The Origin of Species

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The Origin of Species Page 13

by Charles Darwin

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 th
e 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 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
r />   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,

 

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