On the Origin of Species by Means of Natural Selection
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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 common. Hence, the struggle for the production of new
and modified descendants, will mainly lie between the larger groups, which
are all trying to increase in number. One large group will slowly conquer
another large group, reduce its numbers, and thus lessen its chance of
further variation and improvement. Within the same large group, the later
and more highly perfected sub-groups, from branching out and seizing on
many new places in the polity of Nature, will constantly tend to supplant
and destroy the earlier and less improved sub-groups. Small and broken
groups and sub-groups will finally tend to disappear. Looking to the
future, we can predict that the groups of organic beings which are now
large and triumphant, and which are least broken up, that is, which as yet
have suffered least extinction, will for a long period continue to
increase. But which groups will ultimately prevail, no man can predict;
for we well know that many groups, formerly most extensively developed,
have now become extinct. Looking still more remotely to the future, we may
predict that, owing to the continued and steady increase of the larger
groups, a multitude of smaller groups will become utterly extinct, and
leave no modified descendants; and consequently that of the species living
at any one period, extremely few will transmit descendants to a remote
futurity. I shall have to return to this subject in the chapter on
Classification, but I may add that on this view of extremely few of the
more ancient species having transmitted descendants, and on the view of all
the descendants of the same species making a class, we can understand how
it is that there exist but very few classes in each main division of the
animal and vegetable kingdoms. Although extremely few of the most ancient
species may now have living and modified descendants, yet at the most
remote geological period, the earth may have been as well peopled with many
species of many genera, families, orders, and classes, as at the present
day.
Summary of Chapter -- If during the long course of ages and under varying
conditions of life, organic beings vary at all in the several parts of
their organisation, and I think this cannot be disputed; if there be, owing
to the high geometrical powers of increase of each species, at some age,
season, or year, a severe struggle for life, and this certainly cannot be
disputed; then, considering the infinite complexity of the relations of all
organic beings to each other and to their conditions of existence, causing
an infinite diversity in structure, constitution, and habits, to be
advantageous to them, I think it would be a most extraordinary fact if no
variation ever had occurred useful to each being's own welfare, in the same
way as so many variations have occurred useful to man. But if variations
useful to any organic being do occur, assuredly individuals thus
characterised will have the best chance of being preserved in the struggle
for life; and from the strong principle of inheritance they will tend to
produce offspring similarly characterised. This principle of preservation,
I have called, for the sake of brevity, Natural Selection. Natural
selection, on the principle of qualities being inherited at corresponding
ages, can modify the egg, seed, or young, as easily as the adult. Amongst
many animals, sexual selection will give its aid to ordinary selection, by
assuring to the most vigorous and best adapted males the greatest number of
offspring. Sexual selection will also give characters useful to the males
alone, in their struggles with other males.
Whether natural selection has really thus acted in nature, in modifying and
adapting the various forms of life to their several conditions and
stations, must be judged of by the general tenour and balance of evidence
given in the following chapters. But we already see how it entails
extinction; and how largely extinction has acted in the world's history,
geology plainly declares. Natural selection, also, leads to divergence of
character; for more living beings can be supported on the same area the
more they diverge in structure, habits, and constitution, of which we see
proof by looking at the inhabitants of any small spot or at naturalised
productions. Therefore during the modification of the descendants of any
one species, and during the incessant struggle of all species to increase
in numbers, the more diversified these descendants become, the better will
be their chance of succeeding in the battle of life. Thus the small
differences distinguishing varieties of the same species, will steadily
tend to increase till they come to equal the greater differences between
species of the same genus, or even of distinct genera.
We have seen that it is the common, the widely-diffused, and widely-ranging
species, belonging to the larger genera, which vary most; and these will
tend to transmit to their modified offspring that superiority which now
makes them dominant in their own countries. Natural selection, as has just
been remarked, leads to divergence of character and to much extinction of
the less improved and intermediate forms of life. On these principles, I
believe, the nature of the affinities of all organic beings may be
explained. It is a truly wonderful fact--the wonder of which we are apt to
overlook from familiarity--that all animals and all plants throughout all
time and space should be related to each other in group subordinate to
group, in the manner which we everywhere behold--namely, varieties of the
same species most closely related together, species of the same genus less
closely and unequally related together, forming sections and sub-genera,
species of distinct genera much less closely related, and genera related in
different degrees, forming sub-families, families, orders, sub-classes, and
classes. The several subordinate groups in any class cannot be ranked in a
single file, but seem rather to be clustered round points, and these round
other points, and so on in almost endless cycles. On the view that each
species has been independently created, I can see no explanation of this
great fact in the classification of all organic beings; but, to the best of
my judgment, it is explained through inheritance and the complex action of
natural selection, entailing extinction and divergence of character, as we
have seen illustrated in the diagram.
The affinities of all the beings of the same class have sometimes been
represented by a great tree. I believe this simile largely speaks the
truth. The green and budding twigs may represent existing species; and
those produced during each former year may represent the long succession of
extinct species. At each period of growth all the growing twigs have tried
to branch out on all sides, and to overtop and kill the surrounding twigs
and branches, in the same manner as species and groups of species have
tried to overmaster other species in the great battle for life. The limbs
divided into great branches, and these into lesser and lesser branches,
were themselves once, when the tree was small, budding twigs; and this
connexion of the former and present buds by ramifying branches may well
represent the classification of all extinct and living species in groups
subordinate to groups. Of the many twigs which flourished when the tree
was a mere bush, only two or three, now grown into great branches, yet
survive and bear all the other branches; so with the species which lived
during long-past geological periods, very few now have living and modified
descendants. From the first growth of the tree, many a limb and branch has
decayed and dropped off; and these lost branches of various sizes may
represent those whole orders, families, and genera which have now no living
representatives, and which are known to us only from having been found in a
fossil state. As we here and there see a thin straggling branch springing
from a fork low down in a tree, and which by some chance has been favoured
and is still alive on its summit, so we occasionally see an animal like the
Ornithorhynchus or Lepidosiren, which in some small degree connects by its
affinities two large branches of life, and which has apparently been saved
from fatal competition by having inhabited a protected station. As buds
give rise by growth to fresh buds, and these, if vigorous, branch out and
overtop on all sides many a feebler branch, so by generation I believe it
has been with the great Tree of Life, which fills with its dead and broken
branches the crust of the earth, and covers the surface with its ever
branching and beautiful ramifications.
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