rangement within both families is relatively constant. Interfamilial
differences probably represent responses of the arteries to adaptive
structural differences of other systems of the body.

Individual Variation

The term "individual variation" is used here to mean "continuous
non-sex-associated variation" (see Mayr, Linsley, and Usinger, 1953:93)
found between members of the same species or between the two sides of
the same individual. It is hazardous to define individual variation (and
also interspecific differences, as discussed later) in the origin of one
vessel by relating its location to other vessels, because these may
likewise vary in origin. But, by necessity, certain vessels that are
probably less variable (axillary, carotid, and pectoral arteries) have
been considered here as being constant in origin. If these three vessels
are accepted as reference points, individual variants, as well as
interspecific differences, can easily be described in the thoracic and
coracoid arteries and in their various branches.

The thoracic artery in _P. subis_ arose either from the subclavian
artery, or from the coracoid artery. Likewise in other swallows, both of
these origins were found. In doves the thoracic artery arose
consistently from the pectoral stem, lateral to the origin of the
axillary artery.

The coracoid artery in _P. subis_ and other swallows arose from the
subclavian artery, either opposite the base of the axillary artery, or
medial to that vessel. In all doves studied the coracoid artery arose
from the subclavian medial to the axillary artery. I observed much
individual variation in the branches of the coracoid artery (that is to
say, in the vessels of the coracoid complex). In _S. inca_ the sternal
artery arose either from the coracoid artery, or independently from the
subclavian. As mentioned earlier, in members of both families the
vessels to _Mm. coracobrachialis posterior_ and _subcoracoideus_ are
highly variable, arising in swallows from the coracoid artery or from
the subclavian artery, and in doves from either of these two sites or
from the axillary artery. The distribution of these arteries after their
origin is also diverse.

Individual variation in the arteries of the thorax has been recorded
previously. Bhaduri, Biswas, and Das (1957:2) state that, in the
domestic pigeon, "the origin and course of various smaller arteries...
show noticeable variation," although they do not specifically state