A structure used only once in an animal's whole life,
if of high importance to it, might be modified to any extent by
natural selection; for instance, the great jaws possessed by certain
insects, and used exclusively for opening the cocoon--or the hard tip
to the beak of nestling birds, used for breaking the egg. It has been
asserted, that of the best short-beaked tumbler-pigeons more perish in
the egg than are able to get out of it; so that fanciers assist in the
act of hatching. Now, if nature had to make the beak of a full-grown
pigeon very short for the bird's own advantage, the process of
modification would be very slow, and there would be simultaneously the
most rigorous selection of the young birds within the egg, which had
the most powerful and hardest beaks, for all with weak beaks would
inevitably perish: or, more delicate and more easily broken shells
might be selected, the thickness of the shell being known to vary like
every other structure.
SEXUAL SELECTION.
Inasmuch as peculiarities often appear under domestication in one sex
and become hereditarily attached to that sex, the same fact probably
occurs under nature, and if so, natural selection will be able to
modify one sex in its functional relations to the other sex, or in
relation to wholly different habits of life in the two sexes, as is
sometimes the case with insects. And this leads me to say a few words
on what I call Sexual Selection.
Pages:
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119