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where the contest between individual and individual must be very 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 in-
sects on small and uniform islets : also in small ponds of fresh water. Farmers
find that they can raise more food by a rotation of plants belonging to the
most different orders : nature follows what may be called a simultaneous ro-
tation. Most of the animals and plants which Hve close round any small piece
of ground, could live on it (supposing its nature not to be in any way
peculiar), 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, 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 naturalization of plants through man's
agency in foreign lands. It might have been expected that the plants which
would succeed in becoming naturalized in any land would generally have
been closely allied to the indigenes; for these are commonly looked at as spe-
cially created and adapted for their own country. It might also, perhaps, have
been expected that naturalized 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 CandoUe has well remarked, in his great and
admirable work, that floras gain by naturalization, 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 "Man-
ual of the Flora of the Northern United States," 260 naturalized plants are
enumerated, and these belong to 162 genera. We thus see that these natural-



74 THE ORIGIN OF SPECIES

ized plants are of a highly diversified nature. They differ, moreover, to a large
extent, from the indigenes, for out of the 1 62 naturalized genera, no less than
100 genera are not there indigenous, and thus a large proportional addition
is made to the genera now living in the United States.

By considering the nature of the plants or animals which have in any coun-
try struggled successfully with the indigenes, and have there become natural-
ized, we may gain some crude idea in what manner some of the natives would
have to be modified in order to gain an advantage over their compatriots;
and we may at least infer that diversification of structure, amounting to new
generic differences, would be profitable to them.

The advantage of diversification of structure in the inhabitants of the same
region is, in fact, the same as that of the physiological division of labor in the
organs of the same individual body — a subject so well elucidated by Milne
Edwards. No physiologist doubts that a stomach 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 ani-
mals 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 organization but little diversified, could hardly compete
with a set more perfectly diversified in structure. It may be doubted, for in-
stance, whether the AustraUan marsupials, which are divided into groups
differing but Httle from each other, and feebly representing, as Mr. Water-
house and others have remarked, our carnivorous, ruminant, and rodent
mammals, could successfully compete with these well-developed orders. In
the Australian mammals, we see the process of diversification in an early and
incomplete stage of improvement.

THE PROBABLE EFFECTS OF THE ACTION OF NATURAL SELECTION THROUGH
DIVERGENCE OF CHARACTER AND EXTINCTION, ON THE DESCENDANTS OF A
COMMON ANCESTOR

After the foregoing discussion, which has been much compressed, we may
assume that the modified descendants of any one species will succeed 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 prin-
ciple of benefit being derived from divergence of character, combined with
the principles of natural selection and of extinction, tends to act.

The accompanying diagram will aid us in understanding this rather per-
plexing 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 de-
grees, as is so generally the case in nature, and as is represented in the dia-
gram by the letters standing at unequal distances. I have said a large genus,
because, as we saw in the second chapter, on an average more species vary in
large genera than in 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 most widely diffused, vary more than do the



NATURAL SELECTION 75

rare and restricted species. Let (A) be a common, widely- diffused, and vary-
ing species, belonging to a genus large in its own country. The branching and
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 simulta-
neously, 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 profit-
able will be preserved or naturally selected. And here the importance of the
principle of benefit 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 selec-
tion. 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 sup-
posed to have been accumulated to form it into 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 or more generations. After a thousand generations, species
(A) is supposed to have produced two fairly well-marked varieties, namely a^
and m^. These two varieties will generally still be exposed to the same condi-
tions which made their parents variable, and the tendency to variability is in
itself hereditary; consequently they will likewise tend to vary, and commonly
in nearly the same manner as did their parents. Moreover, these two varieties,
being only slightly modified forms, will tend to inherit those advantages
which made their parent (A) more numerous than most of the other inhabit-
ants of the same country; they will also partake of those more general ad-
vantages which made the genus to which the parent species belonged, a large
genus in its own country. And all these circumstances are favorable to the
production of new varieties.

If, then, these two varieties be variable, the most divergent of their varia-
tions will generally be preserved during the next thousand generations. And
after this interval, variety a^ is supposed in the diagram to have produced
variety a^, which will, owing to the principle of divergence, differ more from
(A) than did variety a^. Variety m^ is supposed to have produced two varie-
ties, namely m^ and s^, 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 gen-
erations, 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 of 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 gen^
eration.

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 some-
what irregular, nor that it goes on continuously; it is far more probable that



76



THE ORIGIN OF SPECIES



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78 THE ORIGIN OF SPECIES

each form remains for long periods unaltered, and then again undergoes
modification. Nor do I suppose that the most divergent varieties are invari-
ably preserved : 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 com-
plex 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 increase.
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 allow the
accumulation of a considerable amount of divergent variation.

As all the modified descendants from a common and widely-diffused spe-
cies, 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 multiply-
ing in number as well as diverging in character: this is represented in the
diagram by the several divergent branches proceeding from ( A) . The modi-
fied 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 no doubt the process of modification will be confined to a single
line of descent, and the number of modified descendants will not be in-
creased ; although the amount of divergent modification may have been aug-
mented. This case would be represented in the diagram, if all the lines pro-
ceeding from (A) were removed, excepting that from cD- to a}-^. In the same
way 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, a^°, /^^, and m^°, which, from having diverged in character dur-
ing 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 ex-
cessively small, these three forms may still be only well-marked varieties ; 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 doubtful
or at least 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
a}-^ and m^^, all descended from ( A) . Thus, as I believe, species are multi-
plied, and genera are formed.



NATURAL SELECTION 79

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 (w^^ and z^^) or two species, according to the amount of change
supposed to be represented between the horizontal lines. After fourteen thou-
sand generations, six new species, marked by the letters n^* to z^^, are sup-
posed to have been produced. In any genus, the species which are already
very different in character from each other, will generally tend to produce
the greatest number of modified descendants; for these will have the best
chance of seizing on 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 long but unequal periods continue to
transmit unaltered descendants; and this is shown in the diagram by the
dotted lines unequally prolonged upward.

But during the process of modification, represented in the diagram, an-
other of our principles, namely that of extinction, will have played an im-
portant 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 progenitor. For it should be remembered that
the competition will generally be rnost 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 states of the same 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. If, however, the modified offspring of a species get
into some distinct country, or become quickly adapted to some quite new
station, in which offspring and progenitor 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 ex-
tinct, being replaced by eight new species (a^* to m"), and species (I) will
be replaced by six (n^* to 2^*) 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 ad-
vantage over most of the other species of the genus. Their modified descend-
ants, fourteen in number at the fourteen-thousandth generation, will prob-
ably have inherited some of the same advantages; they have also been modi-



80 THE ORIGIN OF SPECIES

fied 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, extremely probable that they will have taken the
places of, and thus exterminated, not only their parents (A) and (I) , but like-
wise some of the original species which were most nearly related to their
parents. Hence very few of the original species will have transmitted off-
spring to the fourteen-thousandth generation. We may suppose that only one
(F) of the two species (E and F) 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 spe-
cies, will now be fifteen in number. Owing to the divergent tendency of natu-
ral selection, the extreme amount of difference in character between species
a}^ and z'^^ will be much greater than that between the most distinct 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 a^*, q^^, p^^, will be nearly related from having recently
branched off from a^^; b^^ and /^*, from having diverged at an earlier period
from a^, will be in some degree distinct from the three first-named species;
and lastly, o^*, e^^, and m^* will be nearly related one to the other, but, from
being 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 a distinct genus.

The six descendants from (I) will form two sub-genera or genera. But as
the original species (I) differed largely from (A), standing nearly at the
extreme end of the original genus, the six descendants from (I) will, owing
to inheritance alone, 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 impor-
tant consideration), which connected the original species (A) and (I), have
all become, except (F), extinct, and have left no descendants. Hence the six
new species descended from (I), and the eight descendants 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 be descended from some one species
of an earlier genus. In our diagram this is indicated by the broken lines be-
neath the capital letters, converging in sub-branches downward toward a
single point: this point represents a species, the supposed progenitor of our
several new sub-genera and genera.

It is worth while to reflect for a moment on the character of the new spe-
cies F^*, 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. Being descended from a form that stood be-
tween the parent-species (A) and (I), now supposed to be extinct and un-



NATURAL SELECTION 81

known, it will be in some degree intermediate in character between the two
groups descended from these two species. But as these two groups have
gone on diverging in character from the type of their parents, the new
species (f^^) will not be directly intermediate between them, but rather be-
tween types of the two groups; and every naturalist will be able to call such
cases before his mind.

In the diagram each horizontal line has hitherto been supposed to repre-
sent a thousand generations, but each may represent a million or more gen-
erations; it may also represent 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, families, or genera, with those now
living, yet are often, in some degree, intermediate in character between exist-
ing groups; and we can understand this fact, for the extinct species lived at
various remote 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 the diagram, we suppose the amount of
change represented by each successive group of diverging dotted lines to be
great, the forms marked a}^ to p^*, those marked 6^* and /^*, and those marked
o^* to 772^% will form three very distinct genera. We shall also have two very
distinct genera descended from (I), differing widely from the descendants
of (A) . These two groups of genera will thus form two distinct families, or
orders, according to the amount of divergent modification supposed to be
represented in the diagram. And the two new families, or orders, are de-
scended from two species of the original genus, and these are supposed to be
descended from some still more ancient and unknown form.

We have seen that in each country it is the species belonging to 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 number, 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 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 have as yet suffered least extinction, will, for a
long period, continue to increase. But which groups will ultimately prevail,
no man can predict; for we know that many groups, formerly most exten-



82 THE ORIGIN OF SPECIES

sively 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 ex-
tinct, and leave no modified descendants; and consequently, that, of the spe-
cies 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 classi-
fication, but I may add that as, according to this view, extremely few of the
more' ancient species have transmitted descendants to the present day, and
as all the descendants of the same species form a class, we can understand
how it is that there exist so few classes in each main division of the animal
and vegetable kingdoms. Although few of the most ancient species have left
modified descendants, yet, at remote geological periods, the earth may have
been almost as well peopled with species of many genera, families, orders,
and classes, as at the present time.

ON THE DEGREE TO WHICH ORGANIZATION TENDS TO ADVANCE

Natural selection acts exclusively by the preservation and accumulation of
variations, which are beneficial under the organic and inorganic conditions
to which each creature is exposed at all periods of life. The ultimate result is
that each creature tends to become more and more improved in relation to



Online LibraryCharles DarwinThe origin of species → online text (page 11 of 50)