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. As the larger ground-feeding birds seldom
take flight except to escape danger, I believe that the nearly
wingless condition of several birds, which now inhabit or have lately
inhabited several oceanic islands, tenanted by no beast 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 by
kicking it can defend itself from enemies, as well as any of the
smaller quadrupeds. We may imagine that the early progenitor of the
ostrich had habits like those of a bustard, and that as natural
selection increased in successive generations the size and weight of
its body, 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 very
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. In some other genera they are present, but in a
rudimentary condition. In the Ateuchus or sacred beetle of the
Egyptians, they are totally deficient. There is not sufficient
evidence to induce us to believe that mutilations are ever inherited;
and I should prefer explaining the entire absence of the anterior
tarsi in Ateuchus, and their rudimentary condition in some other
genera, by the long-continued effects of disuse in their progenitors;
for as the tarsi are almost always lost in many dung-feeding beetles,
they must be lost early in life, and therefore cannot be much used by
these insects.
Pages:
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174