they construct a different sort of nest from that made by their nearest
allies, and not liable to be injured by their spurs; so that the spurs
have not been removed? Or are we to suppose that the females of these
several species especially require spurs for their defence? It is a
more probable conclusion that both the presence and absence of spurs in
the females result from different laws of inheritance having prevailed,
independently of natural selection. With the many females in which
spurs appear as rudiments, we may conclude that some few of the
successive variations, through which they were developed in the males,
occurred very early in life, and were consequently transferred to the
females. In the other and much rarer cases, in which the females
possess fully developed spurs, we may conclude that all the successive
variations were transferred to them; and that they gradually acquired
and inherited the habit of not disturbing their nests.
The vocal organs and the feathers variously modified for producing
sound, as well as the proper instincts for using them, often differ in
the two sexes, but are sometimes the same in both. Can such differences
be accounted for by the males having acquired these organs and
instincts, whilst the females have been saved from inheriting them, on
account of the danger to which they would have been exposed by
attracting the attention of birds or beasts of prey? This does not seem
to me probable, when we think of the multitude of birds which with
impunity gladden the country with their voices during the spring. (7.
Daines Barrington, however, thought it probable ( Philosophical
Transactions, 1773, p. 164) that few female birds sing, because the
talent would have been dangerous to them during incubation. He adds,
that a similar view may possibly account for the inferiority of the
female to the male in plumage.) It is a safer conclusion that, as vocal
and instrumental organs are of special service only to the males during
their courtship, these organs were developed through sexual selection
and their constant use in that sex alone the successive variations and
the effects of use having been from the first more or less limited in
transmission to the male offspring.
Many analogous cases could be adduced; those for instance of the plumes
on the head being generally longer in the male than in the female,
sometimes of equal length in both sexes, and occasionally absent in the
female, these several cases occurring in the same group of birds. It
would be difficult to account for such a difference between the sexes
472