under
investigation are the White-winged Dove, _Zenaida asiatica_, and the
Mourning Dove, _Zenaidura macroura_. They were chosen to demonstrate
differences and likenesses in proportions of members of the genera.

Ten measurements were taken on each skull, but simple observation
reveals that, in relation to total length of the skull, the beak of the
White-winged Dove is longer than that of the Mourning Dove. Tip of
upper mandible to base of beak averaged 48.6 and 42.9 per cent of the
total length of the skull in the White-winged Dove and Mourning Dove,
respectively. The position of the jugal bar has remained about the same
with respect to the cranial part of the skull, and the entire cranial
part of the skull is almost the same shape in the species studied.

Likewise, in the White-winged Dove the distance from the anterior tip
of the lower mandible to the anterior part of _M. adductor mandibulae
externus_ is relatively longer in relation to the length of the lower
mandible than in the Mourning Dove. Finally, the position of the jugal
with respect to the naso-frontal hinge is about the same in the two
species.

Measurements and calculations indicate that the longer beak of the
White-winged Dove as compared with the Mourning Dove is a function of
the beak itself, not of differences in other parts of the skull.
Measurements of skulls of Eared and Zenaida doves support this view.

OTHER MORPHOLOGICAL FEATURES

In the species dissected, the only variable muscle that I consider
significant in revealing relationships is _M. pseudotemporalis
profundus_. It is markedly enlarged in the White-winged Dove in
relation to the homologous muscle in the Mourning Dove. The muscle is
enlarged in such a manner that a lateral expansion of its mass is
apparent in superficial or dorsal view (compare figures 15 and 16).
This, of course, indicates a muscle with powerful contraction, which
has been unable to enlarge its circumference symmetrically because the
eye is immediately dorsal to the muscle. Therefore it has expanded
laterally. Ventral expansion is blocked by the presence of other
muscles, and medially there is no surface for the insertion of
additional fibers on the orbital process of the quadrate.

The jaw musculature has been known for some time to be highly adaptive
(Beecher, 1951a and b, 1953; Bowman, 1961; Burt, 1930; Engels, 1940 and
Goodman and Fisher, 1962) and it would not be unreasonable, I think, to
expect the jaw mu