d, and Huxley was able to contrast them, or at least to
show how necessary the new embryological method was as a corrective and
a supplement to the older anatomical, or, as he calls it, "gradation"
method. Applied to the "Theory of the Skull," the gradation method
consists in comparing the parts of the skull and vertebral column in
adult animals with respect to their form and connections. "Using the
other method, the investigator traces back skull and vertebral column to
their earliest embryonic states and determines the identity of parts by
their developmental relations" (p. 541). This second method is the final
and ultimate. "The study of the gradations of structure presented by a
series of living beings may have the utmost value in suggesting
homologies, but the study of development alone can finally demonstrate
them" (p. 541). As an example of the utility and, indeed, the necessity
of applying the embryological method Huxley takes the case of the
quadrate bone in birds. This bone had been generally regarded by
anatomists as the equivalent of the tympanic of mammals, on account of
its connection with the tympanum; but Reichert showed (1837) that the
same segment of the first visceral arch developed into the incus in
mammals, and into the quadrate in birds, and that therefore the quadrate
was homologous with the incus. Similarly, on developmental grounds, the
malleus or hammer of mammals is the homologue of the articular of birds,
since both are developed from a portion of Meckel's cartilage identical
in form and connections in the two groups. The homologies of the bones
connected with the jaws in bony fishes had long been a subject of
contention among comparative anatomists; Huxley shows from his personal
observations how the development of the visceral arches throws light
upon these difficulties. The mandibular arch in the developing fish is
abruptly angled, as in the embryo of Tetrapoda; the upper prong of it
ossifies into the palatine and pterygoid; at the angle is formed the
quadrate (jugal, Cuvier), and to the quadrate is articulated the lower
jaw, which ossifies round the lower prong or Meckel's cartilage. The
scheme of development of the jaws is accordingly similar in fish to what
it is in other Vertebrates, and this similarity of development enables
Huxley to recognise what are the true homologues of the quadrate, the
palatine and the pterygoid in adult bony fish, and to prove that the
symplectic and the metapt