THE THEORY OF COURTSHIP.
In the same volumes with the 'Descent of Man' Darwin included his
admirable treatise on sexual selection. This form of selection he had
already dealt with briefly in the 'Origin of Species;' but as in his
opinion it was largely instrumental in producing the minor differences
which separate one race of men from another, he found it necessary to
enlarge and expand it in connection with his account of the rise and
progress of the human species.
Among many animals, and especially in the higher classes of animals, the
males and females do not mate together casually; there is a certain
amount of selection or of courtship. In some cases, as with deer and
antelopes, the males fight with one another for the possession of the
females. In other cases, as with the peacock and the humming-birds, the
males display their beauty and their skill before the eyes of the
assembled females. In the first instance, the victor obtains the mates;
in the second instance, the mates themselves select from the group the
handsomest and most personally pleasing competitor. Sexual selection, of
which these are special cases, depends on the advantage possessed by
certain individuals over others of the same sex and species solely in
respect to the question of mating. In all such instances, the males have
acquired their weapons of offence and defence or their ornamental
decorations, not from being better fitted to survive in the struggle for
existence, but from having gained an advantage over other males of the
same kind, and from having transmitted this advantage to offspring of
their own sex alone.
Just as man can improve the breed of his game-cocks by the selection of
those birds which are victorious in the cockpit, so the strongest and
most vigorous males, or those provided with the best weapons, have
prevailed in the state of nature over their feebler and more cowardly
competitors. Just as man can give beauty, according to his own standard
of taste, to his male poultry, by selecting special birds for their
plumage, their port, their wattles, or their hackles, so female birds in
a state of nature have by a long-continued choice of the more attractive
males added to their beauty and their ornamental adjuncts. In these two
ways, Darwin believed, a limited selection has slowly developed weapons
like the horns of buffaloes, the antlers of stags, the tusks of boars,
and the spurs of game-birds, together with the courage, strength, and
pugnacity always associated with such special organs. It has also
developed the ornamental plumage of the peacock, the argus pheasant, and
the birds of paradise; the song of the lark, the thrush, and the
nightingale; the brilliant hues on the face of the mandrill; and the
attractive perfume of the musk-deer, the snakes, and the scented
butterflies. Wherever one sex possesses any decorative or alluring
adjunct not equally shared by the other, Darwin attributed this special
gift either to the law of battle, or to the long and slowly exerted
selective action of their fastidious mates.
The germ of the doctrine of sexual selection is to be found, like so
many other of Charles Darwin's theories, in a prophetic passage of his
grandfather's 'Zoonomia.' Stags, the Lichfield physician tells us, are
provided with antlers 'for the purpose of combating other stags for the
exclusive possession of the females, who are observed, like the ladies
in the time of chivalry, to attend the car of the victor. The birds
which do not carry food to their young, and do not therefore marry, are
armed with spurs for the purpose of fighting for the exclusive
possession of the females, as cocks and quails. It is certain that these
weapons are not provided for their defence against other adversaries,
because the females of these species are without this armour. The final
cause of this contest among the males seems to be that the strongest and
most active animal should propagate the species, which should thence
become improved.'
It must be noticed, however, that Erasmus Darwin here imports into the
question the metaphysical and teleological notion of the final cause,
implying that the struggle of the males was ordained from without, for
this express and preconceived purpose; whereas Charles Darwin, never
transcending the world of phenomena, more logically regards the struggle
itself as an efficient cause, having for its result the survival of the
strongest or the handsomest as the case may be. This distinction is
fundamental; it marks the gulf between the essentially teleological
spirit of the eighteenth century and the essentially positive spirit of
philosophy and science at the present day.
Here again, too, the immense logical superiority of Charles Darwin's
rigorous and exhaustive inductive method over the loose suggestiveness
of his grandfather Erasmus may easily be observed. For while Erasmus
merely throws out a clever and interesting hint as to the supposed
method and intention of nature, Charles Darwin proves his thesis, point
by point, with almost mathematical exactitude, leaving no objection
unmet behind him, but giving statistical and inductive warrant for every
step in his cumulative argument. He goes carefully into the numerical
proportion of the two sexes in various species; into the relative dates
of arrival in any particular country of the males and females of
migratory birds; into the question whether any individuals ever remain
in the long run unpaired; into the chances of the earliest-mated or most
vigorous couples leaving behind more numerous or stronger offspring to
represent them in the next generation. He collects from every quarter
and from all sources whatever available evidence can be obtained as to
the courtship and rivalry of birds and butterflies, of deer and
antelopes, of fish and lizards. He shows by numerous examples and
quotations how even flies coquet together in their pretty rhythmical
aerial dances; how wasps battle eagerly with one another to secure
possession of their unconcerned mates; how cicadas strive to win their
'voiceless brides' with stridulating music; how sphinx-moths endeavour
to allure their partners with the musky odour of their pencilled wings;
and how emperors and orange-tips display their gorgeous spots and bands
in the broad sunshine before the admiring and attentive eyes of their
observant dames. He traces up the same spirit of rivalry and ostentation
to the cock-pheasant strutting about before the attendant hen, and to
the meeting-places of the blackcock, where all the males of the district
fight with one another and undertake long love-dances in regular
tournaments, while the females stand by and watch the chances and
changes of the contest with affected indifference. Finally, he points
out how similar effects are produced by like causes among the higher
animals, especially among our near relations the monkeys; and then he
proceeds to apply the principles thus firmly grounded to the particular
instance of the human race itself, the primary object of his entire
treatise.
Some of the most interesting of the modifications due to this particular
form of selective action are to be found amongst the insects and other
low types of animal life. The crickets, the locusts, and the
grasshoppers, for example, are all famous for their musical powers; but
the sounds themselves are produced in the different families by very
different and quaintly varied organs. The song of the crickets is evoked
by the scraping of minute teeth on the under side of either wing-cover;
in the case of the locusts, the left wing, which acts as a bow, overlies
the right wing, which serves as a fiddle; while with the grasshoppers,
the leg does duty as the musical instrument, and has a row of
lancet-shaped elastic knobs along its outer surface, which the insect
rubs across the nerves of the wing-covers when it wishes to charm the
ears and rouse the affection of its silent mate. In a South African
species of the same family, the whole body of the male is fairly
converted into a musical instrument, being immensely inflated, hollow,
and distended like a pellucid air-bladder in order to act as an
efficient sounding-board. Among the beetles, taste seems generally to
have specialised itself rather on form than on music or colour, and the
males are here usually remarkable for their singular and very
complicated horns, often compared in various species to those of stags
or rhinoceroses, and entirely absent in the females of most kinds. But
it is among the butterflies and moths that insect æstheticism has
produced its greatest artistic triumphs; for here the beautiful
eye-spots and delicate markings on the expanded wing-membranes are
almost certainly due to sexual selection.
The higher animals display like evidence of the same slow selective
action. The courtship of the stickleback, who dances 'mad with delight'
around the mate he has allured into the nest he prepares for her, has
been observed by dozens of observers both before and since in the
domestic aquarium. The gem-like colours of the male dragonet, the
butterfly wings of certain gurnards, and the decorated tails of some
exotic carps all point in the same direction. Our own larger newt is
adorned during the breeding season with a serrated crest edged with
orange; while in the smaller kind the colours of the body acquire at the
same critical period of love-making a vivid brilliancy. The strange
horns and luridly coloured throat-pouches of tropical lizards are
familiar to all visitors in equatorial climates, and they are confined
exclusively to the male sex. Among birds, the superior beauty of the
male plumage is known to everybody; and their greatest glory invariably
coincides with the special season for the selection of mates. In the
spring, as even our poets have told us, the wanton lapwing gets himself
another crest. The law of battle produces the spur of the game-birds and
the still stranger wing-spurs of certain species of the plover kind.
Æsthetic rivalry is answerable rather for vocal music, and for the
plumage of the umbrella-bird, the lyre-bird, the humming-birds, and the
cock of the rocks. Among mammals, strength rather than beauty seems to
have carried the day; horns, and tusks, and spikes, and antlers are here
the special guerdon of the victorious males. Yet even mammals show
occasional signs of distinctly æsthetic and artistic preferences, as in
the gracefully twisted horns of the koodoo, the scent-glands of the
musk-deer or of certain antelopes, the brilliant hues of the male
mandrill, and the tufts and moustaches of so many monkeys.
It must be frankly conceded that the reception accorded to Darwin's
doctrine of sexual selection, even among the biological public, was far
less unanimous, enthusiastic, and full than that which had been granted
to his more extensive theory of survival of the fittest. Many eminent
naturalists declined from the very outset to accept the conclusions thus
definitely set before them, and others who at first seemed disposed to
bow to the immense weight of Darwin's supreme authority gradually
withdrew their grudging assent from the new doctrine, as they found
their relapse backed up by others, and refused to believe that the
theory of courtship had been fairly proven before the final tribunal of
science. Several critics began by objecting that the whole theory was a
mere afterthought. Darwin, they said, finding that natural selection
did not suffice by itself to explain all the details of structure in
man, had invented sexual selection as a supplementary principle to help
it over the hard places. Those who wrote and spoke in this thoughtless
fashion could have had but a very inadequate idea of Darwin's close
experimental methods of enquiry. As a matter of fact, indeed, they were
entirely wrong; the doctrine of sexual selection itself, already faintly
foreshadowed by Erasmus Darwin in the 'Zoonomia,' had been distinctly
developed in the first edition of the 'Origin of Species' with at least
as much provisional elaboration as any other equally important factor in
the biological drama as set forth in that confessedly introductory work.
Nay, Haeckel had caught gladly at the luminous conception there
expressed, even before the appearance of the 'Descent of Man,' and had
worked it out in his 'Generelle Morphologie,' with great insight, to its
legitimate conclusions in many directions. Indeed, the sole reason why
so much space was devoted to the subject in Darwin's work on human
development was simply because there for the first time an opportunity
arose of utilising his vast store of collected information on this
single aspect of the evolutionary process. It was no afterthought, but a
necessary and inevitable component element of the fully-developed
evolutionary concept.
Still, it cannot be denied that naturalists generally did not accept
with effusion the new clause in the evolutionary creed. Many of them
hesitated; a few acquiesced; the majority more or less openly dissented.
But Darwin's belief remained firm as a rock. 'I am glad you defend
sexual selection,' he wrote a few years later in a private letter; 'I
have no fear about its ultimate fate, though it is now at a discount;'
and in the preface to the second edition of the 'Descent of Man,' he
remarks acutely, 'I have been struck with the likeness of many of the
half-favourable criticisms on sexual selection with those which appeared
at first on natural selection; such as that it would explain some few
details, but certainly was not applicable to the extent to which I have
employed it. My conviction of the power of sexual selection remains
unshaken.... When naturalists have become familiar with the idea, it
will, as I believe, be much more largely accepted; and it has already
been fully and favourably received by several capable judges.'
In spite of the still continued demurrer of not a few among the leading
evolutionists, it is probable, I think, that Darwin's prophecy on this
matter will yet be justified by the verdict of time. For the opposition
to the doctrine of sexual selection proceeds almost invariably, as it
seems to me, from those persons who still desire to erect an efficient
barrier of one sort or another between the human and animal worlds;
while on the contrary the theory in question is almost if not quite
universally accepted by just those rigorously evolutionary biologists
who are freest from preconceptions or special _a priori_ teleological
objections of any kind whatever. The half of the doctrine which deals
with the law of battle, indeed, can hardly be doubted by any competent
naturalist; the other half, which deals with the supposed æsthetic
preferences of the females, is, no doubt, distasteful to certain
thinkers because it seems to imply the existence in the lower animals of
a sense of beauty which many among us are not even now prepared
generously to admit. The desire to arrogate to mankind alone all the
higher faculties either of sense or intellect has probably much to do
with the current disinclination towards the Darwinian idea of sexual
selection. Thinkers who allow themselves to be emotionally swayed by
such extraneous considerations forget that the beautiful is merely that
which pleases; that beauty has no external objective existence; and that
the range of taste, both among ourselves and among animals at large, is
practically infinite. The greatest blow ever aimed at the Darwinian
theory of sexual selection was undoubtedly that dealt out by Mr. Alfred
Russel Wallace (et tu, Brute!) in his able and subtle article on the
Colours of Animals in 'Macmillan's Magazine,' since reprinted in his
delightful work on 'Tropical Nature.' Wallace there urges with his usual
acuteness, ingenuity, and skill several fundamental objections to the
Darwinian hypothesis of no little importance and weight. But it must
always be remembered (with all due respect to the joint discoverer of
natural selection) that Mr. Wallace himself, after publishing his own
admirable essay on the development of man, drew back aghast in the end
from the full consequences of his own admission, and uttered his partial
recantation in the singular words, 'Natural selection could only have
endowed the savage with a brain a little superior to that of an ape.' It
seems probable that in every case an analogous desire to erect a firm
barrier between man and brute by positing the faculty for perceiving
beauty as a special quasi-divine differentia of the human race has been
at the bottom of the still faintly surviving dislike amongst a section
of scientific men to sexual selection. Nevertheless, a candid and
impartial critic would be compelled frankly to admit that Darwin's
admirable theory of courtship has not on the whole proved so generally
acceptable to the biological world up to the present time as his greater
and far more comprehensive theory of survival of the fittest. It still
waits for its final recognition, towards which it is progressing more
rapidly and surely every day it lives.