fertility that we meet with in nature; it is, perhaps, amongst the
simpler flowerless plants that we find the climax. It has been
calculated that a single frond of the beautiful fern so common in our
woods, _Aspidium filix mas_, produces about fourteen million spores.
They serve to distribute the species, and are carried as motes by the
wind, but comparatively few of the millions ever get the length of
germinating at all, much less of attaining to full development into
adult plants. Thus we see that the apparent prodigality of nature is a
real necessity, an indispensable condition of the maintenance of the
species; the fertility of each species is related to the actualities of
elimination to which it is exposed. This is clearly seen when a species
is placed under new and more favourable conditions of life, in which
it has an abundant food-supply and few enemies. This was the case, for
instance, with the horses introduced from Europe into South America,
where they reverted to a feral state, and are now represented by
herds of many thousands roaming the great grassy plains. If the small
singing-birds of a region diminish in number, there is a great increase
of caterpillars and other injurious insects which form part of their
food-supply. The colossal destruction which the much-dreaded nun-moth
from time to time brings about in our woods probably depends in part
on the diminution of one or another of the many animals inimical to
insects; but the occurrence of several years of weather-conditions
favourable to the larvæ must also be taken into account. How
enormously, indeed almost inconceivably, the number of larvæ may
increase under favourable conditions is shown by such devastations
as that in Prussia in 1856, when many square miles of forest were
absolutely eaten up. The caterpillars were so numerous that even from
some distance the falling excrement could be heard rustling like rain,
and ten hundredweights of the eggs were collected, with an average of
20,000 eggs to the half-ounce!
But it would be a great mistake to conclude, from this enormous and
sudden increase in the number of individuals, that the normal number
of individuals is determined by the number of enemies alone. The
average number of individuals in a species depends on many other
conditions, especially on the extent of the available area, and on
the amount of the food-supply in relation to the size of body in the
species. I cannot dwell on this now, but I wish to point out that,
for the continuance of a species, it is indifferent whether it is
'frequent' or 'rare,' if we presuppose that its normal number remains
on an average constant for centuries, that is, that its fertility
suffices to make good the continual losses through enemies and other
causes of elimination. One would be inclined to conclude from such
cases of sudden and enormous increase in the number of individuals as
these caterpillar-blights, that enemies and other causes of destruction
played the major part in the regulation of the normal number of the
species. But this is only apparently the case. Enemies necessitate
a certain fertility in the species on which they prey, so that the
elimination in each generation may be made good; but the number of
pairs capable of reproduction is not thereby decisively determined.
We must not forget that the number of enemies is also, on the other
hand, dependent on the number of victims, and that the normal number of
enemies must rise and fall with that of the species preyed upon.
For this reason, such an enormous increase as that of the caterpillars
cannot last long; it carries its corrective in itself. The appearance
of the caterpillars in such enormous numbers in itself increases the
host of their enemies; singing-birds, ichneumon-flies, beetle-grubs,
and predaceous beetles find abundant and available food, and therefore
reproduce and multiply so rapidly, that, with the help of the
caterpillar's plant-enemies, especially the insect-destroying fungi,
they soon reduce the caterpillars to their normal number, or even
below it. But then the reverse process begins; the enemies of the
caterpillars diminish because their food has become scarce, and their
normal number is lowered, while that of the caterpillars gradually
rises again.
When the number of foxes in a hunting district increases, the number of
the hares that they prey upon diminishes, and, on the other hand, the
decimating of the foxes by Man brings about an increase in the number
of hares in the district. Under natural conditions, that is, without
the intervention of Man, there would be a constant balancing of the
numbers of hares and foxes, for every noteworthy increase of the hares
would be followed by a similar increase of foxes, and this, in its
turn, would diminish the number of hares, so that they would no longer
suffice for the support of so many foxes, and these would decrease in
number again, until the number of hares had again increased because
of the lessened persecution and elimination. In nature the case is not
quite so simple, because the fox does not live on hares alone, and the
hare is not preyed upon only by the fox; but the illustration may serve
to elucidate the point that a moving equilibrium is maintained between
the species of a district, between persecutors and persecuted, in such
a way that the number of individuals in the two species is always
varying a little up and down, and that each influences the other so
that a regulative process results. Throughout periods of considerable
length the average remains the same; that is to say, a _normal number_
is established. This normal strength of population is the mean above
and below which the number of individuals is constantly varying. It is,
of course, seldom that the mutual influences and regulations are so
simple as in the example given; usually several or even many species
interact upon each other, and not beasts of prey and their victims
alone, but the most diverse species of animals and plants, which do
not stand in any obvious relation to one another at all. Moreover, the
physical, and especially the climatic conditions, also cause the normal
number of the species to rise and fall.
The inter-relations between species living together on the same area
are so intricate that I should like to give two other illustrations.
Let us first take Darwin's famous instance of the fertility of clover,
which depends on the number of cats. It is of course only an imaginary
one, but the facts it is based upon are quite correct. The number of
cats living in a village to a certain extent determines the number of
field-mice in the neighbourhood. These again destroy the nests of the
humble-bees, which live in holes in the ground, and thus the number
of humble-bees depends on that of the field-mice and cats. But the
clover must be pollinated by insects if it is to produce fertile seed,
and only the humble-bee has a proboscis long enough to effect the
pollination. Therefore the quantity of clover-seed annually produced
depends on the number of humble-bees, and ultimately upon the number of
cats. And, as a matter of fact, humble-bees were introduced into New
Zealand from England, because without them the clover would produce no
fertile seeds.
On the grassy plains of Paraguay there are no wild cattle and horses,
because of the presence of a fly which has a predilection for laying
its eggs in the navel of the newly-born calves and foals, with the
result that the calves or foals are killed by the emerging maggots. We
may reasonably assume that the numerical strength of this fly-species
depends on the distribution of insect-eating birds, whose numbers
in turn are determined by certain beasts of prey. These again vary
in number in relation to the extent of the forest-land, and this is
determined by the number of ruminants which browse on the young growth
of the woods (Darwin).
That forests can actually be totally destroyed by ruminants is proved
by the case of the island of St. Helena among others. On its discovery
the island was covered with thick wood, but in the course of 200 years
it was transformed into a bare rock by goats and pigs, which devoured
the young growth so completely that trees which were felled or which
died were not replaced.
This point is vividly illustrated by Darwin's observation of a
wide heath on which stood only a few groups of old pine-trees. The
mere fencing in of a portion of the heath sufficed to call forth a
thick growth of young seedling pines within the enclosure, and an
examination of the open part of the heath revealed that the grazing
cattle had eaten up all the young pine-trees which sprang from seed,
and that again and again. In one small space thirty-two little trees
stood concealed in the grass, and several of these showed as many as
twenty-six yearly rings.
How definitely the number of individuals in different species living on
the same area mutually limit and thereby regulate each other, Darwin
sought to illustrate also by the case of the primitive forest, where
the numerous species of plants occur, not mixed together irregularly,
but in a definite proportion. We can find examples of the same kind
wherever the plant-growth of a district has been left to itself. If we
walk along the banks of our little river, the Dreisam, we see a wild
confusion of the most diverse trees, shrubs and herbaceous plants. But,
even though it cannot be demonstrated, we may be certain that these are
represented in definite numerical proportions, dependent on the natural
qualities and requirements of each species, on the number of their
seeds and the facilities for their distribution, on the favourable
or unfavourable season at which they ripen, and on their varying
capacity for taking root in the worst ground, and springing quickly
up, &c. They limit each other mutually, so that the whole flora of the
river-bank will be made up of one per cent. of this species, one per
cent. of that, and, it may be, five per cent. of a third, and the same
combination will repeat itself in the same proportions on the banks of
other rivers of our country in as far as the external conditions are
the same. The same must be true of the fauna of such a plant-thicket;
the animal species also limit one another mutually, and thereby
regulate the number of individuals, which becomes relatively stable
over any area on which the conditions remain the same. That is to say,
a 'normal number' is attained and persists.
Thus we see that the capacity for boundless multiplication inherent in
every species is limited by the co-existence of other species; there
is, metaphorically speaking, a continuous struggle going on between
species, plant and animal alike; each seeks as far as possible to
multiply, and each is hemmed in by the others and as far as possible
prevented from doing so. The 'struggle' is by no means only the
_direct_ limitation of the number of individuals, which consists in
the use of one species by another as food, as in beasts of prey and
their victims, or locusts and plants; it is much more the indirect
limitation--figuratively speaking, the struggle for space, for light,
for moisture among plants, for food among animals. But all this,
important as it is, does not yet exhaust the content of that 'struggle
for existence' to which Darwin and Wallace ascribe the rôle of the
breeder in the process of natural selection. The struggle, that is,
the mutual limiting of species, may indeed restrict a species in
its distribution, and may reduce its normal number possibly to nil.
In other words, it may bring about extinction, but it cannot make
a species other than it is. This can only be done by a struggle
within the limits of the species itself, and this struggle is due
to the fact that of the numerous offspring, on an average those
survive--that is, attain to reproduction--which are the most fit,
whose constitution makes it most possible for them to overcome the
difficulties and dangers of life, and so to reach maturity. We see,
in fact, that a large percentage of each generation in all species
always perishes before attaining maturity. If, then, the decision as to
which is to perish and which is to reach maturity is _not a matter of
chance alone_, but is in part due to the constitution of the growing
individual; if the 'fittest' do _on the average_ survive, and the
'least fit' are on the average eliminated, we have here a process of
selection entirely comparable to that of artificial selection, and
one whose result must be the 'improvement' of the species, whether
that depends on one set of characters or on another. The victorious
qualities, which earlier were peculiar to certain individuals, must
gradually become the common property of the species, if in each
generation the individuals which attained to reproduction all possessed
them, and thus could transmit them to their progeny. But those of the
descendants which did not inherit them would again be at a disadvantage
in the struggle for existence, or rather for reaching maturity, if in
each generation a higher percentage of individuals which possess these
characters reach maturity than of those which do not possess them. This
percentage must increase in each generation, because, in each, natural
selection again chooses out the fittest, and it must finally rise to
100 per cent., that is to say, none but individuals of this fittest
type will be left surviving.
This does not yet exhaust the process, however, for we can infer
from the results of artificial breed-forming that the selected
characters may intensify from generation to generation, and that
they will continue to do so as long as it gives them any advantage
in the struggle for existence, for so long will it lead to the more
frequent survival of its possessors. The increase will only stop when
it has reached the highest degree of usefulness, and in this way new
characters may be formed, just as, in artificial selection, the short
upward-turning feathers of the Jacobin pigeon have been intensified
into the peruke, a feather canopy covering the head.
A few examples of natural selection will make the process clearer. Our
hare is well secured from discovery by his fur of mixed brown, yellow,
white, and black, when he cowers in his form among the dry leaves of
the underwood. It is easy to pass close to him without seeing him.
But if the ground and the bushes are covered with snow, he contrasts
conspicuously with them. Suppose, now, that our climate became colder,
and that the winter brought lasting snow, the hares which had the
largest mixture of white in their fur would have an advantage in their
'struggle for existence' over their darker fellows; they would be less
easily discovered by their enemies--the fox, the badger, the horned
owl, and the wild cat. Of the numerous hares which would annually
become the prey of these enemies, there would be, on an average, more
dark than light individuals. The percentage of light-coloured hares
would, therefore, increase from generation to generation, and the
longer the winter the keener would be the selection between dark and
light hares, until finally none but light ones would remain. At the
same time, the colour of the hares would become increasingly light,
first, because it would happen more and more frequently that two light
hares would pair, and secondly, because, after a time, the struggle for
existence would no longer be between light and dark hares, but between
light hares and still lighter ones. Thus ultimately a race of white
hares would arise, as has actually happened in the Arctic regions and
on the Alps.
Or let us think of a herbaceous plant, in appearance something like a
belladonna, rich in leaves and very juicy, but not poisonous. It would
doubtless be a favourite food with the animals of the forest, and it
would not, therefore, attain to more than a sparse occurrence, since
few of the individuals would be able to form seeds. But now let us
assume that a stuff of very unpleasant taste develops in the stem and
leaves of some of the individuals, as may easily happen through very
slight changes in the chemical metabolism of the plant, what, then,
could result but that such individuals would be less readily eaten than
the others? A process of selection must, therefore, ensue, and the
unpleasant-tasting specimens of the plant would be much more frequently
spared, and consequently would bear seed much oftener than the
palatable ones. Thus the number of unpalatable plants would increase
from year to year. If the stuff in question were not only unpalatable
but poisonous, or gradually became so, a plant would in time be evolved
which would be absolutely safe from being devoured by animals, just as
the deadly nightshade (_Atropa belladonna_) actually is.
Or let us suppose that a stretch of water is inhabited by a species of
carp, which have hitherto had no large enemy, and so have become lazy
and slow, and that there migrates from the sea into this stretch of
water a large species of pike. At first numerous carp will fall victims
to the pike, and the pike will rapidly increase in number. But if all
the carp were not equally lazy and dull-witted, if some of them were
quicker and more intelligent, these would, on an average, become more
rarely the victims of the pike, and numerous individuals with these
better qualities would survive in each generation, till ultimately
there were no others, and the useful characters would gradually become
intensified, and so a more active and wary race of carp would arise.
Let us suppose, however, that the increased activity and wariness would
not alone suffice to preserve the colony from extinction; it might
require also an increased fertility to prevent the normal number from
being permanently lowered; but even this could eventually be brought
about by natural selection, if the nature of the species and the
general conditions of its life permitted. For there are variations of
fertility in every species, and if the chance of seeing some of its
eggs become mature animals were greater for the more fertile female
than for the less fertile, _ceteris paribus_, a process of selection
must take place, which would result in an increase of fertility as far
as that was possible.
Obviously, such processes of natural selection can affect all parts
and characters--size and form of the body, as well as isolated parts,
the external skin and its colour, every internal organ--and not bodily
characters alone, but psychical ones as well, such as intelligence and
instincts. According to this principle, it is only characters which
are biologically indifferent that cannot be altered through natural
selection.
Natural selection can also bring about changes at every age, for the
elimination of individuals begins from the egg, and any kind of egg
which is in some way better able to escape elimination will transmit
its useful characters to its descendants, because the resulting young
animals will thus more frequently reach full development than the
young from other eggs. In the same way, at every succeeding stage of
development, every character favourable to the preservation of the
individual will be maintained and intensified.
We see from all this that natural selection is vastly more powerful
than artificial selection by Man. In the latter, only one character at
a time can be caused to change, while natural selection may influence a
whole group of characters at the same time, as well as all the stages
of development. Through the weeding out of the individuals which are
annually exterminated, it is always on an average the 'fittest' which
survive, that is to say, those which have the greatest number of
bodily parts and rudiments of parts in the fittest possible condition
of development at every stage. The longer this process of selection
continues, the smaller will be the deviations of the individual from
this standard, and the more minute will be the differences of fitness
determining which is to be eliminated and which is to survive to
reproduce its characteristics. In the immeasurable periods of time
which are at the disposal of natural selection, and in the inestimable
numbers of individuals on which it may operate, lie the essential
causes of superiority of natural selection over the artificial
selection of Man.
To sum up briefly: Natural selection depends essentially on the
cumulative augmentation of the most minute useful variations in the
direction of their utility; only the useful is developed and increased,
and great effects are brought about slowly through the summing up of
many very minute steps. Natural selection is a self-regulation of the
species which secures its preservation; its result is the ceaseless
adaptation of the species to its life-conditions. As soon as these
vary, natural selection changes its mode of action, for what was
previously the best is now no longer so; parts that before had to be
large must now perhaps be small, or vice versa; muscle-groups which
were weak must now become strong, and so on. The conditions of life
are, so to speak, the mould into which natural selection is continually
pouring the species anew.
But the philosophical significance of natural selection lies in
the fact, that it shows us how to explain the origin of useful,
well-adapted structures purely by mechanical forces and without
having to fall back on a _directive_ force. We are thus for the first
time in a position to understand, in some degree, the marvellous
adaptation of the organism to an end, without having to call to our
aid any supernaturally intrusive force on the part of the Creator. We
understand now how, in a purely mechanical way, through the forces
always at work in nature, all forms of life must conform to, and adapt
themselves precisely to the conditions of their life, since only the
best possible is preserved, and everything less good is continually
being rejected.
Before I go on to expound in detail the phenomena which we refer to
natural selection, I must briefly state that Darwin did not ascribe to
natural selection by any means all the changes which have taken place
in organisms in the course of time. On the one hand, he ascribed a not
inconsiderable importance to the correlated variations we have already
mentioned; still more, however, he relied on the direct influence of
altered conditions of life, whether these consist in climatic and other
changes in the environment, or in the assumption of new habits, and
the increased or diminished use of individual parts and organs thereby
induced. He recognized the principle so strongly emphasized by Lamarck,
of use and disuse as a cause of heritable increase or decrease of the
exercised or neglected part, though he did so with a certain reserve.
I shall return later to these factors of modification, and shall then
attempt to show that these too are to be referred to processes of
selection, which are, however, of a different order from the phenomena
which the Darwin-Wallace principle of natural selection serves to
interpret. But, in the first instance, it appears to me to be necessary
to show how far the Darwin-Wallace interpretation will suffice, and
in the next lectures we shall occupy ourselves with this question
exclusively.
LECTURE IV
THE COLORATION OF ANIMALS AND ITS RELATION TO THE PROCESSES OF SELECTION
Biological significance of colours--Protective colours of
eggs--Animals of the snow-region--Animals of the desert--Transparent
animals--Green animals--Nocturnal animals--Double
colour-adaptation--Protective marking of caterpillars--Warning
markings--Dimorphism of colouring in caterpillars--Shunting back
of colouring in ontogeny--'Sympathetic' colouring in diurnal
Lepidoptera--In nocturnal Lepidoptera--Theoretical considerations--The
influence of illumination in the production of protective colouring,
_Tropidoderus_--Harmony of protective colouring in minute
details--_Notodonta_--Objections--Imitation of Strange objects,
_Xylina_--Leaf-butterflies, _Kallima_--_Hebomoja_--Nocturnal
Lepidoptera with leaf-markings--Orthoptera resembling
leaves--Caterpillars of the Geometridæ.
WE have seen what Darwin meant by natural selection, and we understand
that this process really implies a transformation of organisms by
slow degrees, in the direction of adaptive fitness--a transformation
which must ensue as necessarily as when a human selector, prompted
by conscious intention, tries to improve an animal in a particular
direction, by always selecting the 'fittest' animals for breeding.
In nature, too, there is selection, because in every generation
the majority succumb in the struggle for life, while on an average
those which survive, attain to reproductive maturity, and transmit
their characters to their descendants, are those which are best
adapted to the conditions of their life--that is, which possess those
variations of most advantage in overcoming the dangers of life.
Since individuals are always variable in some degree, since their
variations can be inherited by their progeny, and since the continually
repeated elimination of the majority of those descendants is a fact,
the inference from these premisses must be correct; there must be a
'natural selection' in the direction of a gradually increasing fitness
and effectiveness of the forms of life.