LIMITS OF HEREDITARY CONTROL 887 
a tendency to form about groups of three would produce just the 
opposite result. A reference to table 13,in which the total counts 
on five specimens are recorded, indicates that, although the ani- 
mals may vary considerably in the total number of scutes, yet 
TABLE 13 
Showing proportion of 3-hair and 4-hair scutes in a small sample of the species 
| NUMBER | NUMBER 
SPECIMEN | BE Oe cea | pane | rome | LEFT MIDDLE RIGHT 
Shell 2...... | 625 391 234 | 2109 AB 77 138 
2) ie | 577 15Qe 418) 2149 70 29 60 
DOI a8 ss | 545 7 AGS 2103 33 9 35 
2 7 ee | 534 19 FGF Me PO) fi 2 10 
9204 2 ve. | 529 26 503 | 2091 13 Os ee 
there is very little difference in their total hair counts. In the 
two extreme cases, shell 2 and female 204, there is a difference of 
96 scutes, but in total hair counts they vary by only 18 points, 
which is a very inconsiderable difference when one considers that 
there are between three and four times as many hairs as scutes. 
Undoubtedly data on a larger number of animals would reveal a 
much greater variation; but the fact remains that, however 
variable the species may be with respect to the scutes (and their 
associated plates), it is comparatively stable with respect to the 
hairs of a given region. A statistical study of the hairs might be 
made, but their enumeration is difficult; for, on the one hand, 
not a few of the adults possess armors from which many of the 
hairs have been lost by abrasions through contact with rocky 
dens, and on the other hand, even advanced foetuses do not have 
hairs sufficiently developed to allow a census of them being taken. 
For this and other reasons it has seemed best to confine our 
studies to the variability and heredity of the scutes, and not to 
attempt a compilation of statistics on hair counts, which at best 
could be only imperfect. It is perhaps sufficient then merely 
to have indicated the line along which the evolution of the species 
is apparently directed. The fact that in all of the regions showing 
a primitive condition of the scutes, as in the case of the belly, 
JOURNAL OF MORPHOLOGY, VOL. 22, No. 4 
