326 
Merism and Sex in " Spinax Niger'' 
conclusion that some process of selection takes place among the ^Ts during 
post-natal existence, leading to a reduction of their variability to the amount 
exhibited by the unselected $s. And this conclusion is in accordance with the 
fact that, whilst the two sexes are produced in equal numbers, the adult (/"s are 
less numerous than the adult $s. (Cf. p. 321.) 
Variability in different Regions. 
That considerable differences in variability, as measured by the standard 
deviation, obtain for various regions of the vertebral series has already become 
apparent. As, however, the number of meristic units which goes to make up 
these different regions differs widely, their relative variability must be compared 
by means of the coefficient of variability ^ = These values are given in 
Table 5 a, from which it appears that the variability shows wide differences 
TABLE 5 a. 
Coefficients of Variation. 
(? adults 
i embryos 
? adults 
? embryos 
Anterior spine 
4-19 

3-47 
4-46 
3-65 
Posterior spine 
2-19 
2-24 
2-20 
2 12 
Whole vertebrae ... 
2-57 
2-96 
2-38 
2-44 
Half vertebrae 
4-71 
4-79 
4-77 
4-26 
Total segments 
1-61 
1-78 
1-59 
1-61 
according to the character chosen — differences on the whole equally well marked 
in the four groups of embryos and adults. The existence of such differences 
is antagonistic to the conception of the landmarks along the vertebral series being 
associated with particular vertebrae, and depending for their various positions on 
any uniform process of augmentation or reduction taking place in that series. To 
this point however reference will be made later on (pp. 335-6). 
Correlated Variability in the Meristic Series. 
With the idea of attempting to throw some light on the processes at work in 
the vertebral column, the various characters have been correlated with one another. 
This has been done separately for the two sexes, as well as for embryos and adults 
apart. The results given in Table 5 b show that there is on the whole considerable 
agreement in the four groups of material. Where the correlation value between 
two characters is high for one group it usually exhibits a similar high value for 
