Suddenly, and, as far as one can see, without any sign from the
female, one of the males pairs with her and all the others
immediately disappear. In these cases the males do not fight or
struggle in any way, and as one watches the ceremony the wonder
arises as to how the moment is determined, and why the pairing
did not take place before. Proximity does not decide the point,
for long beforehand the males often alight close to the female
and brush against her with fluttering wings. I have watched the
process exactly as I have described it in a common Northern
_Noctua_, the antler moth (_Charæax graminis_), and I have seen
the same thing among beetles." (E.B. Poulton, _The Colors of
Animals_, 1890, p. 391.) This author mentions that among some
butterflies the females take the active part. The example here
quoted of courtship among moths illustrates how phenomena which
are with difficulty explicable by the theory of sexual selection
in its original form become at once intelligible when we realize
the importance of tumescence in courtship.

Of the Argentine cow-bird (_Molothrus bonariensis_) Hudson says
(_Argentine Ornithology_, vol. i, p. 73): "The song of the male,
particularly when making love, is accompanied with gestures and
actions somewhat like those of the domestic pigeon. He swells
himself out, beating the ground with his wings, and uttering a
series of deep internal notes, followed by others loud and clear;
and occasionally, when uttering them, he suddenly takes wing and
flies directly away from the female to a distance of fifty yards,
and performs a wide circuit about her in the air, singing all the
time. The homely object of his passion always appears utterly
indifferent to this curious and pretty performance; yet she must
be even more impressionable than most female birds, since she