Chapter 4
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 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.
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 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.
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