As Professor Owen has remarked, there
is no greater anomaly in nature than a bird that cannot fly; yet there are
several in this state. The logger-headed duck of South America can only
flap along the surface of the water, and has its wings in nearly the same
condition as the domestic Aylesbury duck: it is a remarkable fact that the
young birds, according to Mr. Cunningham, can fly, while the adults have
lost this power. As the larger ground-feeding birds seldom take flight
except to escape danger, it is probable that the nearly wingless condition
of several birds, now inhabiting or which lately inhabited several oceanic
islands, tenanted by no beasts of prey, has been caused by disuse. The
ostrich indeed inhabits continents, and is exposed to danger from which it
cannot escape by flight, but it can defend itself, by kicking its enemies,
as efficiently as many quadrupeds. We may believe that the progenitor of
the ostrich genus had habits like those of the bustard, and that, as the
size and weight of its body were increased during successive generations,
its legs were used more and its wings less, until they became incapable of
flight.
Kirby has remarked (and I have observed the same fact) that the anterior
tarsi, or feet, of many male dung-feeding beetles are often broken off; he
examined seventeen specimens in his own collection, and not one had even a
relic left. In the Onites apelles the tarsi are so habitually lost that
the insect has been described as not having them.
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