-girdle fin 
nerves. These correlations (Tables 7 a and b) have been worked out for the 
and % adults and the values obtained are •343 and '370 respectively. On the 
hypothesis under consideration we should expect these correlations to approximate 
to unity and the lowness of the values actually obtained tells strongly against the 
Biometrika m 43 
