THE MOTH AND THE CANDLE
IN order to understand and interpret correctly the operation of
natural selection in producing new species and maintaining them, by
"the preservation of favoured races in the struggle for life" (to use
Darwin's words), we must take a wide and, at the same time, a minutely
accurate survey of the living world. We must seek out the evidences of
this operation and use the imagination in forming conceptions as to the
varied steps of the process and the results which are likely to ensue
from it at different stages and in different conditions. We cannot
interpret the existing structures and behaviour of living things by the
use of a simple formula, such as that set up by some writers who have
not properly studied Mr. Darwin's works, and declare that, according to
him, all structures and behaviours which we observe in living things
are perfect and the finished result of survival of the ideally fittest
variations.
Plants and animals are so complex (as no one has shown more clearly
than Darwin), not only in their structure but in the chemical and
physical action and interaction of their living parts, that in the
course of the ages during which the present species have been, step by
step, fashioned in the endless vicissitudes of a changing world, many
of them have retained structures or chemical constitutions which once
were useful but now are useless, or even positively injurious. Even
injurious structures or behaviours may be retained and inherited by a
species of plant or animal, if, on the whole, the other accompanying
modifications of structure are valuable--that is, of "life-saving"
value, so that, "on the whole," the race is favoured by selection in
the struggle for existence.
In species which have but lately acquired dominance or are brought
by their success into novel conditions, we may, and do, find old
structures and behaviours still persisting which are injurious, not
yet, as it were, "cleaned up" and got rid of as they would be in the
course of further long periods of selection. Such species become
established, and may even acquire a definite stability, because the
injurious structures or behaviours which they have retained are of
little or no account as compared with the other advantageous characters
which the species have developed. The term "disharmonies" is applied to
such injurious characters, consisting in a certain want of harmony (in
minor respects) between the structure of an organism and the conditions
in which, nevertheless, it thrives.
Such species, imperfect because of their "disharmonies," are an
illustration of the fact that Nature herself, in matters relating to
living things, is not averse to compromise. Nature sets the example
of toleration. Toleration may be defended on the ground that it is
the biological method. Nature, though stern and inexorable as to
essentials, yet accepts the faults and defects of some of her children
because of the virtues and excellences which accompany them. The most
highly endowed and successful forms on account of their dominance and
power of spreading into new conditions, are even more likely than
less highly developed kinds to retain concealed defects--disharmonies
which do not lead to the destruction of the species, but occasionally
cause strange embarrassment to it until they are, possibly in the long
process of ages, got rid of by the slow operation of selection and
survival of those individuals in which the injurious character varies
in the direction of diminution and ultimate disappearance.
In man (owing, apparently, to the rapid rate at which he has been
carried along towards dominance over the whole face of the globe by the
development of his intelligence) the bodily structure has failed to
keep pace with and to become perfected, "trimmed up," and completely
adapted to, the newly-acquired habits which his increasing intelligence
has forced on him. His "wisdom teeth" are "disharmonies." They are now
useless and dwindled, weak spots open to the attacks of disease--since
they are no longer needed for grinding coarse vegetable food, and are
consequently no longer kept (by the speedy death of those individuals
in whom they are small) at the full original size and efficiency seen
in the apes. His large intestine is a "disharmony" not yet got rid of
by natural selection, although no longer useful, but, on the contrary,
the seat of poisonous putrefaction and absorption of such poisons. His
tail--a few small vertebræ beneath the skin--is absolutely useless,
and occasionally the seat of dangerous injury or disease. Tails very
generally are liable to become useless in the descendants of animals
in which they were invaluable as "fly-brushes" (cattle, horses, etc.),
as prehensible organs (American monkeys), as concealing cloaks (South
American ant-eater), as aids to swimming or flying, or as ornamental
glories (the big-cats and others). The stumpy tail of the lynx, of
some monkeys, and some lizards and fishes tells of a history in which
the full-sized tail became a "disharmony"--a positive nuisance--and
has been reduced, even if not abolished, by natural selection of
short-tailed or tail-less varieties.
We have to be careful in asserting that any structure or behaviour in
an organism is certainly a "disharmony," for it is very difficult to be
quite sure as to the complete details of the life of a wild creature,
and so to be able to form a conclusion rather than to suggest a
possibility--as to the part played by an apparently injurious structure
or habit in the economy of that creature.
One of the most striking instances of a habit or behaviour which
persists and dominates the life of a wild animal to its own injury
and destruction is that shown by many moths and other insects, which
are attracted at night by a flame (a lamp or an open fire), and fly
into it even when burnt by it, again and again until they are killed.
A burnt child dreads the fire; but a burnt moth or a singed ichneumon
fly seems to enjoy being burnt, and becomes more and more excited by
its dashes into the flame until it finally drops with shrivelled wings
to the ground. My brother told me some years ago of the verandah of
his house in Java in which an open lamp was lit every night. Regularly
two sets of animals, driven and guided by the action of the light on
their nervous mechanism, arrived on the scene. Swarms of moths and
flies dashed in and out of the flame and fell, maimed by the heat, to
the ground. There a strange group had already assembled. Gigantic toads
and wall lizards crept from their holes in the masonry and woodwork,
and awaited the shower of injured insects, which they snapped up in
eager rivalry as the infatuated flame-seekers dropped, hour after
hour, to the floor. The instinct, the nervous mechanism, which
brought the greedy reptiles to the spot was a "harmony," a valuable
guide to nutrition; whilst the flame-seeker's impulse is assuredly a
"disharmony"--a defect in adjustment--leading to death.
It is interesting to inquire into the probable origin of this fatal
desire for close contact with a source of light, a desire so strong as
to be entirely unchecked by the deadly heat accompanying the light.
The May-flies or Ephemerids are delicate little creatures, having four
net-veined wings rarely more than three-quarters of an inch across,
with two or three long filaments hanging from the tail. Three hundred
species are known from all parts of the world, of which forty occur in
the British Islands. They live as wingless, six-legged larvæ in the
water for a couple of years, feeding voraciously. Then one summer's
evening they very rapidly escape from their larval skin and fly over
the water in countless swarms. But only for a few hours. The eggs of
the females are fertilized, and they all, both males and females,
drop dead or dying into the water, where they are greedily devoured
by fishes. The males are far more numerous than the females; in some
species as many as 6000 males to one female have been counted. They
are attracted to an extraordinary degree by lights (flames or electric
lamps) set up for nocturnal illumination by civilized man, and in some
districts they are collected by fishermen in this way for use as food
for fish, or were so in Holland in the eighteenth century according
to Swammerdam's statement in his "Biblia Naturæ." Why do they thus
seek artificial lights? There is some indication of an explanation
in the fact that two tropical species of May-flies are known which,
like the glow-worms and fire-flies, produce light in their bodies. The
May-flies, especially the males, have unusually large and prominent
eyes, as is the case with phosphorescent fishes and some other
light-producing animals, and it appears probable that in the now rare
instances of self-luminous May-flies, the sexes are attracted to one
another by the light they produce, as is the case in other luminous
insects. It seems probable that the ancestral May-flies, of which many
remarkable kinds have been discovered in the fossilized condition in
strata as far back in time as those of the coal-measures, were all
self-luminous, and acquired an overpowering instinct of seeking the
light given out by other individuals as a necessary step towards sexual
congress. In the course of ages other senses (probably smell and touch)
have been called in to bring the fluttering insects into association.
The power of producing light, being no longer needed, has disappeared
from all but two rare species. But the urgent erotic instinct, the
nervous mechanism, which drove the ancient May-flies towards the
dancing lights of other May-flies, has remained unaltered in all the
living species of the group. It is a "disharmony" which has not been of
sufficient destructive importance to be "cleared away" or suppressed
by natural selection. In pre-human times, nocturnal fires and lights
were too uncommon to cause much disaster to the May-flies. But now that
mankind sets up everywhere his nocturnal flames and electric lamps, the
previously unimportant useless survival of an overpowering impulse to
rush to nocturnal lights, reveals itself as a serious and death-dealing
"disharmony." We must suppose, on this theory, that the other insects,
such as moths and certain flies (by no means all insects), which also
madly fly into nocturnal lights to their own destruction, have had
luminous ancestors and a similar early history. This is a legitimate
supposition, since there are several very distinct kinds of insects
known at the present day which are luminous at night, although no
existing moths or butterflies are known to be so.
A fact bearing on the explanation of the insects' perilous rush to
flame is that birds when migrating are attracted by the great brilliant
lamps of lighthouses, and, flying towards them, strike against their
glass coverings, and are killed in considerable numbers. In that case,
it may be that the flying towards the sun has become instinctive, and
that the bright light of the lighthouse acts upon a certain number of
birds (perhaps the less well-adjusted individuals) so as to call forth
the same response in the direction of flight as that exercised by the
sun's globe. The truth or error of this suggestion should be tested by
an examination of the species of birds which kill themselves against
lighthouse lanterns, and a knowledge of the season and direction of
their migration.
As to luminous or phosphorescent (often called "luminescent") insects
and other animals, there are a great many curious and interesting facts
known. There are luminescent bacteria (common on old meat bones and
dead fish and in the sea generally), animalcules of various species,
jelly-fish, star-fish, worms, shell-fish, and crustaceans and true
fishes. Inhabitants of the great depths of the ocean of all kinds
are usually luminescent. The light is caused by the oxidation of a
peculiar fatty substance. Without free oxygen there is no luminescence,
and yet no heat is produced but merely light, as when a stick of
damp phosphorus glows. The luminescence of living things (often, but
undesirably, called phosphorescence) is a process differing greatly
from that called "phosphorescence" in minerals and crystals, such as
the emission of light by a lump of white loaf-sugar when crushed.
You may see that kind of phosphorescence by standing in front of a
looking-glass in a dark room and crushing a lump of loaf-sugar with
the teeth, keeping the lips raised. It seems that in many organisms
luminescence occurs without any consequent use or service to the
organism. But in higher forms the power of emitting light has been
seized upon by natural selection having become of value in attracting
the individuals of a species to one another, or in attracting prey, or
again in scaring enemies. The luminescent matter is concentrated in
certain definite organs, and the access to it of oxygen and even its
formation are controlled by the nervous system.
Among insects far better known than the rare luminescent May-flies,
are the glow-worms, a family of beetles of which several species are
known besides our own familiar one, called Lampyris noctiluca. The
fire-flies of Southern Europe--Luciola italica--are small beetles
allied to the glow-worm, but both sexes fly and both are luminous,
whilst in the common glow-worm the female is wingless, and the flying
male, who is guided to the female by her light (which she can vary in
intensity), gives but a feeble light. The swarms of Italian fire-flies
consist of as many as a hundred males to one female, and the males are
far more brilliant than the females. My fellow-student Moseley showed
some in oxygen gas at the Royal Society's soirée many years ago. The
gas greatly increased their brilliancy. Many valuable experiments
in search of an explanation of the brilliance of the male Luciolæ
and their excess in number could be carried out in North Italy. A
peculiar grub-like female glow-worm, three inches long, is found in
South America, which produces a red light at each end of the body and
numerous points of green light on each side of it. It is called the
"railway-beetle" in Paraguay.
Another family of beetles besides the Lampyrids, or glow-worms, is
celebrated for the brilliant luminescence of some of its species.
These are the click-beetles, or Elaterids (see Fig. 22, C). In South
America there are upwards of a hundred species of this group, showing
various degrees of luminosity. The "Cucujos" (Pyrophorus noctilucus) of
tropical America is one of the most abundant and largest. It is as much
as an inch and a half long, and has three "lamps," or luminous organs,
one on each side of the body and one below the tail. The light given
off is extremely beautiful, and the live insects are used by the women
for ornament and by the country-folk as lamps on nocturnal excursions.
Erroneously the term "fire-fly" is applied to these beetles; it should
be reserved for the little Italian Luciola, which swarms, as countless
thousands of dancing lights, in the nights of early summer over the
marsh lands of North Italy. I have seen it at the end of June as far
north as Bonn, on the Rhine. In Australia a small true "fly"[7]--that
is to say, a two-winged fly or Dipteron like our gnats, midges, and
house-flies--is known, the maggot of which is luminous. And in New
Zealand there is another of which both the maggot and the perfect
insect are luminous. The grub is called the New Zealand glow-worm.[8]
[7] Known to entomologists as Ceroplatus mastersi.
[8] Boletophila luminosa of entomologists.
There are grounds for believing that the luminescence of some of these
insects serves them not to attract one another, but to scare would-be
predatory foes, such as birds, bats, and reptiles. I have heard a story
(which I should like to have confirmed) that in some part of tropical
Asia a certain kind of bird collects half a dozen or so of a species
of glow-worm and places them at the entrance to its nest, so as to
scare nocturnal animals which might attack its eggs or its young. It
is a noteworthy fact that a point of light in the dark may act in two
opposite ways on animals which see it--either it attracts or it repels
them. The physiologist calls this positive and negative "photo-taxis"
(light-guidance). And we have the similarly positive and negative
influence of chemical taste and smell, called "chemo-taxis," and a
similarly contrasted positive and negative "hygrotaxis," or directive
influence of moisture upon the movements of animals and plants.