mains have been found of a
massive bird, about eight feet high, with a head larger than that of
any living animal except the elephant, rhinoceros, and hippopotamus
(Chamberlin).

The absence of early Eocene remains prevents us from tracing the lines
of our vast and varied bird-kingdom to their Mesozoic beginnings.
And when we appeal to the zoologist to supply the missing links of
relationship, by a comparison of the structures of living birds, we
receive only uncertain and very general suggestions. [*] He tells us that
the ostrich-group (especially the emus and cassowaries) are one of the
most primitive stocks of the bird world, and that the ancient Dinornis
group and the recently extinct moas seem to be offshoots of that stock.
The remaining many thousand species of Carinate birds (or flying birds
with a keel [carina]-shaped breast-bone for the attachment of the flying
muscles) are then gathered into two great branches, which are "traceable
to a common stock" (Pycraft), and branch in their turn along the later
lines of development. One of these lines--the pelicans, cormorants,
etc.--seems to be a continuation of the Ichthyornis type of the
Cretaceous, with the Odontopteryx as an Eocene offshoot; the divers,
penguins, grebes, and petrels represent another ancient stock, which
may be related to the Hesperornis group of the Cretaceous. Dr. Chalmers
Mitchell thinks that the "screamers" of South America are the nearest
representatives of the common ancestor of the keel-breasted birds. But
even to give the broader divisions of the 19,000 species of living birds
would be of little interest to the general reader.

* The best treatment of the subject will be found in W. P.
Pycraft's History of Birds, 1910.

The special problems of bird-evolution are as numerous and unsettled
as those of the insects. There is the same dispute as to "protective
colours" and "recognition marks", the same uncertainty as to the origin
of such instinctive practices as migration and nesting. The general
feeling is that the annual migration had its origin in the overcrowding
of the regions in which birds could live all the year round. They
therefore pushed northward in the spring and remained north until the
winter impoverishment drove them south again. On this view each group
would be returning to its ancestral home, led by the older birds, in the
great migration flights. The curious paths they follow are believed by
some authorities to