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.
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