"Type 4: attachment to the subclavian, but with a common
root for both the coracoid and thoracic.

"Type 5: attachment to the subclavian medial to both the
axillary and coracoid.

"Type 6: two separate thoracic arteries are present; the
primary thoracic is the same as type 1 above, while the
secondary thoracic is the same as type 3 or type 4 above."

Possibly the thoracic artery has undergone migration but apparent
differences in its origin might well be due to differences in other
vessels of the thoracic area. Additionally, there seems to be no reason
to assume that the lateral position of the thoracic artery is the
primitive one, or that the medial is the derived position, as is implied
by the phrase "medial migration." Although the lateral site of
attachment (type 1) is predominant in the lower orders of birds, and the
medial attachment is found primarily in Passeriformes, a fact which may
indicate that type 1 is the more primitive, it must nevertheless be kept
in mind that a sequence of a single morphological character does not
necessarily represent the phylogenetic sequence of the character itself
(see Mayr, 1955:41).

Also, a given arterial arrangement might be independently derived more
than once. If such has been the case, similarities in arterial
arrangements in different taxa would sometimes be "chance similarities,"
that is to say, "resemblance in characteristics developed in separate
taxa by independent causes and without causal relationship involving the
similarity as such" (Simpson, 1961:79).

The particular arrangement of the arteries of the thoracic area also
seems to be of limited value as a clue to taxonomic relationships. If
the origin of any artery is determined by skeletal and muscular
features, as I suggest, the artery perhaps ought not be considered as a
separate character, but as part of a "character complex" that varies as
a unit (see Mayr, Linsley, and Usinger, 1953:123). The skeleton offers a
potential fossil record for consideration. Changes in the skeleton and
muscles, great enough to affect the blood vessels, would probably be
detected more easily than would the resulting vascular changes. Also, I
did not find as much individual variation in the skeleton and muscles in
the area studied as I did in the vascular system. In other words, within
the bounds established by the skeletal and muscular features, the artery
still exhibited individual variation i