mM CONTRIBUTION TO THE STUDY OF VARIATION: 
(SKELETAL VARIATIONS OF NECTURUS MACULATUS RAF.) 
HERMON C. BUMPUS. 
“Both heredity and variation are in urgent need of causal explanation.”’— Roux, Wheeler. 
In this communication, based upon the comparative exami- 
nation of skeletons of Necturus, an effort is made to answer 
the following questions: 
I. What is the per cent of homeceotic variation ! in the attach- 
ment of the pelvic arch to the axial skeleton; is there true 
meristic variation, and is homceotic associated with meristic 
variation ? 
II. Is there a ratio between the absolute length of the 
animal and the number of vertebrz? 
III. Why does the variation tend towards forward rather 
than backward homceosis ? 
IV. Can an explanation be given for the frequent occurrence 
of oblique or unsymmetrical sacra? 
V.-Is the position of the pelvic arch dependent upon the 
ordinal position of some one segment (sacrum) of the vertebral 
column, or is its position determined by the location of some 
topographical point ? 
VI. Are there variations in the position of the pectoral arch, 
and are these correlated with variations in the pelvic arch? 
VII. Are there other skeletal variations associated with 
pelvic variation ? 
1 Bateson (’94) states that “‘ Homeeotic variation in the spinal column consists 
in the assumption by one or more vertebrz of a structure which in the type is 
proper to vertebre in a different ordinal position in the series. Examples of this 
are seen in the . . . occurrence of a vertebra, normally lumbar, in the likeness of 
a sacral vertebra, having its transverse process modified to support the pelvic 
girdle. ... In using the expression, Homeeosis, ... we may speak of the 
variation as occurring from before backwards or from behind forwards, according 
as the segment to whose form an approach is made stands in the normal series 
behind or in front of the segment whose variation is being considered. The 
formation of a cervical rib on the VII vertebra is thus a backward Homeeosis for 
the VII vertebra thus makes an approach to the characters of the VIII,” etc. 
