334 
Merism and Sex in " Spiiiax Niger" 
No. of 
oases * 
1 canal 110 
.2 canals 86 
Difference 
fl canal 184 
\2 canals 
112 
Difference 
The possession of two canals is associated with a markedly more rostral position 
of the girdle, the difference being half a metamere for the ^fs and a third of one 
in the case of the $ s. Now it has already appeared that the pelvic girdle is more 
rostrally situated in the than in the %. Consequently we should be led to 
expect a larger proportion of fish with two canals among the </s, and this condition 
, ^ ^ , ^ . r^^^ HO. of (/s with 2 canals . 86 78-2 
IS what actually obtains. Ihe r-r— -r is -r,-/- T^^, whilst the 
no. ol (/s with 1 canal 114 100 
no. of 2 s with 2 canals . 112 60-9 , , . . , 
To ^rr^i 1 IS 7-5-7 = , cases ot two canals being distinctly more 
no. of $s with 1 canal 184 100 ° 
numerous among the </s. Reasons have already been given (p. 331-2) for supposing 
that the different positions of the pelvic girdle are, at any rate in the main, due to 
a process of homoeosis, and that such homoeosis is for the most part backward 
homoeosis (cf. Bateson, '94, pp. Ill et seq.). Now the preponderance of two canals 
among the more rostrally situated girdles is explicable on the supposition that this 
homoeosis is in some measure ontogenetic. The " Anlage " of the girdle is relatively 
larger than its ultimate condition. Its width is somewhat greater than the interval 
between two successive limb nerves, some one of which it must necessarily enclose. 
Supposing then that rostral migration of the girdle occurs, that structure will at 
first abut against and ultimately enclose the nerve immediately anterior to it, 
whilst at the same time retaining as it were its grasp of the nerve which orginally 
perforated it. That some such process does actually occur is rendered probable by 
the observations of Braus ('Ol, p. 590 and Taf XXIII, Fig. 8) on the arrangement 
of the cartilage cells. Hence the variations in the number of the girdle canals 
support the view that the different positions of the girdle are largely due to 
homoeosis, and that such homoeosis is to some extent ontogenetic, though this is 
probably confined to the earlier stages of development. 
The Hypothesis of Excalation. 
In most linear meristic series certain members become specialized in a definite 
manner and acquire thereby an individuality that serves to distinguish them from 
the rest of the series. In Spinax such specialization occurs in the case of those 
vertebrae which enter into a definite relation with the fins, paired and unpaired. 
On the hypothesis under consideration it is supposed that such specialized 
* As the number of canals may differ ou the two sides of the same animal each side has been con- 
sidered sepaiately as a "case." 
