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[Bullseye]

Part Two - Sexual Selection
Chapter VIII - Principles of Sexual Selection

  WITH animals which have their sexes separated, the males necessarily
differ from the females in their organs of reproduction; and these are
the primary sexual characters. But the sexes often differ in what
Hunter has called secondary sexual characters, which are not
directly connected with the act of reproduction; for instance, the
male possesses certain organs of sense or locomotion, of which the
female is quite destitute, or has them more highly-developed, in order
that he may readily find or reach her; or again the male has special
organs of prehension for holding her securely. These latter organs, of
infinitely diversified kinds, graduate into those which are commonly
ranked as primary, and in some cases can hardly be distinguished
from them; we see instances of this in the complex appendages at the
apex of the abdomen in male insects. Unless indeed we confine the term
"primary" to the reproductive glands, it is scarcely possible to
decide which ought to be called primary and which secondary.
  The female often differs from the male in having organs for the
nourishment or protection of her young, such as the mammary glands
of mammals, and the abdominal sacks of the marsupials. In some few
cases also the male possesses similar organs, which are wanting in the
female, such as the receptacles for the ova in certain male fishes,
and those temporarily developed in certain male frogs. The females
of most bees are provided with a special apparatus for collecting
and carrying pollen, and their ovipositor is modified into a sting for
the defense of the larvae and the community. Many similar cases
could be given, but they do not here concern us. There are, however,
other sexual differences quite unconnected with the primary
reproductive organs, and it is with these that we are more
especially concerned such as the greater size, strength, and pugnacity
of the male, his weapons of offence or means of defence against
rivals, his gaudy colouring and various ornaments, his power of
song, and other such characters.
  Besides the primary and secondary sexual differences, such as the
foregoing, the males and females of some animals differ in
structures related to different habits of life, and not at all, or
only indirectly, to the reproductive functions. Thus the females of
certain flies (Culicidae and Tabanidae) are blood-suckers, whilst
the males, living on flowers, have mouths destitute of mandibles.* The
males of certain moths and of some crustaceans (e. g. Tanais) have
imperfect, closed mouths, and cannot feed. The complemental males of
certain cirripedes live like epiphytic plants either on the female
or the hermaphrodite form, and are destitute of a mouth and of
prehensile limbs. In these cases it is the male which has been
modified, and has lost certain important organs, which the females
possess. In other cases it is the female which has lost such parts;
for instance, the female glow-worm is destitute of wings, as also
are many female moths, some of which never leave their cocoons. Many
female parasitic crustaceans have lost their natatory legs. In some
weevil-beetles (Curculionidae) there is a great difference between the
male and female in the length of the rostrum or snout;*(2) but the
meaning of this and of many analogous differences, is not at all
understood. Differences of structure between the two sexes in relation
to different habits of life are generally confined to the lower
animals; but with some few birds the beak of the male differs from
that of the female. In the Huia of New Zealand the difference is
wonderfully great, and we hear from Dr. Buller*(3) that the male
uses his strong beak in chiselling the larvae of insects out of
decayed wood, whilst the female probes the softer parts with her far
longer, much curved and pliant beak: and thus they mutually aid each
other. In most cases, differences of structure between the sexes are
more or less directly connected with the propagation of the species:
thus a female, which has to nourish a multitude of ova, requires
more food than the male, and consequently requires special means for
procuring it. A male animal, which lives for a very short time,
might lose its organs for procuring food through disuse, without
detriment; but he would retain his locomotive organs in a perfect
state, so that he might reach the female. The female, on the other
hand, might safely lose her organs for flying, swimming, or walking,
if she gradually acquired habits which rendered such powers useless.

  * Westwood, Modern Classification of Insects, vol. ii., 1840, p.
541. For the statement about Tanais, mentioned below, I am indebted to
Fritz Muller.
  *(2) Kirby and Spence, Introduction to Entomology, vol. iii.,
1826, p. 309.
  *(3) Birds of New Zealand, 1872, p. 66.

  We are, however, here concerned only with sexual selection. This
depends on the advantage which certain individuals have over others of
the same sex and species solely in respect of reproduction. When, as
in the cases above mentioned, the two sexes differ in structure in
relation to different habits of life, they have no doubt been modified
through natural selection, and by inheritance, limited to one and
the same sex. So again the primary sexual organs, and those for
nourishing or protecting the young, come under the same influence; for
those individuals which generated or nourished their offspring best,
would leave, ceteris paribus, the greatest number to inherit their
superiority; whilst those which generated or nourished their offspring
badly, would leave but few to inherit their weaker powers. As the male
has to find the female, he requires organs of sense and locomotion,
but if these organs are necessary for the other purposes of life, as
is generally the case, they will have been developed through natural
selection. When the male has found the female, he sometimes absolutely
requires prehensile organs to hold her; thus Dr. Wallace informs me
that the males of certain moths cannot unite with the females if their
tarsi or feet are broken. The males of many oceanic crustaceans,
when adult, have their legs and antennae modified in an
extraordinary manner for the prehension of the female; hence we may
suspect that it is because these animals are washed about by the waves
of the open sea, that they require these organs in order to
propagate their kind, and if so, their development has been the result
of ordinary or natural selection. Some animals extremely low in the
scale have been modified for this same purpose; thus the males of
certain parasitic worms, when fully grown, have the lower surface of
the terminal part of their bodies roughened like a rasp, and with this
they coil round and permanently hold the females.*

  * M. Perrier advances this case (Revue Scientifique, Feb. 1, 1873,
p. 865) as one fatal to the belief in sexual election, inasmuch as
he supposes that I attribute all the differences between the sexes
to sexual selection. This distinguished naturalist, therefore, like so
many other Frenchmen, has not taken the trouble to understand even the
first principles of sexual selection. An English naturalist insists
that the claspers of certain male animals could not have been
developed through the choice of the female! Had I not met with this
remark, I should not have thought it possible for any one to have read
this chapter and to have imagined that I maintain that the choice of
the female had anything to do with the development of the prehensile
organs in the male.