232 
EDWIN S. GOODRICH. 
limb is normally supplied by the sixteenth, seventeenth and 
eighteenth spinal nerves, and the girdle is attached to the 
sixteenth vertebra. Occasionally, however, while the plexus 
remains in the same position the sacral vertebra is the 
seventeenth. Thus the limb and girdle, which presumably 
developed from the same segments in the two cases, become 
connected with the vertebral column one segment farther back,, 
their real place of origin being’ shown by the nerves. It is 
only such slight secondary shiftings that the theory of 
migration during ontogeny can explain. 
There remain three other possible explanations of the 
varying position of limbs to be considered : the theory of 
intercalation and excalation, the theory of re-division, and the 
theory of progressive modification or transposition. 
The theory of intercalation has been admirably dealt with 
by von Jhering in a classical inemoir (17). According to this 
view, the relative shifting of a limb and its plexus along 
a series of segments in different animals is explained on the 
supposition that one or more segments have dropped out or 
new segments have been added between pre-existing 
segments. This process is taken to account not only for 
the apparent motion of a structure up or down the segmental 
series, its change of position, but also for the extension 
of a limb or plexus over more or fewer segments than it 
originally occupied. It is necessary to suppose that segments 
can be intercalated or excalated at any point in front of a 
plexus, within the plexus, or behind it. 
Yon Jhering first deals with such simple cases as Sorex, 
where a whole segment seems to have been intercalated in 
front, some individuals having thirteen and others fourteen 
dorsal vertebrae, the whole pelvic plexus and sacrum in the 
latter being situated one segment farther back. Or, again, 
Cynocephalus,in which the last lumbar vertebra and its nerve 
appear to have dropped out, leaving the plexus reduced 
by one nerve, and the pre-sacral vertebrae twenty-five instead 
of t-wenty-six in number. At first sight the theory seems to. 
afford a plausible explanation of such variations. Less easy 
