On the Origin of Species
by
Charles Darwin
Part 2 out of 9
scarcely more than a truism, for if a variety were found to have a
wider range than that of its supposed parent-species, their
denominations ought to be reversed. But there is also reason to
believe, that those species which are very closely allied to other
species, and in so far resemble varieties, often have much restricted
ranges. For instance, Mr. H. C. Watson has marked for me in the
well-sifted London Catalogue of plants (4th edition) 63 plants which
are therein ranked as species, but which he considers as so closely
allied to other species as to be of doubtful value: these 63 reputed
species range on an average over 6.9 of the provinces into which Mr.
Watson has divided Great Britain. Now, in this same catalogue, 53
acknowledged varieties are recorded, and these range over 7.7
provinces; whereas, the species to which these varieties belong range
over 14.3 provinces. So that the acknowledged varieties have very
nearly the same restricted average range, as have those very closely
allied forms, marked for me by Mr. Watson as doubtful species, but
which are almost universally ranked by British botanists as good and
true species.
Finally, then, varieties have the same general characters as species,
for they cannot be distinguished from species,--except, firstly, by
the discovery of intermediate linking forms, and the occurrence of
such links cannot affect the actual characters of the forms which they
connect; and except, secondly, by a certain amount of difference, for
two forms, if differing very little, are generally ranked as
varieties, notwithstanding that intermediate linking forms have not
been discovered; but the amount of difference considered necessary to
give to two forms the rank of species is quite indefinite. In genera
having more than the average number of species in any country, the
species of these genera have more than the average number of
varieties. In large genera the species are apt to be closely, but
unequally, allied together, forming little clusters round certain
species. Species very closely allied to other species apparently have
restricted ranges. In all these several respects the species of large
genera present a strong analogy with varieties. And we can clearly
understand these analogies, if species have once existed as varieties,
and have thus originated: whereas, these analogies are utterly
inexplicable if each species has been independently created.
We have, also, seen that it is the most flourishing and dominant
species of the larger genera which on an average vary most; and
varieties, as we shall hereafter see, tend to become converted into
new and distinct species. The larger genera thus tend to become
larger; and throughout nature the forms of life which are now dominant
tend to become still more dominant by leaving many modified and
dominant descendants. But by steps hereafter to be explained, the
larger genera also tend to break up into smaller genera. And thus, the
forms of life throughout the universe become divided into groups
subordinate to groups.
CHAPTER 3. STRUGGLE FOR EXISTENCE.
Bears on natural selection.
The term used in a wide sense.
Geometrical powers of increase.
Rapid increase of naturalised animals and plants.
Nature of the checks to increase.
Competition universal.
Effects of climate.
Protection from the number of individuals.
Complex relations of all animals and plants throughout nature.
Struggle for life most severe between individuals and varieties of the
same species; often severe between species of the same genus.
The relation of organism to organism the most important of all
relations.
Before entering on the subject of this chapter, I must make a few
preliminary remarks, to show how the struggle for existence bears on
Natural Selection. It has been seen in the last chapter that amongst
organic beings in a state of nature there is some individual
variability; indeed I am not aware that this has ever been disputed.
It is immaterial for us whether a multitude of doubtful forms be
called species or sub-species or varieties; what rank, for instance,
the two or three hundred doubtful forms of British plants are entitled
to hold, if the existence of any well-marked varieties be admitted.
But the mere existence of individual variability and of some few
well-marked varieties, though necessary as the foundation for the
work, helps us but little in understanding how species arise in
nature. How have all those exquisite adaptations of one part of the
organisation to another part, and to the conditions of life, and of
one distinct organic being to another being, been perfected? We see
these beautiful co-adaptations most plainly in the woodpecker and
missletoe; and only a little less plainly in the humblest parasite
which clings to the hairs of a quadruped or feathers of a bird; in the
structure of the beetle which dives through the water; in the plumed
seed which is wafted by the gentlest breeze; in short, we see
beautiful adaptations everywhere and in every part of the organic
world.
Again, it may be asked, how is it that varieties, which I have called
incipient species, become ultimately converted into good and distinct
species, which in most cases obviously differ from each other far more
than do the varieties of the same species? How do those groups of
species, which constitute what are called distinct genera, and which
differ from each other more than do the species of the same genus,
arise? All these results, as we shall more fully see in the next
chapter, follow inevitably from the struggle for life. Owing to this
struggle for life, any variation, however slight and from whatever
cause proceeding, if it be in any degree profitable to an individual
of any species, in its infinitely complex relations to other organic
beings and to external nature, will tend to the preservation of that
individual, and will generally be inherited by its offspring. The
offspring, also, will thus have a better chance of surviving, for, of
the many individuals of any species which are periodically born, but a
small number can survive. I have called this principle, by which each
slight variation, if useful, is preserved, by the term of Natural
Selection, in order to mark its relation to man's power of selection.
We have seen that man by selection can certainly produce great
results, and can adapt organic beings to his own uses, through the
accumulation of slight but useful variations, given to him by the hand
of Nature. But Natural Selection, as we shall hereafter see, is a
power incessantly ready for action, and is as immeasurably superior to
man's feeble efforts, as the works of Nature are to those of Art.
We will now discuss in a little more detail the struggle for
existence. In my future work this subject shall be treated, as it well
deserves, at much greater length. The elder De Candolle and Lyell have
largely and philosophically shown that all organic beings are exposed
to severe competition. In regard to plants, no one has treated this
subject with more spirit and ability than W. Herbert, Dean of
Manchester, evidently the result of his great horticultural knowledge.
Nothing is easier than to admit in words the truth of the universal
struggle for life, or more difficult--at least I have found it
so--than constantly to bear this conclusion in mind. Yet unless it be
thoroughly engrained in the mind, I am convinced that the whole
economy of nature, with every fact on distribution, rarity, abundance,
extinction, and variation, will be dimly seen or quite misunderstood.
We behold the face of nature bright with gladness, we often see
superabundance of food; we do not see, or we forget, that the birds
which are idly singing round us mostly live on insects or seeds, and
are thus constantly destroying life; or we forget how largely these
songsters, or their eggs, or their nestlings, are destroyed by birds
and beasts of prey; we do not always bear in mind, that though food
may be now superabundant, it is not so at all seasons of each
recurring year.
I should premise that I use the term Struggle for Existence in a large
and metaphorical sense, including dependence of one being on another,
and including (which is more important) not only the life of the
individual, but success in leaving progeny. Two canine animals in a
time of dearth, may be truly said to struggle with each other which
shall get food and live. But a plant on the edge of a desert is said
to struggle for life against the drought, though more properly it
should be said to be dependent on the moisture. A plant which annually
produces a thousand seeds, of which on an average only one comes to
maturity, may be more truly said to struggle with the plants of the
same and other kinds which already clothe the ground. The missletoe is
dependent on the apple and a few other trees, but can only in a
far-fetched sense be said to struggle with these trees, for if too
many of these parasites grow on the same tree, it will languish and
die. But several seedling missletoes, growing close together on the
same branch, may more truly be said to struggle with each other. As
the missletoe is disseminated by birds, its existence depends on
birds; and it may metaphorically be said to struggle with other
fruit-bearing plants, in order to tempt birds to devour and thus
disseminate its seeds rather than those of other plants. In these
several senses, which pass into each other, I use for convenience sake
the general term of struggle for existence.
A struggle for existence inevitably follows from the high rate at
which all organic beings tend to increase. Every being, which during
its natural lifetime produces several eggs or seeds, must suffer
destruction during some period of its life, and during some season or
occasional year, otherwise, on the principle of geometrical increase,
its numbers would quickly become so inordinately great that no country
could support the product. Hence, as more individuals are produced
than can possibly survive, there must in every case be a struggle for
existence, either one individual with another of the same species, or
with the individuals of distinct species, or with the physical
conditions of life. It is the doctrine of Malthus applied with
manifold force to the whole animal and vegetable kingdoms; for in this
case there can be no artificial increase of food, and no prudential
restraint from marriage. Although some species may be now increasing,
more or less rapidly, in numbers, all cannot do so, for the world
would not hold them.
There is no exception to the rule that every organic being naturally
increases at so high a rate, that if not destroyed, the earth would
soon be covered by the progeny of a single pair. Even slow-breeding
man has doubled in twenty-five years, and at this rate, in a few
thousand years, there would literally not be standing room for his
progeny. Linnaeus has calculated that if an annual plant produced only
two seeds--and there is no plant so unproductive as this--and their
seedlings next year produced two, and so on, then in twenty years
there would be a million plants. The elephant is reckoned to be the
slowest breeder of all known animals, and I have taken some pains to
estimate its probable minimum rate of natural increase: it will be
under the mark to assume that it breeds when thirty years old, and
goes on breeding till ninety years old, bringing forth three pair of
young in this interval; if this be so, at the end of the fifth century
there would be alive fifteen million elephants, descended from the
first pair.
But we have better evidence on this subject than mere theoretical
calculations, namely, the numerous recorded cases of the astonishingly
rapid increase of various animals in a state of nature, when
circumstances have been favourable to them during two or three
following seasons. Still more striking is the evidence from our
domestic animals of many kinds which have run wild in several parts of
the world: if the statements of the rate of increase of slow-breeding
cattle and horses in South America, and latterly in Australia, had not
been well authenticated, they would have been quite incredible. So it
is with plants: cases could be given of introduced plants which have
become common throughout whole islands in a period of less than ten
years. Several of the plants now most numerous over the wide plains of
La Plata, clothing square leagues of surface almost to the exclusion
of all other plants, have been introduced from Europe; and there are
plants which now range in India, as I hear from Dr. Falconer, from
Cape Comorin to the Himalaya, which have been imported from America
since its discovery. In such cases, and endless instances could be
given, no one supposes that the fertility of these animals or plants
has been suddenly and temporarily increased in any sensible degree.
The obvious explanation is that the conditions of life have been very
favourable, and that there has consequently been less destruction of
the old and young, and that nearly all the young have been enabled to
breed. In such cases the geometrical ratio of increase, the result of
which never fails to be surprising, simply explains the
extraordinarily rapid increase and wide diffusion of naturalised
productions in their new homes.
In a state of nature almost every plant produces seed, and amongst
animals there are very few which do not annually pair. Hence we may
confidently assert, that all plants and animals are tending to
increase at a geometrical ratio, that all would most rapidly stock
every station in which they could any how exist, and that the
geometrical tendency to increase must be checked by destruction at
some period of life. Our familiarity with the larger domestic animals
tends, I think, to mislead us: we see no great destruction falling on
them, and we forget that thousands are annually slaughtered for food,
and that in a state of nature an equal number would have somehow to be
disposed of.
The only difference between organisms which annually produce eggs or
seeds by the thousand, and those which produce extremely few, is, that
the slow-breeders would require a few more years to people, under
favourable conditions, a whole district, let it be ever so large. The
condor lays a couple of eggs and the ostrich a score, and yet in the
same country the condor may be the more numerous of the two: the
Fulmar petrel lays but one egg, yet it is believed to be the most
numerous bird in the world. One fly deposits hundreds of eggs, and
another, like the hippobosca, a single one; but this difference does
not determine how many individuals of the two species can be supported
in a district. A large number of eggs is of some importance to those
species, which depend on a rapidly fluctuating amount of food, for it
allows them rapidly to increase in number. But the real importance of
a large number of eggs or seeds is to make up for much destruction at
some period of life; and this period in the great majority of cases is
an early one. If an animal can in any way protect its own eggs or
young, a small number may be produced, and yet the average stock be
fully kept up; but if many eggs or young are destroyed, many must be
produced, or the species will become extinct. It would suffice to keep
up the full number of a tree, which lived on an average for a thousand
years, if a single seed were produced once in a thousand years,
supposing that this seed were never destroyed, and could be ensured to
germinate in a fitting place. So that in all cases, the average number
of any animal or plant depends only indirectly on the number of its
eggs or seeds.
In looking at Nature, it is most necessary to keep the foregoing
considerations always in mind--never to forget that every single
organic being around us may be said to be striving to the utmost to
increase in numbers; that each lives by a struggle at some period of
its life; that heavy destruction inevitably falls either on the young
or old, during each generation or at recurrent intervals. Lighten any
check, mitigate the destruction ever so little, and the number of the
species will almost instantaneously increase to any amount. The face
of Nature may be compared to a yielding surface, with ten thousand
sharp wedges packed close together and driven inwards by incessant
blows, sometimes one wedge being struck, and then another with greater
force.
What checks the natural tendency of each species to increase in number
is most obscure. Look at the most vigorous species; by as much as it
swarms in numbers, by so much will its tendency to increase be still
further increased. We know not exactly what the checks are in even one
single instance. Nor will this surprise any one who reflects how
ignorant we are on this head, even in regard to mankind, so
incomparably better known than any other animal. This subject has been
ably treated by several authors, and I shall, in my future work,
discuss some of the checks at considerable length, more especially in
regard to the feral animals of South America. Here I will make only a
few remarks, just to recall to the reader's mind some of the chief
points. Eggs or very young animals seem generally to suffer most, but
this is not invariably the case. With plants there is a vast
destruction of seeds, but, from some observations which I have made, I
believe that it is the seedlings which suffer most from germinating in
ground already thickly stocked with other plants. Seedlings, also, are
destroyed in vast numbers by various enemies; for instance, on a piece
of ground three feet long and two wide, dug and cleared, and where
there could be no choking from other plants, I marked all the
seedlings of our native weeds as they came up, and out of the 357 no
less than 295 were destroyed, chiefly by slugs and insects. If turf
which has long been mown, and the case would be the same with turf
closely browsed by quadrupeds, be let to grow, the more vigorous
plants gradually kill the less vigorous, though fully grown, plants:
thus out of twenty species growing on a little plot of turf (three
feet by four) nine species perished from the other species being
allowed to grow up freely.
The amount of food for each species of course gives the extreme limit
to which each can increase; but very frequently it is not the
obtaining food, but the serving as prey to other animals, which
determines the average numbers of a species. Thus, there seems to be
little doubt that the stock of partridges, grouse, and hares on any
large estate depends chiefly on the destruction of vermin. If not one
head of game were shot during the next twenty years in England, and,
at the same time, if no vermin were destroyed, there would, in all
probability, be less game than at present, although hundreds of
thousands of game animals are now annually killed. On the other hand,
in some cases, as with the elephant and rhinoceros, none are destroyed
by beasts of prey: even the tiger in India most rarely dares to attack
a young elephant protected by its dam.
Climate plays an important part in determining the average numbers of
a species, and periodical seasons of extreme cold or drought, I
believe to be the most effective of all checks. I estimated that the
winter of 1854-55 destroyed four-fifths of the birds in my own
grounds; and this is a tremendous destruction, when we remember that
ten per cent. is an extraordinarily severe mortality from epidemics
with man. The action of climate seems at first sight to be quite
independent of the struggle for existence; but in so far as climate
chiefly acts in reducing food, it brings on the most severe struggle
between the individuals, whether of the same or of distinct species,
which subsist on the same kind of food. Even when climate, for
instance extreme cold, acts directly, it will be the least vigorous,
or those which have got least food through the advancing winter, which
will suffer most. When we travel from south to north, or from a damp
region to a dry, we invariably see some species gradually getting
rarer and rarer, and finally disappearing; and the change of climate
being conspicuous, we are tempted to attribute the whole effect to its
direct action. But this is a very false view: we forget that each
species, even where it most abounds, is constantly suffering enormous
destruction at some period of its life, from enemies or from
competitors for the same place and food; and if these enemies or
competitors be in the least degree favoured by any slight change of
climate, they will increase in numbers, and, as each area is already
fully stocked with inhabitants, the other species will decrease. When
we travel southward and see a species decreasing in numbers, we may
feel sure that the cause lies quite as much in other species being
favoured, as in this one being hurt. So it is when we travel
northward, but in a somewhat lesser degree, for the number of species
of all kinds, and therefore of competitors, decreases northwards;
hence in going northward, or in ascending a mountain, we far oftener
meet with stunted forms, due to the DIRECTLY injurious action of
climate, than we do in proceeding southwards or in descending a
mountain. When we reach the Arctic regions, or snow-capped summits, or
absolute deserts, the struggle for life is almost exclusively with the
elements.
That climate acts in main part indirectly by favouring other species,
we may clearly see in the prodigious number of plants in our gardens
which can perfectly well endure our climate, but which never become
naturalised, for they cannot compete with our native plants, nor
resist destruction by our native animals.
When a species, owing to highly favourable circumstances, increases
inordinately in numbers in a small tract, epidemics--at least, this
seems generally to occur with our game animals--often ensue: and here
we have a limiting check independent of the struggle for life. But
even some of these so-called epidemics appear to be due to parasitic
worms, which have from some cause, possibly in part through facility
of diffusion amongst the crowded animals, been disproportionably
favoured: and here comes in a sort of struggle between the parasite
and its prey.
On the other hand, in many cases, a large stock of individuals of the
same species, relatively to the numbers of its enemies, is absolutely
necessary for its preservation. Thus we can easily raise plenty of
corn and rape-seed, etc., in our fields, because the seeds are in
great excess compared with the number of birds which feed on them; nor
can the birds, though having a superabundance of food at this one
season, increase in number proportionally to the supply of seed, as
their numbers are checked during winter: but any one who has tried,
knows how troublesome it is to get seed from a few wheat or other such
plants in a garden; I have in this case lost every single seed. This
view of the necessity of a large stock of the same species for its
preservation, explains, I believe, some singular facts in nature, such
as that of very rare plants being sometimes extremely abundant in the
few spots where they do occur; and that of some social plants being
social, that is, abounding in individuals, even on the extreme
confines of their range. For in such cases, we may believe, that a
plant could exist only where the conditions of its life were so
favourable that many could exist together, and thus save each other
from utter destruction. I should add that the good effects of frequent
intercrossing, and the ill effects of close interbreeding, probably
come into play in some of these cases; but on this intricate subject I
will not here enlarge.
Many cases are on record showing how complex and unexpected are the
checks and relations between organic beings, which have to struggle
together in the same country. I will give only a single instance,
which, though a simple one, has interested me. In Staffordshire, on
the estate of a relation where I had ample means of investigation,
there was a large and extremely barren heath, which had never been
touched by the hand of man; but several hundred acres of exactly the
same nature had been enclosed twenty-five years previously and planted
with Scotch fir. The change in the native vegetation of the planted
part of the heath was most remarkable, more than is generally seen in
passing from one quite different soil to another: not only the
proportional numbers of the heath-plants were wholly changed, but
twelve species of plants (not counting grasses and carices) flourished
in the plantations, which could not be found on the heath. The effect
on the insects must have been still greater, for six insectivorous
birds were very common in the plantations, which were not to be seen
on the heath; and the heath was frequented by two or three distinct
insectivorous birds. Here we see how potent has been the effect of the
introduction of a single tree, nothing whatever else having been done,
with the exception that the land had been enclosed, so that cattle
could not enter. But how important an element enclosure is, I plainly
saw near Farnham, in Surrey. Here there are extensive heaths, with a
few clumps of old Scotch firs on the distant hill-tops: within the
last ten years large spaces have been enclosed, and self-sown firs are
now springing up in multitudes, so close together that all cannot
live.
When I ascertained that these young trees had not been sown or
planted, I was so much surprised at their numbers that I went to
several points of view, whence I could examine hundreds of acres of
the unenclosed heath, and literally I could not see a single Scotch
fir, except the old planted clumps. But on looking closely between the
stems of the heath, I found a multitude of seedlings and little trees,
which had been perpetually browsed down by the cattle. In one square
yard, at a point some hundred yards distant from one of the old
clumps, I counted thirty-two little trees; and one of them, judging
from the rings of growth, had during twenty-six years tried to raise
its head above the stems of the heath, and had failed. No wonder that,
as soon as the land was enclosed, it became thickly clothed with
vigorously growing young firs. Yet the heath was so extremely barren
and so extensive that no one would ever have imagined that cattle
would have so closely and effectually searched it for food.
Here we see that cattle absolutely determine the existence of the
Scotch fir; but in several parts of the world insects determine the
existence of cattle. Perhaps Paraguay offers the most curious instance
of this; for here neither cattle nor horses nor dogs have ever run
wild, though they swarm southward and northward in a feral state; and
Azara and Rengger have shown that this is caused by the greater number
in Paraguay of a certain fly, which lays its eggs in the navels of
these animals when first born. The increase of these flies, numerous
as they are, must be habitually checked by some means, probably by
birds. Hence, if certain insectivorous birds (whose numbers are
probably regulated by hawks or beasts of prey) were to increase in
Paraguay, the flies would decrease--then cattle and horses would
become feral, and this would certainly greatly alter (as indeed I have
observed in parts of South America) the vegetation: this again would
largely affect the insects; and this, as we just have seen in
Staffordshire, the insectivorous birds, and so onwards in
ever-increasing circles of complexity. We began this series by
insectivorous birds, and we have ended with them. Not that in nature
the relations can ever be as simple as this. Battle within battle must
ever be recurring with varying success; and yet in the long-run the
forces are so nicely balanced, that the face of nature remains uniform
for long periods of time, though assuredly the merest trifle would
often give the victory to one organic being over another. Nevertheless
so profound is our ignorance, and so high our presumption, that we
marvel when we hear of the extinction of an organic being; and as we
do not see the cause, we invoke cataclysms to desolate the world, or
invent laws on the duration of the forms of life!
I am tempted to give one more instance showing how plants and animals,
most remote in the scale of nature, are bound together by a web of
complex relations. I shall hereafter have occasion to show that the
exotic Lobelia fulgens, in this part of England, is never visited by
insects, and consequently, from its peculiar structure, never can set
a seed. Many of our orchidaceous plants absolutely require the visits
of moths to remove their pollen-masses and thus to fertilise them. I
have, also, reason to believe that humble-bees are indispensable to
the fertilisation of the heartsease (Viola tricolor), for other bees
do not visit this flower. From experiments which I have tried, I have
found that the visits of bees, if not indispensable, are at least
highly beneficial to the fertilisation of our clovers; but humble-bees
alone visit the common red clover (Trifolium pratense), as other bees
cannot reach the nectar. Hence I have very little doubt, that if the
whole genus of humble-bees became extinct or very rare in England, the
heartsease and red clover would become very rare, or wholly disappear.
The number of humble-bees in any district depends in a great degree on
the number of field-mice, which destroy their combs and nests; and Mr.
H. Newman, who has long attended to the habits of humble-bees,
believes that "more than two thirds of them are thus destroyed all
over England." Now the number of mice is largely dependent, as every
one knows, on the number of cats; and Mr. Newman says, "Near villages
and small towns I have found the nests of humble-bees more numerous
than elsewhere, which I attribute to the number of cats that destroy
the mice." Hence it is quite credible that the presence of a feline
animal in large numbers in a district might determine, through the
intervention first of mice and then of bees, the frequency of certain
flowers in that district!
In the case of every species, many different checks, acting at
different periods of life, and during different seasons or years,
probably come into play; some one check or some few being generally
the most potent, but all concurring in determining the average number
or even the existence of the species. In some cases it can be shown
that widely-different checks act on the same species in different
districts. When we look at the plants and bushes clothing an entangled
bank, we are tempted to attribute their proportional numbers and kinds
to what we call chance. But how false a view is this! Every one has
heard that when an American forest is cut down, a very different
vegetation springs up; but it has been observed that the trees now
growing on the ancient Indian mounds, in the Southern United States,
display the same beautiful diversity and proportion of kinds as in the
surrounding virgin forests. What a struggle between the several kinds
of trees must here have gone on during long centuries, each annually
scattering its seeds by the thousand; what war between insect and
insect--between insects, snails, and other animals with birds and
beasts of prey--all striving to increase, and all feeding on each
other or on the trees or their seeds and seedlings, or on the other
plants which first clothed the ground and thus checked the growth of
the trees! Throw up a handful of feathers, and all must fall to the
ground according to definite laws; but how simple is this problem
compared to the action and reaction of the innumerable plants and
animals which have determined, in the course of centuries, the
proportional numbers and kinds of trees now growing on the old Indian
ruins!
The dependency of one organic being on another, as of a parasite on
its prey, lies generally between beings remote in the scale of nature.
This is often the case with those which may strictly be said to
struggle with each other for existence, as in the case of locusts and
grass-feeding quadrupeds. But the struggle almost invariably will be
most severe between the individuals of the same species, for they
frequent the same districts, require the same food, and are exposed to
the same dangers. In the case of varieties of the same species, the
struggle will generally be almost equally severe, and we sometimes see
the contest soon decided: for instance, if several varieties of wheat
be sown together, and the mixed seed be resown, some of the varieties
which best suit the soil or climate, or are naturally the most
fertile, will beat the others and so yield more seed, and will
consequently in a few years quite supplant the other varieties. To
keep up a mixed stock of even such extremely close varieties as the
variously coloured sweet-peas, they must be each year harvested
separately, and the seed then mixed in due proportion, otherwise the
weaker kinds will steadily decrease in numbers and disappear. So again
with the varieties of sheep: it has been asserted that certain
mountain-varieties will starve out other mountain-varieties, so that
they cannot be kept together. The same result has followed from
keeping together different varieties of the medicinal leech. It may
even be doubted whether the varieties of any one of our domestic
plants or animals have so exactly the same strength, habits, and
constitution, that the original proportions of a mixed stock could be
kept up for half a dozen generations, if they were allowed to struggle
together, like beings in a state of nature, and if the seed or young
were not annually sorted.
As species of the same genus have usually, though by no means
invariably, some similarity in habits and constitution, and always in
structure, the struggle will generally be more severe between species
of the same genus, when they come into competition with each other,
than between species of distinct genera. We see this in the recent
extension over parts of the United States of one species of swallow
having caused the decrease of another species. The recent increase of
the missel-thrush in parts of Scotland has caused the decrease of the
song-thrush. How frequently we hear of one species of rat taking the
place of another species under the most different climates! In Russia
the small Asiatic cockroach has everywhere driven before it its great
congener. One species of charlock will supplant another, and so in
other cases. We can dimly see why the competition should be most
severe between allied forms, which fill nearly the same place in the
economy of nature; but probably in no one case could we precisely say
why one species has been victorious over another in the great battle
of life.
A corollary of the highest importance may be deduced from the
foregoing remarks, namely, that the structure of every organic being
is related, in the most essential yet often hidden manner, to that of
all other organic beings, with which it comes into competition for
food or residence, or from which it has to escape, or on which it
preys. This is obvious in the structure of the teeth and talons of the
tiger; and in that of the legs and claws of the parasite which clings
to the hair on the tiger's body. But in the beautifully plumed seed of
the dandelion, and in the flattened and fringed legs of the
water-beetle, the relation seems at first confined to the elements of
air and water. Yet the advantage of plumed seeds no doubt stands in
the closest relation to the land being already thickly clothed by
other plants; so that the seeds may be widely distributed and fall on
unoccupied ground. In the water-beetle, the structure of its legs, so
well adapted for diving, allows it to compete with other aquatic
insects, to hunt for its own prey, and to escape serving as prey to
other animals.
The store of nutriment laid up within the seeds of many plants seems
at first sight to have no sort of relation to other plants. But from
the strong growth of young plants produced from such seeds (as peas
and beans), when sown in the midst of long grass, I suspect that the
chief use of the nutriment in the seed is to favour the growth of the
young seedling, whilst struggling with other plants growing vigorously
all around.
Look at a plant in the midst of its range, why does it not double or
quadruple its numbers? We know that it can perfectly well withstand a
little more heat or cold, dampness or dryness, for elsewhere it ranges
into slightly hotter or colder, damper or drier districts. In this
case we can clearly see that if we wished in imagination to give the
plant the power of increasing in number, we should have to give it
some advantage over its competitors, or over the animals which preyed
on it. On the confines of its geographical range, a change of
constitution with respect to climate would clearly be an advantage to
our plant; but we have reason to believe that only a few plants or
animals range so far, that they are destroyed by the rigour of the
climate alone. Not until we reach the extreme confines of life, in the
arctic regions or on the borders of an utter desert, will competition
cease. The land may be extremely cold or dry, yet there will be
competition between some few species, or between the individuals of
the same species, for the warmest or dampest spots.
Hence, also, we can see that when a plant or animal is placed in a new
country amongst new competitors, though the climate may be exactly the
same as in its former home, yet the conditions of its life will
generally be changed in an essential manner. If we wished to increase
its average numbers in its new home, we should have to modify it in a
different way to what we should have done in its native country; for
we should have to give it some advantage over a different set of
competitors or enemies.
It is good thus to try in our imagination to give any form some
advantage over another. Probably in no single instance should we know
what to do, so as to succeed. It will convince us of our ignorance on
the mutual relations of all organic beings; a conviction as necessary,
as it seems to be difficult to acquire. All that we can do, is to keep
steadily in mind that each organic being is striving to increase at a
geometrical ratio; that each at some period of its life, during some
season of the year, during each generation or at intervals, has to
struggle for life, and to suffer great destruction. When we reflect on
this struggle, we may console ourselves with the full belief, that the
war of nature is not incessant, that no fear is felt, that death is
generally prompt, and that the vigorous, the healthy, and the happy
survive and multiply.
CHAPTER 4.
NATURAL SELECTION.
Natural Selection: its power compared with man's selection, its power
on characters of trifling importance, its power at all ages and on
both sexes.
Sexual Selection.
On the generality of intercrosses between individuals of the same
species.
Circumstances favourable and unfavourable to Natural Selection,
namely, intercrossing, isolation, number of individuals.
Slow action.
Extinction caused by Natural Selection.
Divergence of Character, related to the diversity of inhabitants of
any small area, and to naturalisation.
Action of Natural Selection, through Divergence of Character and
Extinction, on the descendants from a common parent.
Explains the Grouping of all organic beings.
How will the struggle for existence, discussed too briefly in the last
chapter, act in regard to variation? Can the principle of selection,
which we have seen is so potent in the hands of man, apply in nature?
I think we shall see that it can act most effectually. Let it be borne
in mind in what an endless number of strange peculiarities our
domestic productions, and, in a lesser degree, those under nature,
vary; and how strong the hereditary tendency is. Under domestication,
it may be truly said that the whole organisation becomes in some
degree plastic. Let it be borne in mind how infinitely complex and
close-fitting are the mutual relations of all organic beings to each
other and to their physical conditions of life. Can it, then, be
thought improbable, seeing that variations useful to man have
undoubtedly occurred, that other variations useful in some way to each
being in the great and complex battle of life, should sometimes occur
in the course of thousands of generations? If such do occur, can we
doubt (remembering that many more individuals are born than can
possibly survive) that individuals having any advantage, however
slight, over others, would have the best chance of surviving and of
procreating their kind? On the other hand, we may feel sure that any
variation in the least degree injurious would be rigidly destroyed.
This preservation of favourable variations and the rejection of
injurious variations, I call Natural Selection. Variations neither
useful nor injurious would not be affected by natural selection, and
would be left a fluctuating element, as perhaps we see in the species
called polymorphic.
We shall best understand the probable course of natural selection by
taking the case of a country undergoing some physical change, for
instance, of climate. The proportional numbers of its inhabitants
would almost immediately undergo a change, and some species might
become extinct. We may conclude, from what we have seen of the
intimate and complex manner in which the inhabitants of each country
are bound together, that any change in the numerical proportions of
some of the inhabitants, independently of the change of climate
itself, would most seriously affect many of the others. If the country
were open on its borders, new forms would certainly immigrate, and
this also would seriously disturb the relations of some of the former
inhabitants. Let it be remembered how powerful the influence of a
single introduced tree or mammal has been shown to be. But in the case
of an island, or of a country partly surrounded by barriers, into
which new and better adapted forms could not freely enter, we should
then have places in the economy of nature which would assuredly be
better filled up, if some of the original inhabitants were in some
manner modified; for, had the area been open to immigration, these
same places would have been seized on by intruders. In such case,
every slight modification, which in the course of ages chanced to
arise, and which in any way favoured the individuals of any of the
species, by better adapting them to their altered conditions, would
tend to be preserved; and natural selection would thus have free scope
for the work of improvement.
We have reason to believe, as stated in the first chapter, that a
change in the conditions of life, by specially acting on the
reproductive system, causes or increases variability; and in the
foregoing case the conditions of life are supposed to have undergone a
change, and this would manifestly be favourable to natural selection,
by giving a better chance of profitable variations occurring; and
unless profitable variations do occur, natural selection can do
nothing. Not that, as I believe, any extreme amount of variability is
necessary; as man can certainly produce great results by adding up in
any given direction mere individual differences, so could Nature, but
far more easily, from having incomparably longer time at her disposal.
Nor do I believe that any great physical change, as of climate, or any
unusual degree of isolation to check immigration, is actually
necessary to produce new and unoccupied places for natural selection
to fill up by modifying and improving some of the varying inhabitants.
For as all the inhabitants of each country are struggling together
with nicely balanced forces, extremely slight modifications in the
structure or habits of one inhabitant would often give it an advantage
over others; and still further modifications of the same kind would
often still further increase the advantage. No country can be named in
which all the native inhabitants are now so perfectly adapted to each
other and to the physical conditions under which they live, that none
of them could anyhow be improved; for in all countries, the natives
have been so far conquered by naturalised productions, that they have
allowed foreigners to take firm possession of the land. And as
foreigners have thus everywhere beaten some of the natives, we may
safely conclude that the natives might have been modified with
advantage, so as to have better resisted such intruders.
As man can produce and certainly has produced a great result by his
methodical and unconscious means of selection, what may not nature
effect? Man can act only on external and visible characters: nature
cares nothing for appearances, except in so far as they may be useful
to any being. She can act on every internal organ, on every shade of
constitutional difference, on the whole machinery of life. Man selects
only for his own good; Nature only for that of the being which she
tends. Every selected character is fully exercised by her; and the
being is placed under well-suited conditions of life. Man keeps the
natives of many climates in the same country; he seldom exercises each
selected character in some peculiar and fitting manner; he feeds a
long and a short beaked pigeon on the same food; he does not exercise
a long-backed or long-legged quadruped in any peculiar manner; he
exposes sheep with long and short wool to the same climate. He does
not allow the most vigorous males to struggle for the females. He does
not rigidly destroy all inferior animals, but protects during each
varying season, as far as lies in his power, all his productions. He
often begins his selection by some half-monstrous form; or at least by
some modification prominent enough to catch his eye, or to be plainly
useful to him. Under nature, the slightest difference of structure or
constitution may well turn the nicely-balanced scale in the struggle
for life, and so be preserved. How fleeting are the wishes and efforts
of man! how short his time! and consequently how poor will his
products be, compared with those accumulated by nature during whole
geological periods. Can we wonder, then, that nature's productions
should be far "truer" in character than man's productions; that they
should be infinitely better adapted to the most complex conditions of
life, and should plainly bear the stamp of far higher workmanship?
It may be said that natural selection is daily and hourly
scrutinising, throughout the world, every variation, even the
slightest; rejecting that which is bad, preserving and adding up all
that is good; silently and insensibly working, whenever and wherever
opportunity offers, at the improvement of each organic being in
relation to its organic and inorganic conditions of life. We see
nothing of these slow changes in progress, until the hand of time has
marked the long lapse of ages, and then so imperfect is our view into
long past geological ages, that we only see that the forms of life are
now different from what they formerly were.
Although natural selection can act only through and for the good of
each being, yet characters and structures, which we are apt to
consider as of very trifling importance, may thus be acted on. When we
see leaf-eating insects green, and bark-feeders mottled-grey; the
alpine ptarmigan white in winter, the red-grouse the colour of
heather, and the black-grouse that of peaty earth, we must believe
that these tints are of service to these birds and insects in
preserving them from danger. Grouse, if not destroyed at some period
of their lives, would increase in countless numbers; they are known to
suffer largely from birds of prey; and hawks are guided by eyesight to
their prey,--so much so, that on parts of the Continent persons are
warned not to keep white pigeons, as being the most liable to
destruction. Hence I can see no reason to doubt that natural selection
might be most effective in giving the proper colour to each kind of
grouse, and in keeping that colour, when once acquired, true and
constant. Nor ought we to think that the occasional destruction of an
animal of any particular colour would produce little effect: we should
remember how essential it is in a flock of white sheep to destroy
every lamb with the faintest trace of black. In plants the down on the
fruit and the colour of the flesh are considered by botanists as
characters of the most trifling importance: yet we hear from an
excellent horticulturist, Downing, that in the United States
smooth-skinned fruits suffer far more from a beetle, a curculio, than
those with down; that purple plums suffer far more from a certain
disease than yellow plums; whereas another disease attacks
yellow-fleshed peaches far more than those with other coloured flesh.
If, with all the aids of art, these slight differences make a great
difference in cultivating the several varieties, assuredly, in a state
of nature, where the trees would have to struggle with other trees and
with a host of enemies, such differences would effectually settle
which variety, whether a smooth or downy, a yellow or purple fleshed
fruit, should succeed.
In looking at many small points of difference between species, which,
as far as our ignorance permits us to judge, seem to be quite
unimportant, we must not forget that climate, food, etc., probably
produce some slight and direct effect. It is, however, far more
necessary to bear in mind that there are many unknown laws of
correlation of growth, which, when one part of the organisation is
modified through variation, and the modifications are accumulated by
natural selection for the good of the being, will cause other
modifications, often of the most unexpected nature.
As we see that those variations which under domestication appear at
any particular period of life, tend to reappear in the offspring at
the same period;--for instance, in the seeds of the many varieties of
our culinary and agricultural plants; in the caterpillar and cocoon
stages of the varieties of the silkworm; in the eggs of poultry, and
in the colour of the down of their chickens; in the horns of our sheep
and cattle when nearly adult;--so in a state of nature, natural
selection will be enabled to act on and modify organic beings at any
age, by the accumulation of profitable variations at that age, and by
their inheritance at a corresponding age. If it profit a plant to have
its seeds more and more widely disseminated by the wind, I can see no
greater difficulty in this being effected through natural selection,
than in the cotton-planter increasing and improving by selection the
down in the pods on his cotton-trees. Natural selection may modify and
adapt the larva of an insect to a score of contingencies, wholly
different from those which concern the mature insect. These
modifications will no doubt affect, through the laws of correlation,
the structure of the adult; and probably in the case of those insects
which live only for a few hours, and which never feed, a large part of
their structure is merely the correlated result of successive changes
in the structure of their larvae. So, conversely, modifications in the
adult will probably often affect the structure of the larva; but in
all cases natural selection will ensure that modifications consequent
on other modifications at a different period of life, shall not be in
the least degree injurious: for if they became so, they would cause
the extinction of the species.
Natural selection will modify the structure of the young in relation
to the parent, and of the parent in relation to the young. In social
animals it will adapt the structure of each individual for the benefit
of the community; if each in consequence profits by the selected
change. What natural selection cannot do, is to modify the structure
of one species, without giving it any advantage, for the good of
another species; and though statements to this effect may be found in
works of natural history, I cannot find one case which will bear
investigation. A structure used only once in an animal's whole life,
if of high importance to it, might be modified to any extent by
natural selection; for instance, the great jaws possessed by certain
insects, and used exclusively for opening the cocoon--or the hard tip
to the beak of nestling birds, used for breaking the egg. It has been
asserted, that of the best short-beaked tumbler-pigeons more perish in
the egg than are able to get out of it; so that fanciers assist in the
act of hatching. Now, if nature had to make the beak of a full-grown
pigeon very short for the bird's own advantage, the process of
modification would be very slow, and there would be simultaneously the
most rigorous selection of the young birds within the egg, which had
the most powerful and hardest beaks, for all with weak beaks would
inevitably perish: or, more delicate and more easily broken shells
might be selected, the thickness of the shell being known to vary like
every other structure.
SEXUAL SELECTION.
Inasmuch as peculiarities often appear under domestication in one sex
and become hereditarily attached to that sex, the same fact probably
occurs under nature, and if so, natural selection will be able to
modify one sex in its functional relations to the other sex, or in
relation to wholly different habits of life in the two sexes, as is
sometimes the case with insects. And this leads me to say a few words
on what I call Sexual Selection. This depends, not on a struggle for
existence, but on a struggle between the males for possession of the
females; the result is not death to the unsuccessful competitor, but
few or no offspring. Sexual selection is, therefore, less rigorous
than natural selection. Generally, the most vigorous males, those
which are best fitted for their places in nature, will leave most
progeny. But in many cases, victory will depend not on general vigour,
but on having special weapons, confined to the male sex. A hornless
stag or spurless cock would have a poor chance of leaving offspring.
Sexual selection by always allowing the victor to breed might surely
give indomitable courage, length to the spur, and strength to the wing
to strike in the spurred leg, as well as the brutal cock-fighter, who
knows well that he can improve his breed by careful selection of the
best cocks. How low in the scale of nature this law of battle
descends, I know not; male alligators have been described as fighting,
bellowing, and whirling round, like Indians in a war-dance, for the
possession of the females; male salmons have been seen fighting all
day long; male stag-beetles often bear wounds from the huge mandibles
of other males. The war is, perhaps, severest between the males of
polygamous animals, and these seem oftenest provided with special
weapons. The males of carnivorous animals are already well armed;
though to them and to others, special means of defence may be given
through means of sexual selection, as the mane to the lion, the
shoulder-pad to the boar, and the hooked jaw to the male salmon; for
the shield may be as important for victory, as the sword or spear.
Amongst birds, the contest is often of a more peaceful character. All
those who have attended to the subject, believe that there is the
severest rivalry between the males of many species to attract by
singing the females. The rock-thrush of Guiana, birds of Paradise, and
some others, congregate; and successive males display their gorgeous
plumage and perform strange antics before the females, which standing
by as spectators, at last choose the most attractive partner. Those
who have closely attended to birds in confinement well know that they
often take individual preferences and dislikes: thus Sir R. Heron has
described how one pied peacock was eminently attractive to all his hen
birds. It may appear childish to attribute any effect to such
apparently weak means: I cannot here enter on the details necessary to
support this view; but if man can in a short time give elegant
carriage and beauty to his bantams, according to his standard of
beauty, I can see no good reason to doubt that female birds, by
selecting, during thousands of generations, the most melodious or
beautiful males, according to their standard of beauty, might produce
a marked effect. I strongly suspect that some well-known laws with
respect to the plumage of male and female birds, in comparison with
the plumage of the young, can be explained on the view of plumage
having been chiefly modified by sexual selection, acting when the
birds have come to the breeding age or during the breeding season; the
modifications thus produced being inherited at corresponding ages or
seasons, either by the males alone, or by the males and females; but I
have not space here to enter on this subject.
Thus it is, as I believe, that when the males and females of any
animal have the same general habits of life, but differ in structure,
colour, or ornament, such differences have been mainly caused by
sexual selection; that is, individual males have had, in successive
generations, some slight advantage over other males, in their weapons,
means of defence, or charms; and have transmitted these advantages to
their male offspring. Yet, I would not wish to attribute all such
sexual differences to this agency: for we see peculiarities arising
and becoming attached to the male sex in our domestic animals (as the
wattle in male carriers, horn-like protuberances in the cocks of
certain fowls, etc.), which we cannot believe to be either useful to
the males in battle, or attractive to the females. We see analogous
cases under nature, for instance, the tuft of hair on the breast of
the turkey-cock, which can hardly be either useful or ornamental to
this bird;--indeed, had the tuft appeared under domestication, it
would have been called a monstrosity.
ILLUSTRATIONS OF THE ACTION OF NATURAL SELECTION.
In order to make it clear how, as I believe, natural selection acts, I
must beg permission to give one or two imaginary illustrations. Let us
take the case of a wolf, which preys on various animals, securing some
by craft, some by strength, and some by fleetness; and let us suppose
that the fleetest prey, a deer for instance, had from any change in
the country increased in numbers, or that other prey had decreased in
numbers, during that season of the year when the wolf is hardest
pressed for food. I can under such circumstances see no reason to
doubt that the swiftest and slimmest wolves would have the best chance
of surviving, and so be preserved or selected,--provided always that
they retained strength to master their prey at this or at some other
period of the year, when they might be compelled to prey on other
animals. I can see no more reason to doubt this, than that man can
improve the fleetness of his greyhounds by careful and methodical
selection, or by that unconscious selection which results from each
man trying to keep the best dogs without any thought of modifying the
breed.
Even without any change in the proportional numbers of the animals on
which our wolf preyed, a cub might be born with an innate tendency to
pursue certain kinds of prey. Nor can this be thought very improbable;
for we often observe great differences in the natural tendencies of
our domestic animals; one cat, for instance, taking to catch rats,
another mice; one cat, according to Mr. St. John, bringing home winged
game, another hares or rabbits, and another hunting on marshy ground
and almost nightly catching woodcocks or snipes. The tendency to catch
rats rather than mice is known to be inherited. Now, if any slight
innate change of habit or of structure benefited an individual wolf,
it would have the best chance of surviving and of leaving offspring.
Some of its young would probably inherit the same habits or structure,
and by the repetition of this process, a new variety might be formed
which would either supplant or coexist with the parent-form of wolf.
Or, again, the wolves inhabiting a mountainous district, and those
frequenting the lowlands, would naturally be forced to hunt different
prey; and from the continued preservation of the individuals best
fitted for the two sites, two varieties might slowly be formed. These
varieties would cross and blend where they met; but to this subject of
intercrossing we shall soon have to return. I may add, that, according
to Mr. Pierce, there are two varieties of the wolf inhabiting the
Catskill Mountains in the United States, one with a light
greyhound-like form, which pursues deer, and the other more bulky,
with shorter legs, which more frequently attacks the shepherd's
flocks.
Let us now take a more complex case. Certain plants excrete a sweet
juice, apparently for the sake of eliminating something injurious from
their sap: this is effected by glands at the base of the stipules in
some Leguminosae, and at the back of the leaf of the common laurel.
This juice, though small in quantity, is greedily sought by insects.
Let us now suppose a little sweet juice or nectar to be excreted by
the inner bases of the petals of a flower. In this case insects in
seeking the nectar would get dusted with pollen, and would certainly
often transport the pollen from one flower to the stigma of another
flower. The flowers of two distinct individuals of the same species
would thus get crossed; and the act of crossing, we have good reason
to believe (as will hereafter be more fully alluded to), would produce
very vigorous seedlings, which consequently would have the best chance
of flourishing and surviving. Some of these seedlings would probably
inherit the nectar-excreting power. Those individual flowers which had
the largest glands or nectaries, and which excreted most nectar, would
be oftenest visited by insects, and would be oftenest crossed; and so
in the long-run would gain the upper hand. Those flowers, also, which
had their stamens and pistils placed, in relation to the size and
habits of the particular insects which visited them, so as to favour
in any degree the transportal of their pollen from flower to flower,
would likewise be favoured or selected. We might have taken the case
of insects visiting flowers for the sake of collecting pollen instead
of nectar; and as pollen is formed for the sole object of
fertilisation, its destruction appears a simple loss to the plant; yet
if a little pollen were carried, at first occasionally and then
habitually, by the pollen-devouring insects from flower to flower, and
a cross thus effected, although nine-tenths of the pollen were
destroyed, it might still be a great gain to the plant; and those
individuals which produced more and more pollen, and had larger and
larger anthers, would be selected.
When our plant, by this process of the continued preservation or
natural selection of more and more attractive flowers, had been
rendered highly attractive to insects, they would, unintentionally on
their part, regularly carry pollen from flower to flower; and that
they can most effectually do this, I could easily show by many
striking instances. I will give only one--not as a very striking case,
but as likewise illustrating one step in the separation of the sexes
of plants, presently to be alluded to. Some holly-trees bear only male
flowers, which have four stamens producing rather a small quantity of
pollen, and a rudimentary pistil; other holly-trees bear only female
flowers; these have a full-sized pistil, and four stamens with
shrivelled anthers, in which not a grain of pollen can be detected.
Having found a female tree exactly sixty yards from a male tree, I put
the stigmas of twenty flowers, taken from different branches, under
the microscope, and on all, without exception, there were
pollen-grains, and on some a profusion of pollen. As the wind had set
for several days from the female to the male tree, the pollen could
not thus have been carried. The weather had been cold and boisterous,
and therefore not favourable to bees, nevertheless every female flower
which I examined had been effectually fertilised by the bees,
accidentally dusted with pollen, having flown from tree to tree in
search of nectar. But to return to our imaginary case: as soon as the
plant had been rendered so highly attractive to insects that pollen
was regularly carried from flower to flower, another process might
commence. No naturalist doubts the advantage of what has been called
the "physiological division of labour;" hence we may believe that it
would be advantageous to a plant to produce stamens alone in one
flower or on one whole plant, and pistils alone in another flower or
on another plant. In plants under culture and placed under new
conditions of life, sometimes the male organs and sometimes the female
organs become more or less impotent; now if we suppose this to occur
in ever so slight a degree under nature, then as pollen is already
carried regularly from flower to flower, and as a more complete
separation of the sexes of our plant would be advantageous on the
principle of the division of labour, individuals with this tendency
more and more increased, would be continually favoured or selected,
until at last a complete separation of the sexes would be effected.
Let us now turn to the nectar-feeding insects in our imaginary case:
we may suppose the plant of which we have been slowly increasing the
nectar by continued selection, to be a common plant; and that certain
insects depended in main part on its nectar for food. I could give
many facts, showing how anxious bees are to save time; for instance,
their habit of cutting holes and sucking the nectar at the bases of
certain flowers, which they can, with a very little more trouble,
enter by the mouth. Bearing such facts in mind, I can see no reason to
doubt that an accidental deviation in the size and form of the body,
or in the curvature and length of the proboscis, etc., far too slight
to be appreciated by us, might profit a bee or other insect, so that
an individual so characterised would be able to obtain its food more
quickly, and so have a better chance of living and leaving
descendants. Its descendants would probably inherit a tendency to a
similar slight deviation of structure. The tubes of the corollas of
the common red and incarnate clovers (Trifolium pratense and
incarnatum) do not on a hasty glance appear to differ in length; yet
the hive-bee can easily suck the nectar out of the incarnate clover,
but not out of the common red clover, which is visited by humble-bees
alone; so that whole fields of the red clover offer in vain an
abundant supply of precious nectar to the hive-bee. Thus it might be a
great advantage to the hive-bee to have a slightly longer or
differently constructed proboscis. On the other hand, I have found by
experiment that the fertility of clover greatly depends on bees
visiting and moving parts of the corolla, so as to push the pollen on
to the stigmatic surface. Hence, again, if humble-bees were to become
rare in any country, it might be a great advantage to the red clover
to have a shorter or more deeply divided tube to its corolla, so that
the hive-bee could visit its flowers. Thus I can understand how a
flower and a bee might slowly become, either simultaneously or one
after the other, modified and adapted in the most perfect manner to
each other, by the continued preservation of individuals presenting
mutual and slightly favourable deviations of structure.
I am well aware that this doctrine of natural selection, exemplified
in the above imaginary instances, is open to the same objections which
were at first urged against Sir Charles Lyell's noble views on "the
modern changes of the earth, as illustrative of geology;" but we now
very seldom hear the action, for instance, of the coast-waves, called
a trifling and insignificant cause, when applied to the excavation of
gigantic valleys or to the formation of the longest lines of inland
cliffs. Natural selection can act only by the preservation and
accumulation of infinitesimally small inherited modifications, each
profitable to the preserved being; and as modern geology has almost
banished such views as the excavation of a great valley by a single
diluvial wave, so will natural selection, if it be a true principle,
banish the belief of the continued creation of new organic beings, or
of any great and sudden modification in their structure.
ON THE INTERCROSSING OF INDIVIDUALS.
I must here introduce a short digression. In the case of animals and
plants with separated sexes, it is of course obvious that two
individuals must always unite for each birth; but in the case of
hermaphrodites this is far from obvious. Nevertheless I am strongly
inclined to believe that with all hermaphrodites two individuals,
either occasionally or habitually, concur for the reproduction of
their kind. This view, I may add, was first suggested by Andrew
Knight. We shall presently see its importance; but I must here treat
the subject with extreme brevity, though I have the materials prepared
for an ample discussion. All vertebrate animals, all insects, and some
other large groups of animals, pair for each birth. Modern research
has much diminished the number of supposed hermaphrodites, and of real
hermaphrodites a large number pair; that is, two individuals regularly
unite for reproduction, which is all that concerns us. But still there
are many hermaphrodite animals which certainly do not habitually pair,
and a vast majority of plants are hermaphrodites. What reason, it may
be asked, is there for supposing in these cases that two individuals
ever concur in reproduction? As it is impossible here to enter on
details, I must trust to some general considerations alone.
In the first place, I have collected so large a body of facts,
showing, in accordance with the almost universal belief of breeders,
that with animals and plants a cross between different varieties, or
between individuals of the same variety but of another strain, gives
vigour and fertility to the offspring; and on the other hand, that
CLOSE interbreeding diminishes vigour and fertility; that these facts
alone incline me to believe that it is a general law of nature
(utterly ignorant though we be of the meaning of the law) that no
organic being self-fertilises itself for an eternity of generations;
but that a cross with another individual is occasionally--perhaps at
very long intervals--indispensable.
On the belief that this is a law of nature, we can, I think,
understand several large classes of facts, such as the following,
which on any other view are inexplicable. Every hybridizer knows how
unfavourable exposure to wet is to the fertilisation of a flower, yet
what a multitude of flowers have their anthers and stigmas fully
exposed to the weather! but if an occasional cross be indispensable,
the fullest freedom for the entrance of pollen from another individual
will explain this state of exposure, more especially as the plant's
own anthers and pistil generally stand so close together that
self-fertilisation seems almost inevitable. Many flowers, on the other
hand, have their organs of fructification closely enclosed, as in the
great papilionaceous or pea-family; but in several, perhaps in all,
such flowers, there is a very curious adaptation between the structure
of the flower and the manner in which bees suck the nectar; for, in
doing this, they either push the flower's own pollen on the stigma, or
bring pollen from another flower. So necessary are the visits of bees
to papilionaceous flowers, that I have found, by experiments published
elsewhere, that their fertility is greatly diminished if these visits
be prevented. Now, it is scarcely possible that bees should fly from
flower to flower, and not carry pollen from one to the other, to the
great good, as I believe, of the plant. Bees will act like a
camel-hair pencil, and it is quite sufficient just to touch the
anthers of one flower and then the stigma of another with the same
brush to ensure fertilisation; but it must not be supposed that bees
would thus produce a multitude of hybrids between distinct species;
for if you bring on the same brush a plant's own pollen and pollen
from another species, the former will have such a prepotent effect,
that it will invariably and completely destroy, as has been shown by
Gartner, any influence from the foreign pollen.
When the stamens of a flower suddenly spring towards the pistil, or
slowly move one after the other towards it, the contrivance seems
adapted solely to ensure self-fertilisation; and no doubt it is useful
for this end: but, the agency of insects is often required to cause
the stamens to spring forward, as Kolreuter has shown to be the case
with the barberry; and curiously in this very genus, which seems to
have a special contrivance for self-fertilisation, it is well known
that if very closely-allied forms or varieties are planted near each
other, it is hardly possible to raise pure seedlings, so largely do
they naturally cross. In many other cases, far from there being any
aids for self-fertilisation, there are special contrivances, as I
could show from the writings of C. C. Sprengel and from my own
observations, which effectually prevent the stigma receiving pollen
from its own flower: for instance, in Lobelia fulgens, there is a
really beautiful and elaborate contrivance by which every one of the
infinitely numerous pollen-granules are swept out of the conjoined
anthers of each flower, before the stigma of that individual flower is
ready to receive them; and as this flower is never visited, at least
in my garden, by insects, it never sets a seed, though by placing
pollen from one flower on the stigma of another, I raised plenty of
seedlings; and whilst another species of Lobelia growing close by,
which is visited by bees, seeds freely. In very many other cases,
though there be no special mechanical contrivance to prevent the
stigma of a flower receiving its own pollen, yet, as C. C. Sprengel
has shown, and as I can confirm, either the anthers burst before the
stigma is ready for fertilisation, or the stigma is ready before the
pollen of that flower is ready, so that these plants have in fact
separated sexes, and must habitually be crossed. How strange are these
facts! How strange that the pollen and stigmatic surface of the same
flower, though placed so close together, as if for the very purpose of
self-fertilisation, should in so many cases be mutually useless to
each other! How simply are these facts explained on the view of an
occasional cross with a distinct individual being advantageous or
indispensable!
If several varieties of the cabbage, radish, onion, and of some other
plants, be allowed to seed near each other, a large majority, as I
have found, of the seedlings thus raised will turn out mongrels: for
instance, I raised 233 seedling cabbages from some plants of different
varieties growing near each other, and of these only 78 were true to
their kind, and some even of these were not perfectly true. Yet the
pistil of each cabbage-flower is surrounded not only by its own six
stamens, but by those of the many other flowers on the same plant.
How, then, comes it that such a vast number of the seedlings are
mongrelized? I suspect that it must arise from the pollen of a
distinct VARIETY having a prepotent effect over a flower's own pollen;
and that this is part of the general law of good being derived from
the intercrossing of distinct individuals of the same species. When
distinct SPECIES are crossed the case is directly the reverse, for a
plant's own pollen is always prepotent over foreign pollen; but to
this subject we shall return in a future chapter.
In the case of a gigantic tree covered with innumerable flowers, it
may be objected that pollen could seldom be carried from tree to tree,
and at most only from flower to flower on the same tree, and that
flowers on the same tree can be considered as distinct individuals
only in a limited sense. I believe this objection to be valid, but
that nature has largely provided against it by giving to trees a
strong tendency to bear flowers with separated sexes. When the sexes
are separated, although the male and female flowers may be produced on
the same tree, we can see that pollen must be regularly carried from
flower to flower; and this will give a better chance of pollen being
occasionally carried from tree to tree. That trees belonging to all
Orders have their sexes more often separated than other plants, I find
to be the case in this country; and at my request Dr. Hooker tabulated
the trees of New Zealand, and Dr. Asa Gray those of the United States,
and the result was as I anticipated. On the other hand, Dr. Hooker has
recently informed me that he finds that the rule does not hold in
Australia; and I have made these few remarks on the sexes of trees
simply to call attention to the subject.
Turning for a very brief space to animals: on the land there are some
hermaphrodites, as land-mollusca and earth-worms; but these all pair.
As yet I have not found a single case of a terrestrial animal which
fertilises itself. We can understand this remarkable fact, which
offers so strong a contrast with terrestrial plants, on the view of an
occasional cross being indispensable, by considering the medium in
which terrestrial animals live, and the nature of the fertilising
element; for we know of no means, analogous to the action of insects
and of the wind in the case of plants, by which an occasional cross
could be effected with terrestrial animals without the concurrence of
two individuals. Of aquatic animals, there are many self-fertilising
hermaphrodites; but here currents in the water offer an obvious means
for an occasional cross. And, as in the case of flowers, I have as yet
failed, after consultation with one of the highest authorities,
namely, Professor Huxley, to discover a single case of an
hermaphrodite animal with the organs of reproduction so perfectly
enclosed within the body, that access from without and the occasional
influence of a distinct individual can be shown to be physically
impossible. Cirripedes long appeared to me to present a case of very
great difficulty under this point of view; but I have been enabled, by
a fortunate chance, elsewhere to prove that two individuals, though
both are self-fertilising hermaphrodites, do sometimes cross.
It must have struck most naturalists as a strange anomaly that, in the
case of both animals and plants, species of the same family and even
of the same genus, though agreeing closely with each other in almost
their whole organisation, yet are not rarely, some of them
hermaphrodites, and some of them unisexual. But if, in fact, all
hermaphrodites do occasionally intercross with other individuals, the
difference between hermaphrodites and unisexual species, as far as
function is concerned, becomes very small.
From these several considerations and from the many special facts
which I have collected, but which I am not here able to give, I am
strongly inclined to suspect that, both in the vegetable and animal
kingdoms, an occasional intercross with a distinct individual is a law
of nature. I am well aware that there are, on this view, many cases of
difficulty, some of which I am trying to investigate. Finally then, we
may conclude that in many organic beings, a cross between two
individuals is an obvious necessity for each birth; in many others it
occurs perhaps only at long intervals; but in none, as I suspect, can
self-fertilisation go on for perpetuity.
CIRCUMSTANCES FAVOURABLE TO NATURAL SELECTION.
This is an extremely intricate subject. A large amount of inheritable
and diversified variability is favourable, but I believe mere
individual differences suffice for the work. A large number of
individuals, by giving a better chance for the appearance within any
given period of profitable variations, will compensate for a lesser
amount of variability in each individual, and is, I believe, an
extremely important element of success. Though nature grants vast
periods of time for the work of natural selection, she does not grant
an indefinite period; for as all organic beings are striving, it may
be said, to seize on each place in the economy of nature, if any one
species does not become modified and improved in a corresponding
degree with its competitors, it will soon be exterminated.
In man's methodical selection, a breeder selects for some definite
object, and free intercrossing will wholly stop his work. But when
many men, without intending to alter the breed, have a nearly common
standard of perfection, and all try to get and breed from the best
animals, much improvement and modification surely but slowly follow
from this unconscious process of selection, notwithstanding a large
amount of crossing with inferior animals. Thus it will be in nature;
for within a confined area, with some place in its polity not so
perfectly occupied as might be, natural selection will always tend to
preserve all the individuals varying in the right direction, though in
different degrees, so as better to fill up the unoccupied place. But
if the area be large, its several districts will almost certainly
present different conditions of life; and then if natural selection be
modifying and improving a species in the several districts, there will
be intercrossing with the other individuals of the same species on the
confines of each. And in this case the effects of intercrossing can
hardly be counterbalanced by natural selection always tending to
modify all the individuals in each district in exactly the same manner
to the conditions of each; for in a continuous area, the conditions
will generally graduate away insensibly from one district to another.
The intercrossing will most affect those animals which unite for each
birth, which wander much, and which do not breed at a very quick rate.
Hence in animals of this nature, for instance in birds, varieties will
generally be confined to separated countries; and this I believe to be
the case. In hermaphrodite organisms which cross only occasionally,
and likewise in animals which unite for each birth, but which wander
little and which can increase at a very rapid rate, a new and improved
variety might be quickly formed on any one spot, and might there
maintain itself in a body, so that whatever intercrossing took place
would be chiefly between the individuals of the same new variety. A
local variety when once thus formed might subsequently slowly spread
to other districts. On the above principle, nurserymen always prefer
getting seed from a large body of plants of the same variety, as the
chance of intercrossing with other varieties is thus lessened.
Even in the case of slow-breeding animals, which unite for each birth,
we must not overrate the effects of intercrosses in retarding natural
selection; for I can bring a considerable catalogue of facts, showing
that within the same area, varieties of the same animal can long
remain distinct, from haunting different stations, from breeding at
slightly different seasons, or from varieties of the same kind
preferring to pair together.
Intercrossing plays a very important part in nature in keeping the
individuals of the same species, or of the same variety, true and
uniform in character. It will obviously thus act far more efficiently
with those animals which unite for each birth; but I have already
attempted to show that we have reason to believe that occasional
intercrosses take place with all animals and with all plants. Even if
these take place only at long intervals, I am convinced that the young
thus produced will gain so much in vigour and fertility over the
offspring from long-continued self-fertilisation, that they will have
a better chance of surviving and propagating their kind; and thus, in
the long run, the influence of intercrosses, even at rare intervals,
will be great. If there exist organic beings which never intercross,
uniformity of character can be retained amongst them, as long as their
conditions of life remain the same, only through the principle of
inheritance, and through natural selection destroying any which depart
from the proper type; but if their conditions of life change and they
undergo modification, uniformity of character can be given to their
modified offspring, solely by natural selection preserving the same
favourable variations.
Isolation, also, is an important element in the process of natural
selection. In a confined or isolated area, if not very large, the
organic and inorganic conditions of life will generally be in a great
degree uniform; so that natural selection will tend to modify all the
individuals of a varying species throughout the area in the same
manner in relation to the same conditions. Intercrosses, also, with
the individuals of the same species, which otherwise would have
inhabited the surrounding and differently circumstanced districts,
will be prevented. But isolation probably acts more efficiently in
checking the immigration of better adapted organisms, after any
physical change, such as of climate or elevation of the land, etc.;
and thus new places in the natural economy of the country are left
open for the old inhabitants to struggle for, and become adapted to,
through modifications in their structure and constitution. Lastly,
isolation, by checking immigration and consequently competition, will
give time for any new variety to be slowly improved; and this may
sometimes be of importance in the production of new species. If,
however, an isolated area be very small, either from being surrounded
by barriers, or from having very peculiar physical conditions, the
total number of the individuals supported on it will necessarily be
very small; and fewness of individuals will greatly retard the
production of new species through natural selection, by decreasing the
chance of the appearance of favourable variations.
If we turn to nature to test the truth of these remarks, and look at
any small isolated area, such as an oceanic island, although the total
number of the species inhabiting it, will be found to be small, as we
shall see in our chapter on geographical distribution; yet of these
species a very large proportion are endemic,--that is, have been
produced there, and nowhere else. Hence an oceanic island at first
sight seems to have been highly favourable for the production of new
species. But we may thus greatly deceive ourselves, for to ascertain
whether a small isolated area, or a large open area like a continent,
has been most favourable for the production of new organic forms, we
ought to make the comparison within equal times; and this we are
incapable of doing.
Although I do not doubt that isolation is of considerable importance
in the production of new species, on the whole I am inclined to
believe that largeness of area is of more importance, more especially
in the production of species, which will prove capable of enduring for
a long period, and of spreading widely. Throughout a great and open
area, not only will there be a better chance of favourable variations
arising from the large number of individuals of the same species there
supported, but the conditions of life are infinitely complex from the
large number of already existing species; and if some of these many
species become modified and improved, others will have to be improved
in a corresponding degree or they will be exterminated. Each new form,
also, as soon as it has been much improved, will be able to spread
over the open and continuous area, and will thus come into competition
with many others. Hence more new places will be formed, and the
competition to fill them will be more severe, on a large than on a
small and isolated area. Moreover, great areas, though now continuous,
owing to oscillations of level, will often have recently existed in a
broken condition, so that the good effects of isolation will
generally, to a certain extent, have concurred. Finally, I conclude
that, although small isolated areas probably have been in some
respects highly favourable for the production of new species, yet that
the course of modification will generally have been more rapid on
large areas; and what is more important, that the new forms produced
on large areas, which already have been victorious over many
competitors, will be those that will spread most widely, will give
rise to most new varieties and species, and will thus play an
important part in the changing history of the organic world.
We can, perhaps, on these views, understand some facts which will be
again alluded to in our chapter on geographical distribution; for
instance, that the productions of the smaller continent of Australia
have formerly yielded, and apparently are now yielding, before those
of the larger Europaeo-Asiatic area. Thus, also, it is that
continental productions have everywhere become so largely naturalised
on islands. On a small island, the race for life will have been less
severe, and there will have been less modification and less
extermination. Hence, perhaps, it comes that the flora of Madeira,
according to Oswald Heer, resembles the extinct tertiary flora of
Europe. All fresh-water basins, taken together, make a small area
compared with that of the sea or of the land; and, consequently, the
competition between fresh-water productions will have been less severe
than elsewhere; new forms will have been more slowly formed, and old
forms more slowly exterminated. And it is in fresh water that we find
seven genera of Ganoid fishes, remnants of a once preponderant order:
and in fresh water we find some of the most anomalous forms now known
in the world, as the Ornithorhynchus and Lepidosiren, which, like
fossils, connect to a certain extent orders now widely separated in
the natural scale. These anomalous forms may almost be called living
fossils; they have endured to the present day, from having inhabited a
confined area, and from having thus been exposed to less severe
competition.
To sum up the circumstances favourable and unfavourable to natural
selection, as far as the extreme intricacy of the subject permits. I
conclude, looking to the future, that for terrestrial productions a
large continental area, which will probably undergo many oscillations
of level, and which consequently will exist for long periods in a
broken condition, will be the most favourable for the production of
many new forms of life, likely to endure long and to spread widely.
For the area will first have existed as a continent, and the
inhabitants, at this period numerous in individuals and kinds, will
have been subjected to very severe competition. When converted by
subsidence into large separate islands, there will still exist many
individuals of the same species on each island: intercrossing on the
confines of the range of each species will thus be checked: after
physical changes of any kind, immigration will be prevented, so that
new places in the polity of each island will have to be filled up by
modifications of the old inhabitants; and time will be allowed for the
varieties in each to become well modified and perfected. When, by
renewed elevation, the islands shall be re-converted into a
continental area, there will again be severe competition: the most
favoured or improved varieties will be enabled to spread: there will
be much extinction of the less improved forms, and the relative
proportional numbers of the various inhabitants of the renewed
continent will again be changed; and again there will be a fair field
for natural selection to improve still further the inhabitants, and
thus produce new species.
That natural selection will always act with extreme slowness, I fully
admit. Its action depends on there being places in the polity of
nature, which can be better occupied by some of the inhabitants of the
country undergoing modification of some kind. The existence of such
places will often depend on physical changes, which are generally very
slow, and on the immigration of better adapted forms having been
checked. But the action of natural selection will probably still
oftener depend on some of the inhabitants becoming slowly modified;
the mutual relations of many of the other inhabitants being thus
disturbed. Nothing can be effected, unless favourable variations
occur, and variation itself is apparently always a very slow process.
The process will often be greatly retarded by free intercrossing. Many
will exclaim that these several causes are amply sufficient wholly to
stop the action of natural selection. I do not believe so. On the
other hand, I do believe that natural selection will always act very
slowly, often only at long intervals of time, and generally on only a
very few of the inhabitants of the same region at the same time. I
further believe, that this very slow, intermittent action of natural
selection accords perfectly well with what geology tells us of the
rate and manner at which the inhabitants of this world have changed.
Slow though the process of selection may be, if feeble man can do much
by his powers of artificial selection, I can see no limit to the
amount of change, to the beauty and infinite complexity of the
coadaptations between all organic beings, one with another and with
their physical conditions of life, which may be effected in the long
course of time by nature's power of selection.
EXTINCTION.
This subject will be more fully discussed in our chapter on Geology;
but it must be here alluded to from being intimately connected with
natural selection. Natural selection acts solely through the
preservation of variations in some way advantageous, which
consequently endure. But as from the high geometrical powers of
increase of all organic beings, each area is already fully stocked
with inhabitants, it follows that as each selected and favoured form
increases in number, so will the less favoured forms decrease and
become rare. Rarity, as geology tells us, is the precursor to
extinction. We can, also, see that any form represented by few
individuals will, during fluctuations in the seasons or in the number
of its enemies, run a good chance of utter extinction. But we may go
further than this; for as new forms are continually and slowly being
produced, unless we believe that the number of specific forms goes on
perpetually and almost indefinitely increasing, numbers inevitably
must become extinct. That the number of specific forms has not
indefinitely increased, geology shows us plainly; and indeed we can
see reason why they should not have thus increased, for the number of
places in the polity of nature is not indefinitely great,--not that we
have any means of knowing that any one region has as yet got its
maximum of species. Probably no region is as yet fully stocked, for at
the Cape of Good Hope, where more species of plants are crowded
together than in any other quarter of the world, some foreign plants
have become naturalised, without causing, as far as we know, the
extinction of any natives.
Furthermore, the species which are most numerous in individuals will
have the best chance of producing within any given period favourable
variations. We have evidence of this, in the facts given in the second
chapter, showing that it is the common species which afford the
greatest number of recorded varieties, or incipient species. Hence,
rare species will be less quickly modified or improved within any
given period, and they will consequently be beaten in the race for
life by the modified descendants of the commoner species.
From these several considerations I think it inevitably follows, that
as new species in the course of time are formed through natural
selection, others will become rarer and rarer, and finally extinct.
The forms which stand in closest competition with those undergoing
modification and improvement, will naturally suffer most. And we have
seen in the chapter on the Struggle for Existence that it is the most
closely-allied forms,--varieties of the same species, and species of
the same genus or of related genera,--which, from having nearly the
same structure, constitution, and habits, generally come into the
severest competition with each other. Consequently, each new variety
or species, during the progress of its formation, will generally press
hardest on its nearest kindred, and tend to exterminate them. We see
the same process of extermination amongst our domesticated
productions, through the selection of improved forms by man. Many
curious instances could be given showing how quickly new breeds of
cattle, sheep, and other animals, and varieties of flowers, take the
place of older and inferior kinds. In Yorkshire, it is historically
known that the ancient black cattle were displaced by the long-horns,
and that these "were swept away by the short-horns" (I quote the words
of an agricultural writer) "as if by some murderous pestilence."
DIVERGENCE OF CHARACTER.
The principle, which I have designated by this term, is of high
importance on my theory, and explains, as I believe, several important
facts. In the first place, varieties, even strongly-marked ones,
though having somewhat of the character of species--as is shown by the
hopeless doubts in many cases how to rank them--yet certainly differ
from each other far less than do good and distinct species.
Nevertheless, according to my view, varieties are species in the
process of formation, or are, as I have called them, incipient
species. How, then, does the lesser difference between varieties
become augmented into the greater difference between species? That
this does habitually happen, we must infer from most of the
innumerable species throughout nature presenting well-marked
differences; whereas varieties, the supposed prototypes and parents of
future well-marked species, present slight and ill-defined
differences. Mere chance, as we may call it, might cause one variety
to differ in some character from its parents, and the offspring of
this variety again to differ from its parent in the very same
character and in a greater degree; but this alone would never account
for so habitual and large an amount of difference as that between
varieties of the same species and species of the same genus.
As has always been my practice, let us seek light on this head from
our domestic productions. We shall here find something analogous. A
fancier is struck by a pigeon having a slightly shorter beak; another
fancier is struck by a pigeon having a rather longer beak; and on the
acknowledged principle that "fanciers do not and will not admire a
medium standard, but like extremes," they both go on (as has actually
occurred with tumbler-pigeons) choosing and breeding from birds with
longer and longer beaks, or with shorter and shorter beaks. Again, we
may suppose that at an early period one man preferred swifter horses;
another stronger and more bulky horses. The early differences would be
very slight; in the course of time, from the continued selection of
swifter horses by some breeders, and of stronger ones by others, the
differences would become greater, and would be noted as forming two
sub-breeds; finally, after the lapse of centuries, the sub-breeds
would become converted into two well-established and distinct breeds.
As the differences slowly become greater, the inferior animals with
intermediate characters, being neither very swift nor very strong,
will have been neglected, and will have tended to disappear. Here,
then, we see in man's productions the action of what may be called the
principle of divergence, causing differences, at first barely
appreciable, steadily to increase, and the breeds to diverge in
character both from each other and from their common parent.
But how, it may be asked, can any analogous principle apply in nature?
I believe it can and does apply most efficiently, from the simple
circumstance that the more diversified the descendants from any one
species become in structure, constitution, and habits, by so much will
they be better enabled to seize on many and widely diversified places
in the polity of nature, and so be enabled to increase in numbers.
We can clearly see this in the case of animals with simple habits.
Take the case of a carnivorous quadruped, of which the number that can
be supported in any country has long ago arrived at its full average.
If its natural powers of increase be allowed to act, it can succeed in
increasing (the country not undergoing any change in its conditions)
only by its varying descendants seizing on places at present occupied
by other animals: some of them, for instance, being enabled to feed on
new kinds of prey, either dead or alive; some inhabiting new stations,
climbing trees, frequenting water, and some perhaps becoming less
carnivorous. The more diversified in habits and structure the
descendants of our carnivorous animal became, the more places they
would be enabled to occupy. What applies to one animal will apply
throughout all time to all animals--that is, if they vary--for
otherwise natural selection can do nothing. So it will be with plants.
It has been experimentally proved, that if a plot of ground be sown
with one species of grass, and a similar plot be sown with several
distinct genera of grasses, a greater number of plants and a greater
weight of dry herbage can thus be raised. The same has been found to
hold good when first one variety and then several mixed varieties of
wheat have been sown on equal spaces of ground. Hence, if any one
species of grass were to go on varying, and those varieties were
continually selected which differed from each other in at all the same
manner as distinct species and genera of grasses differ from each
other, a greater number of individual plants of this species of grass,
including its modified descendants, would succeed in living on the
same piece of ground. And we well know that each species and each
variety of grass is annually sowing almost countless seeds; and thus,
as it may be said, is striving its utmost to increase its numbers.
Consequently, I cannot doubt that in the course of many thousands of
generations, the most distinct varieties of any one species of grass
would always have the best chance of succeeding and of increasing in
numbers, and thus of supplanting the less distinct varieties; and
varieties, when rendered very distinct from each other, take the rank
of species.
The truth of the principle, that the greatest amount of life can be
supported by great diversification of structure, is seen under many
natural circumstances. In an extremely small area, especially if
freely open to immigration, and where the contest between individual
and individual must be severe, we always find great diversity in its
inhabitants. For instance, I found that a piece of turf, three feet by
four in size, which had been exposed for many years to exactly the
same conditions, supported twenty species of plants, and these
belonged to eighteen genera and to eight orders, which shows how much
these plants differed from each other. So it is with the plants and
insects on small and uniform islets; and so in small ponds of fresh
water. Farmers find that they can raise most food by a rotation of
plants belonging to the most different orders: nature follows what may
be called a simultaneous rotation. Most of the animals and plants
which live close round any small piece of ground, could live on it
(supposing it not to be in any way peculiar in its nature), and may be
said to be striving to the utmost to live there; but, it is seen, that
where they come into the closest competition with each other, the
advantages of diversification of structure, with the accompanying
differences of habit and constitution, determine that the inhabitants,
which thus jostle each other most closely, shall, as a general rule,
belong to what we call different genera and orders.
The same principle is seen in the naturalisation of plants through
man's agency in foreign lands. It might have been expected that the
plants which have succeeded in becoming naturalised in any land would
generally have been closely allied to the indigenes; for these are
commonly looked at as specially created and adapted for their own
country. It might, also, perhaps have been expected that naturalised
plants would have belonged to a few groups more especially adapted to
certain stations in their new homes. But the case is very different;
and Alph. De Candolle has well remarked in his great and admirable
work, that floras gain by naturalisation, proportionally with the
number of the native genera and species, far more in new genera than
in new species. To give a single instance: in the last edition of Dr.
Asa Gray's 'Manual of the Flora of the Northern United States,' 260
naturalised plants are enumerated, and these belong to 162 genera. We
thus see that these naturalised plants are of a highly diversified
nature. They differ, moreover, to a large extent from the indigenes,
for out of the 162 genera, no less than 100 genera are not there
indigenous, and thus a large proportional addition is made to the
genera of these States.
By considering the nature of the plants or animals which have
struggled successfully with the indigenes of any country, and have
there become naturalised, we can gain some crude idea in what manner
some of the natives would have had to be modified, in order to have
gained an advantage over the other natives; and we may, I think, at
least safely infer that diversification of structure, amounting to new
generic differences, would have been profitable to them.
The advantage of diversification in the inhabitants of the same region
is, in fact, the same as that of the physiological division of labour
in the organs of the same individual body--a subject so well
elucidated by Milne Edwards. No physiologist doubts that a stomach by
being adapted to digest vegetable matter alone, or flesh alone, draws
most nutriment from these substances. So in the general economy of any
land, the more widely and perfectly the animals and plants are
diversified for different habits of life, so will a greater number of
individuals be capable of there supporting themselves. A set of
animals, with their organisation but little diversified, could hardly
compete with a set more perfectly diversified in structure. It may be
doubted, for instance, whether the Australian marsupials, which are
divided into groups differing but little from each other, and feebly
representing, as Mr. Waterhouse and others have remarked, our
carnivorous, ruminant, and rodent mammals, could successfully compete
with these well-pronounced orders. In the Australian mammals, we see
the process of diversification in an early and incomplete stage of
development. After the foregoing discussion, which ought to have been
much amplified, we may, I think, assume that the modified descendants
of any one species will succeed by so much the better as they become
more diversified in structure, and are thus enabled to encroach on
places occupied by other beings. Now let us see how this principle of
great benefit being derived from divergence of character, combined
with the principles of natural selection and of extinction, will tend
to act.
The accompanying diagram will aid us in understanding this rather
perplexing subject. Let A to L represent the species of a genus large
in its own country; these species are supposed to resemble each other
in unequal degrees, as is so generally the case in nature, and as is
represented in the diagram by the letters standing at unequal
distances. I have said a large genus, because we have seen in the
second chapter, that on an average more of the species of large genera
vary than of small genera; and the varying species of the large genera
present a greater number of varieties. We have, also, seen that the
species, which are the commonest and the most widely-diffused, vary
more than rare species with restricted ranges. Let (A) be a common,
widely-diffused, and varying species, belonging to a genus large in
its own country. The little fan of diverging dotted lines of unequal
lengths proceeding from (A), may represent its varying offspring. The
variations are supposed to be extremely slight, but of the most
diversified nature; they are not supposed all to appear
simultaneously, but often after long intervals of time; nor are they
all supposed to endure for equal periods. Only those variations which
are in some way profitable will be preserved or naturally selected.
And here the importance of the principle of benefit being derived from
divergence of character comes in; for this will generally lead to the
most different or divergent variations (represented by the outer
dotted lines) being preserved and accumulated by natural selection.
When a dotted line reaches one of the horizontal lines, and is there
marked by a small numbered letter, a sufficient amount of variation is
supposed to have been accumulated to have formed a fairly well-marked
variety, such as would be thought worthy of record in a systematic
work.
The intervals between the horizontal lines in the diagram, may
represent each a thousand generations; but it would have been better
if each had represented ten thousand generations. After a thousand
generations, species (A) is supposed to have produced two fairly
well-marked varieties, namely a1 and m1. These two varieties will
generally continue to be exposed to the same conditions which made
their parents variable, and the tendency to variability is in itself
hereditary, consequently they will tend to vary, and generally to vary
in nearly the same manner as their parents varied. Moreover, these two
varieties, being only slightly modified forms, will tend to inherit
those advantages which made their common parent (A) more numerous than
most of the other inhabitants of the same country; they will likewise
partake of those more general advantages which made the genus to which
the parent-species belonged, a large genus in its own country. And
these circumstances we know to be favourable to the production of new
varieties.
If, then, these two varieties be variable, the most divergent of their
variations will generally be preserved during the next thousand
generations. And after this interval, variety a1 is supposed in the
diagram to have produced variety a2, which will, owing to the
principle of divergence, differ more from (A) than did variety a1.
Variety m1 is supposed to have produced two varieties, namely m2 and
s2, differing from each other, and more considerably from their common
parent (A). We may continue the process by similar steps for any
length of time; some of the varieties, after each thousand
generations, producing only a single variety, but in a more and more
modified condition, some producing two or three varieties, and some
failing to produce any. Thus the varieties or modified descendants,
proceeding from the common parent (A), will generally go on increasing
in number and diverging in character. In the diagram the process is
represented up to the ten-thousandth generation, and under a condensed
and simplified form up to the fourteen-thousandth generation.
But I must here remark that I do not suppose that the process ever
goes on so regularly as is represented in the diagram, though in
itself made somewhat irregular. I am far from thinking that the most
divergent varieties will invariably prevail and multiply: a medium
form may often long endure, and may or may not produce more than one
modified descendant; for natural selection will always act according
to the nature of the places which are either unoccupied or not
perfectly occupied by other beings; and this will depend on infinitely
complex relations. But as a general rule, the more diversified in
structure the descendants from any one species can be rendered, the
more places they will be enabled to seize on, and the more their
modified progeny will be increased. In our diagram the line of
succession is broken at regular intervals by small numbered letters
marking the successive forms which have become sufficiently distinct
to be recorded as varieties. But these breaks are imaginary, and might
have been inserted anywhere, after intervals long enough to have
allowed the accumulation of a considerable amount of divergent
variation.
As all the modified descendants from a common and widely-diffused
species, belonging to a large genus, will tend to partake of the same
advantages which made their parent successful in life, they will
generally go on multiplying in number as well as diverging in
character: this is represented in the diagram by the several divergent
branches proceeding from (A). The modified offspring from the later
and more highly improved branches in the lines of descent, will, it is
probable, often take the place of, and so destroy, the earlier and
less improved branches: this is represented in the diagram by some of
the lower branches not reaching to the upper horizontal lines. In some
cases I do not doubt that the process of modification will be confined
to a single line of descent, and the number of the descendants will
not be increased; although the amount of divergent modification may
have been increased in the successive generations. This case would be
represented in the diagram, if all the lines proceeding from (A) were
removed, excepting that from a1 to a10. In the same way, for instance,
the English race-horse and English pointer have apparently both gone
on slowly diverging in character from their original stocks, without
either having given off any fresh branches or races.
After ten thousand generations, species (A) is supposed to have
produced three forms, a10, f10, and m10, which, from having diverged
in character during the successive generations, will have come to
differ largely, but perhaps unequally, from each other and from their
common parent. If we suppose the amount of change between each
horizontal line in our diagram to be excessively small, these three
forms may still be only well-marked varieties; or they may have
arrived at the doubtful category of sub-species; but we have only to
suppose the steps in the process of modification to be more numerous
or greater in amount, to convert these three forms into well-defined
species: thus the diagram illustrates the steps by which the small
differences distinguishing varieties are increased into the larger
differences distinguishing species. By continuing the same process for
a greater number of generations (as shown in the diagram in a
condensed and simplified manner), we get eight species, marked by the
letters between a14 and m14, all descended from (A). Thus, as I
believe, species are multiplied and genera are formed.
In a large genus it is probable that more than one species would vary.
In the diagram I have assumed that a second species (I) has produced,
by analogous steps, after ten thousand generations, either two
well-marked varieties (w10 and z10) or two species, according to the
amount of change supposed to be represented between the horizontal
lines. After fourteen thousand generations, six new species, marked by
the letters n14 to z14, are supposed to have been produced. In each
genus, the species, which are already extremely different in
character, will generally tend to produce the greatest number of
modified descendants; for these will have the best chance of filling
new and widely different places in the polity of nature: hence in the
diagram I have chosen the extreme species (A), and the nearly extreme
species (I), as those which have largely varied, and have given rise
to new varieties and species. The other nine species (marked by
capital letters) of our original genus, may for a long period continue
transmitting unaltered descendants; and this is shown in the diagram
by the dotted lines not prolonged far upwards from want of space.
But during the process of modification, represented in the diagram,
another of our principles, namely that of extinction, will have played
an important part. As in each fully stocked country natural selection
necessarily acts by the selected form having some advantage in the
struggle for life over other forms, there will be a constant tendency
in the improved descendants of any one species to supplant and
exterminate in each stage of descent their predecessors and their
original parent. For it should be remembered that the competition will
generally be most severe between those forms which are most nearly
related to each other in habits, constitution, and structure. Hence
all the intermediate forms between the earlier and later states, that
is between the less and more improved state of a species, as well as
the original parent-species itself, will generally tend to become
extinct. So it probably will be with many whole collateral lines of
descent, which will be conquered by later and improved lines of
descent. If, however, the modified offspring of a species get into
some distinct country, or become quickly adapted to some quite new
station, in which child and parent do not come into competition, both
may continue to exist.
If then our diagram be assumed to represent a considerable amount of
modification, species (A) and all the earlier varieties will have
become extinct, having been replaced by eight new species (a14 to
m14); and (I) will have been replaced by six (n14 to z14) new species.
But we may go further than this. The original species of our genus
were supposed to resemble each other in unequal degrees, as is so
generally the case in nature; species (A) being more nearly related to
B, C, and D, than to the other species; and species (I) more to G, H,
K, L, than to the others. These two species (A) and (I), were also
supposed to be very common and widely diffused species, so that they
must originally have had some advantage over most of the other species
of the genus. Their modified descendants, fourteen in number at the
fourteen-thousandth generation, will probably have inherited some of
the same advantages: they have also been modified and improved in a
diversified manner at each stage of descent, so as to have become
adapted to many related places in the natural economy of their
country. It seems, therefore, to me extremely probable that they will
have taken the places of, and thus exterminated, not only their
parents (A) and (I), but likewise some of the original species which
were most nearly related to their parents. Hence very few of the
original species will have transmitted offspring to the
fourteen-thousandth generation. We may suppose that only one (F), of
the two species which were least closely related to the other nine
original species, has transmitted descendants to this late stage of
descent.
The new species in our diagram descended from the original eleven
species, will now be fifteen in number. Owing to the divergent
tendency of natural selection, the extreme amount of difference in
character between species a14 and z14 will be much greater than that
between the most different of the original eleven species. The new
species, moreover, will be allied to each other in a widely different
manner. Of the eight descendants from (A) the three marked a14, q14,
p14, will be nearly related from having recently branched off from
a10; b14 and f14, from having diverged at an earlier period from a5,
will be in some degree distinct from the three first-named species;
and lastly, o14, e14, and m14, will be nearly related one to the
other, but from having diverged at the first commencement of the
process of modification, will be widely different from the other five
species, and may constitute a sub-genus or even a distinct genus.
The six descendants from (I) will form two sub-genera or even genera.
But as the original species (I) differed largely from (A), standing
nearly at the extreme points of the original genus, the six
descendants from (I) will, owing to inheritance, differ considerably
from the eight descendants from (A); the two groups, moreover, are
supposed to have gone on diverging in different directions. The
intermediate species, also (and this is a very important
consideration), which connected the original species (A) and (I), have
all become, excepting (F), extinct, and have left no descendants.
Hence the six new species descended from (I), and the eight descended
from (A), will have to be ranked as very distinct genera, or even as
distinct sub-families.
Thus it is, as I believe, that two or more genera are produced by
descent, with modification, from two or more species of the same
genus. And the two or more parent-species are supposed to have
descended from some one species of an earlier genus. In our diagram,
this is indicated by the broken lines, beneath the capital letters,
converging in sub-branches downwards towards a single point; this
point representing a single species, the supposed single parent of our
several new sub-genera and genera.
It is worth while to reflect for a moment on the character of the new
species F14, which is supposed not to have diverged much in character,
but to have retained the form of (F), either unaltered or altered only
in a slight degree. In this case, its affinities to the other fourteen
new species will be of a curious and circuitous nature. Having
descended from a form which stood between the two parent-species (A)
and (I), now supposed to be extinct and unknown, it will be in some
degree intermediate in character between the two groups descended from
these species. But as these two groups have gone on diverging in
character from the type of their parents, the new species (F14) will
not be directly intermediate between them, but rather between types of
the two groups; and every naturalist will be able to bring some such
case before his mind.
In the diagram, each horizontal line has hitherto been supposed to
represent a thousand generations, but each may represent a million or
hundred million generations, and likewise a section of the successive
strata of the earth's crust including extinct remains. We shall, when
we come to our chapter on Geology, have to refer again to this
subject, and I think we shall then see that the diagram throws light
on the affinities of extinct beings, which, though generally belonging
to the same orders, or families, or genera, with those now living, yet
are often, in some degree, intermediate in character between existing
groups; and we can understand this fact, for the extinct species lived
at very ancient epochs when the branching lines of descent had
diverged less.
I see no reason to limit the process of modification, as now
explained, to the formation of genera alone. If, in our diagram, we
suppose the amount of change represented by each successive group of
diverging dotted lines to be very great, the forms marked a14 to p14,
those marked b14 and f14, and those marked o14 to m14, will form three
very distinct genera. We shall also have two very distinct genera
descended from (I) and as these latter two genera, both from continued
divergence of character and from inheritance from a different parent,
will differ widely from the three genera descended from (A), the two
little groups of genera will form two distinct families, or even
orders, according to the amount of divergent modification supposed to
be represented in the diagram. And the two new families, or orders,
will have descended from two species of the original genus; and these
two species are supposed to have descended from one species of a still
more ancient and unknown genus.
We have seen that in each country it is the species of the larger
genera which oftenest present varieties or incipient species. This,
indeed, might have been expected; for as natural selection acts
through one form having some advantage over other forms in the
struggle for existence, it will chiefly act on those which already
have some advantage; and the largeness of any group shows that its
species have inherited from a common ancestor some advantage in
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