No. 2.] THE STUDY OF VARIATION. 467 
appendages with the general dimensions of the vertebrate body, 
though the radiographs of Necturus, procured by the X-rays, 
make the necessary process of measurement extremely simple. 
For a standard of measurement I have considered the dis- 
tance from the first to the XXX vertebra, no matter what the 
actual length of the animal may be, to be represented by 100; 
the XXX vertebra being selected because the terminal portions 
of the animals are liable to injury. I have taken the inter- 
vertebral space between the XIX and XX as the locus of a 
variable, and I find by making accurate measurements on the 
negatives that the position of this intervertebral space varies 
in different animals in a very appreciable way. In certain ani- 
mals the trunk, the part lying anteriorly to the selected inter- 
vertebral space, is relatively longer, while in others the caudal 
portion is relatively longer. The distance from the XX to 
the XXX vertebra may in some specimens (Nos. 4, 67, 95) 
be only 29% of the entire measured length, while in others it 
may be as great as 35% (Nos. 58, 61, 74). 
On Plate C these relative measurements of the several 
specimens are given, each ordinate representing the length 
from the first to the XXX vertebra, and the upper of the two 
black lines that cross the plate represents the fluctuating inter- 
vertebral septum between the XIX and XX vertebre. In by 
far the larger number of cases the caudal region is one-third, 
33%, of the length. Specimen No. 1 has a trunk slightly 
longer and a caudal region correspondingly shorter than the 
average. The trunk of No. 2 is shorter and the tail longer. 
No. 3 is like No. 2. No. 4 has an uncommonly long trunk 
and short tail, etc. 
If this curve is followed across the plate, it will be noted 
that the first nineteen vertebraze may suffer regional expansion 
and contraction, with correlative contraction and expansion 
of the caudal region, and to such an extent as to give an 
amplitude of variation to the dividing line between the XIX 
and XX vertebre, amounting to 585 (35% minus 29%) of 
the average distance between the I and XXX vertebrae. The 
amplitude of variation in the column itself, absolutely inde- 
pendent of the limbs, is sufficient, then, to include within 
