take as typical of Huxley's
contributions to vertebrate anatomy, is his classical study on the
classification of birds. The great group of birds contains a larger
number of species than is known in any other group of vertebrates,
and, in this vast assemblage of forms there is strikingly little
anatomical difference. The ostrich and the humming-bird might perhaps
be taken as types of the extremest differences to be found, and yet,
although these differ in size, plumage, adaptations, habits, mode of
life, and almost everything that can separate living things, the two
conform so closely to the common type of bird structure that knowledge
of the anatomy of one would be a sufficient guide, down to minute
details, for dissection of the other. None the less, there are
hundreds of thousands of species of birds between these two types. It
is not surprising that to reduce this vast assemblage of similar
creatures to an ordered system of classification has proved one of the
most difficult tasks attempted by zooelogists. Before Huxley, it had
been attempted by a number of distinguished zooelogists; but, for the
most part, these had relied too much on merely external characters and
on superficial modifications in obvious relation to habits. When
Huxley, in the course of a set of lectures on Comparative Anatomy, was
about to approach the subject of birds he was asked by a zooelogist how
he proposed to treat them. "I intend," he replied, "to treat them as
extinct animals." By that he meant that it was his purpose to make a
prolonged study of their skeletal structures the basis of his
grouping, following the lines which Cuvier, Owen, and he himself had
pursued so successfully in the case of the fossil remains of
vertebrates. The result was that this first systematic study of even
one set of the anatomical characters of the group completely reformed
the method by which all subsequent workers have tried to grapple with
the problem; ornithology was raised from a process akin to
stamp-collecting to a reasoned scientific study. The immediate
practical results were equally important. He was able to shew that
among the innumerable known forms there were three grades of
structure. The lowest had already been recognised and named by
Haeckel; it consisted of the Saururae, or reptile-like, birds, and
contained a single fossil form, Archaeopteryx, distinguished from all
living birds by the presence of a hand-like wing in which the
metacarpal bones