472 BUMPUS. [ VoL. XII. 
homeeotic individuals towards variations in other directions, 
leads to the conclusion that homeceosis is only an index of 
general instability on the part of the skeleton as a whole. 
(1) This principle was illustrated in Section VI, where it was 
shown that variations in the position of the sacral vertebra are 
accompanied by variations, in a definite direction, in the pectoral 
region. 
(2) The principle was also illustrated in Section I, when it 
was shown that homoeotic specimens tend towards numerical 
increase in the number of vertebral segments. — Before passing 
on to other possible illustrations, let us examine this present 
phenomenon more in detail. 
According to Bateson’s law, forward homeeosis, involving one 
vertebra, should yield a column of oze more than the normal 
number of vertebrze. But if we examine the curve of lengths 
in terms of vertebra, on Plate C (the lower of the curves 
drawn in black), we shall find that whereas the normal speci- 
mens average only 45 vertebrz, the homceotic average not one 
more, 46, but ¢wo and ¢hree more, 47 and 48, and that the 
amplitude of variation, moreover, is much greater among the 
latter than among the normal specimens. It is clear, then, 
that there is an added increase in the variation of the vertebrze 
among homoeotic specimens. 
(3) A third illustration of the principle of general variability 
was given in Section V, where it was shown that abnormalities 
in the relative lengths of the anterior and posterior portions of 
the body tend to gather about homceotic individuals. 
(4) Dr. Parker ('96) has made use of the first haemal arch in 
his comparisons of the vertebral columns of Necturus, showing 
that its position is subject to considerable variation. Let us 
see if homceotic specimens present greater or less variation in 
the position of this arch than do the normal examples. 
Of sixty-three normal specimens giving satisfactory radio- 
graphs of the first hamal arch, 
10 examples (16%) have the arch attached to the XXII vertebra. 
51 “c (81%) iT “ “ “ CO DOCU ue “ 
7 “ 707 74 (73 (73 6c“ 66a ee XOXO, “ 
a (3% 
and of thirty-five homceotic specimens, 
Ai 
