The Descent of Man

is furnished with an additional pair of muscles.
(45. Bishop, in Todd's 'Cyclopaedia of Anatomy
and Physiology,' vol. iv. p. 1499.) In one of the
ducks, however, namely Anas punctata, the
bony enlargement is only a little more developed
in the male than in the female. (46. Prof.
Newton, 'Proc. Zoolog. Soc.' 1871, p. 651.) But
the meaning of these differences in the trachea
of the two sexes of the Anatidae is not understood;
for the male is not always the more vociferous;
thus with the common duck, the male
hisses, whilst the female utters a loud quack.
(47. The spoonbill (Platalea) has its trachea
convoluted into a figure of eight, and yet this
bird (Jerdon, 'Birds of India,' vol. iii. p. 763) is
mute; but Mr. Blyth informs me that the convolutions
are not constantly present, so that perhaps
they are now tending towards abortion.)
In both sexes of one of the cranes (Grus virgo)
the trachea penetrates the sternum, but presents
"certain sexual modifications." In the male
of the black stork there is also a well-marked