The Origin of Species

by Charles Darwin

why intermediate varieties should not endure for very long periods;--why as

  a general rule they should be exterminated and disappear, sooner than the

  forms which they originally linked together.

  For any form existing in lesser numbers would, as already remarked, run a

  greater chance of being exterminated than one existing in large numbers;

  and in this particular case the intermediate form would be eminently liable

  to the inroads of closely allied forms existing on both sides of it. But a

  far more important consideration, as I believe, is that, during the process

  of further modification, by which two varieties are supposed on my theory

  to be converted and perfected into two distinct species, the two which

  exist in larger numbers from inhabiting larger areas, will have a great

  advantage over the intermediate variety, which exists in smaller numbers in

  a narrow and intermediate zone. For forms existing in larger numbers will

  always have a better chance, within any given period, of presenting further

  favourable variations for natural selection to seize on, than will the

  rarer forms which exist in lesser numbers. Hence, the more common forms,

  in the race for life, will tend to beat and supplant the less common forms,

  for these will be more slowly modified and improved. It is the same

  principle which, as I believe, accounts for the common species in each

  country, as shown in the second chapter, presenting on an average a greater

  number of well-marked varieties than do the rarer species. I may

  illustrate what I mean by supposing three varieties of sheep to be kept,

  one adapted to an extensive mountainous region; a second to a comparatively

  narrow, hilly tract; and a third to wide plains at the base; and that the

  inhabitants are all trying with equal steadiness and skill to improve their

  stocks by selection; the chances in this case will be strongly in favour of

  the great holders on the mountains or on the plains improving their breeds

  more quickly than the small holders on the intermediate narrow, hilly

  tract; and consequently the improved mountain or plain breed will soon take

  the place of the less improved hill breed; and thus the two breeds, which

  originally existed in greater numbers, will come into close contact with

  each other, without the interposition of the supplanted, intermediate

  hill-variety.

  To sum up, I believe that species come to be tolerably well-defined

  objects, and do not at any one period present an inextricable chaos of

  varying and intermediate links: firstly, because new varieties are very

  slowly formed, for variation is a very slow process, and natural selection

  can do nothing until favourable variations chance to occur, and until a

  place in the natural polity of the country can be better filled by some

  modification of some one or more of its inhabitants. And such new places

  will depend on slow changes of climate, or on the occasional immigration of

  new inhabitants, and, probably, in a still more important degree, on some

  of the old inhabitants becoming slowly modified, with the new forms thus

  produced and the old ones acting and reacting on each other. So that, in

  any one region and at any one time, we ought only to see a few species

  presenting slight modifications of structure in some degree permanent; and

  this assuredly we do see.

  Secondly, areas now continuous must often have existed within the recent

  period in isolated portions, in which many forms, more especially amongst

  the classes which unite for each birth and wander much, may have separately

  been rendered sufficiently distinct to rank as representative species. In

  this case, intermediate varieties between the several representative

  species and their common parent, must formerly have existed in each broken

  portion of the land, but these links will have been supplanted and

  exterminated during the process of natural selection, so that they will no

  longer exist in a living state.

  Thirdly, when two or more varieties have been formed in different portions

  of a strictly continuous area, intermediate varieties will, it is probable,

  at first have been formed in the intermediate zones, but they will

  generally have had a short duration. For these intermediate varieties

  will, from reasons already assigned (namely from what we know of the actual

  distribution of closely allied or representative species, and likewise of

  acknowledged varieties), exist in the intermediate zones in lesser numbers

  than the varieties which they tend to connect. From this cause alone the

  intermediate varieties will be liable to accidental extermination; and

  during the process of further modification through natural selection, they

  will almost certainly be beaten and supplanted by the forms which they

  connect; for these from existing in greater numbers will, in the aggregate,

  present more variation, and thus be further improved through natural

  selection and gain further advantages.

  Lastly, looking not to any one time, but to all time, if my theory be true,

  numberless intermediate varieties, linking most closely all the species of

  the same group together, must assuredly have existed; but the very process

  of natural selection constantly tends, as has been so often remarked, to

  exterminate the parent forms and the intermediate links. Consequently

  evidence of their former existence could be found only amongst fossil

  remains, which are preserved, as we shall in a future chapter attempt to

  show, in an extremely imperfect and intermittent record.

  On the origin and transitions of organic beings with peculiar habits and

  structure. -- It has been asked by the opponents of such views as I hold,

  how, for instance, a land carnivorous animal could have been converted into

  one with aquatic habits; for how could the animal in its transitional state

  have subsisted? It would be easy to show that within the same group

  carnivorous animals exist having every intermediate grade between truly

  aquatic and strictly terrestrial habits; and as each exists by a struggle

  for life, it is clear that each is well adapted in its habits to its place

  in nature. Look at the Mustela vison of North America, which has webbed

  feet and which resembles an otter in its fur, short legs, and form of tail;

  during summer this animal dives for and preys on fish, but during the long

  winter it leaves the frozen waters, and preys like other polecats on mice

  and land animals. If a different case had been taken, and it had been

  asked how an insectivorous quadruped could possibly have been converted

  into a flying bat, the question would have been far more difficult, and I

  could have given no answer. Yet I think such difficulties have very little

  weight.

  Here, as on other occasions, I lie under a heavy disadvantage, for out of

  the many striking cases which I have collected, I can give only one or two

  instances of transitional habits and structures in closely allied species

  of the same genus; and of diversified habits, either constant or

  occasional, in the same species. And it seems to me that nothing less than

  a long list of such cases is sufficient to lessen the d
ifficulty in any

  particular case like that of the bat.

  Look at the family of squirrels; here we have the finest gradation from

  animals with their tails only slightly flattened, and from others, as Sir

  J. Richardson has remarked, with the posterior part of their bodies rather

  wide and with the skin on their flanks rather full, to the so-called flying

  squirrels; and flying squirrels have their limbs and even the base of the

  tail united by a broad expanse of skin, which serves as a parachute and

  allows them to glide through the air to an astonishing distance from tree

  to tree. We cannot doubt that each structure is of use to each kind of

  squirrel in its own country, by enabling it to escape birds or beasts of

  prey, or to collect food more quickly, or, as there is reason to believe,

  by lessening the danger from occasional falls. But it does not follow from

  this fact that the structure of each squirrel is the best that it is

  possible to conceive under all natural conditions. Let the climate and

  vegetation change, let other competing rodents or new beasts of prey

  immigrate, or old ones become modified, and all analogy would lead us to

  believe that some at least of the squirrels would decrease in numbers or

  become exterminated, unless they also became modified and improved in

  structure in a corresponding manner. Therefore, I can see no difficulty,

  more especially under changing conditions of life, in the continued

  preservation of individuals with fuller and fuller flank-membranes, each

  modification being useful, each being propagated, until by the accumulated

  effects of this process of natural selection, a perfect so-called flying

  squirrel was produced.

  Now look at the Galeopithecus or flying lemur, which formerly was falsely

  ranked amongst bats. It has an extremely wide flank-membrane, stretching

  from the corners of the jaw to the tail, and including the limbs and the

  elongated fingers: the flank membrane is, also, furnished with an extensor

  muscle. Although no graduated links of structure, fitted for gliding

  through the air, now connect the Galeopithecus with the other Lemuridae,

  yet I can see no difficulty in supposing that such links formerly existed,

  and that each had been formed by the same steps as in the case of the less

  perfectly gliding squirrels; and that each grade of structure had been

  useful to its possessor. Nor can I see any insuperable difficulty in

  further believing it possible that the membrane-connected fingers and

  fore-arm of the Galeopithecus might be greatly lengthened by natural

  selection; and this, as far as the organs of flight are concerned, would

  convert it into a bat. In bats which have the wing-membrane extended from

  the top of the shoulder to the tail, including the hind-legs, we perhaps

  see traces of an apparatus originally constructed for gliding through the

  air rather than for flight.

  If about a dozen genera of birds had become extinct or were unknown, who

  would have ventured to have surmised that birds might have existed which

  used their wings solely as flappers, like the logger-headed duck

  (Micropterus of Eyton); as fins in the water and front legs on the land,

  like the penguin; as sails, like the ostrich; and functionally for no

  purpose, like the Apteryx. Yet the structure of each of these birds is

  good for it, under the conditions of life to which it is exposed, for each

  has to live by a struggle; but it is not necessarily the best possible

  under all possible conditions. It must not be inferred from these remarks

  that any of the grades of wing-structure here alluded to, which perhaps may

  all have resulted from disuse, indicate the natural steps by which birds

  have acquired their perfect power of flight; but they serve, at least, to

  show what diversified means of transition are possible.

  Seeing that a few members of such water-breathing classes as the Crustacea

  and Mollusca are adapted to live on the land, and seeing that we have

  flying birds and mammals, flying insects of the most diversified types, and

  formerly had flying reptiles, it is conceivable that flying-fish, which now

  glide far through the air, slightly rising and turning by the aid of their

  fluttering fins, might have been modified into perfectly winged animals.

  If this had been effected, who would have ever imagined that in an early

  transitional state they had been inhabitants of the open ocean, and had

  used their incipient organs of flight exclusively, as far as we know, to

  escape being devoured by other fish?

  When we see any structure highly perfected for any particular habit, as the

  wings of a bird for flight, we should bear in mind that animals displaying

  early transitional grades of the structure will seldom continue to exist to

  the present day, for they will have been supplanted by the very process of

  perfection through natural selection. Furthermore, we may conclude that

  transitional grades between structures fitted for very different habits of

  life will rarely have been developed at an early period in great numbers

  and under many subordinate forms. Thus, to return to our imaginary

  illustration of the flying-fish, it does not seem probable that fishes

  capable of true flight would have been developed under many subordinate

  forms, for taking prey of many kinds in many ways, on the land and in the

  water, until their organs of flight had come to a high stage of perfection,

  so as to have given them a decided advantage over other animals in the

  battle for life. Hence the chance of discovering species with transitional

  grades of structure in a fossil condition will always be less, from their

  having existed in lesser numbers, than in the case of species with fully

  developed structures.

  I will now give two or three instances of diversified and of changed habits

  in the individuals of the same species. When either case occurs, it would

  be easy for natural selection to fit the animal, by some modification of

  its structure, for its changed habits, or exclusively for one of its

  several different habits. But it is difficult to tell, and immaterial for

  us, whether habits generally change first and structure afterwards; or

  whether slight modifications of structure lead to changed habits; both

  probably often change almost simultaneously. Of cases of changed habits it

  will suffice merely to allude to that of the many British insects which now

  feed on exotic plants, or exclusively on artificial substances. Of

  diversified habits innumerable instances could be given: I have often

  watched a tyrant flycatcher (Saurophagus sulphuratus) in South America,

  hovering over one spot and then proceeding to another, like a kestrel, and

  at other times standing stationary on the margin of water, and then dashing

  like a kingfisher at a fish. In our own country the larger titmouse (Parus

  major) may be seen climbing branches, almost like a creeper; it often, like

  a shrike, kills small birds by blows on the head; and I have many times

  seen and heard it hammering the seeds of the yew on a branch, and thus

  breaking them like a nuthatch. In North America the black bear was seen by
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br />   Hearne swimming for hours with widely open mouth, thus catching, like a

  whale, insects in the water. Even in so extreme a case as this, if the

  supply of insects were constant, and if better adapted competitors did not

  already exist in the country, I can see no difficulty in a race of bears

  being rendered, by natural selection, more and more aquatic in their

  structure and habits, with larger and larger mouths, till a creature was

  produced as monstrous as a whale.

  As we sometimes see individuals of a species following habits widely

  different from those both of their own species and of the other species of

  the same genus, we might expect, on my theory, that such individuals would

  occasionally have given rise to new species, having anomalous habits, and

  with their structure either slightly or considerably modified from that of

  their proper type. And such instances do occur in nature. Can a more

  striking instance of adaptation be given than that of a woodpecker for

  climbing trees and for seizing insects in the chinks of the bark? Yet in

  North America there are woodpeckers which feed largely on fruit, and others

  with elongated wings which chase insects on the wing; and on the plains of

  La Plata, where not a tree grows, there is a woodpecker, which in every

  essential part of its organisation, even in its colouring, in the harsh

  tone of its voice, and undulatory flight, told me plainly of its close

  blood-relationship to our common species; yet it is a woodpecker which

  never climbs a tree!

  Petrels are the most aerial and oceanic of birds, yet in the quiet Sounds

  of Tierra del Fuego, the Puffinuria berardi, in its general habits, in its

  astonishing power of diving, its manner of swimming, and of flying when

  unwillingly it takes flight, would be mistaken by any one for an auk or

  grebe; nevertheless, it is essentially a petrel, but with many parts of its

  organisation profoundly modified. On the other hand, the acutest observer

  by examining the dead body of the water-ouzel would never have suspected

  its sub-aquatic habits; yet this anomalous member of the strictly

  terrestrial thrush family wholly subsists by diving,--grasping the stones

  with its feet and using its wings under water.

  He who believes that each being has been created as we now see it, must

  occasionally have felt surprise when he has met with an animal having

  habits and structure not at all in agreement. What can be plainer than

  that the webbed feet of ducks and geese are formed for swimming? yet there

  are upland geese with webbed feet which rarely or never go near the water;

  and no one except Audubon has seen the frigate-bird, which has all its four

  toes webbed, alight on the surface of the sea. On the other hand, grebes

  and coots are eminently aquatic, although their toes are only bordered by

  membrane. What seems plainer than that the long toes of grallatores are

  formed for walking over swamps and floating plants, yet the water-hen is

  nearly as aquatic as the coot; and the landrail nearly as terrestrial as

  the quail or partridge. In such cases, and many others could be given,

  habits have changed without a corresponding change of structure. The

  webbed feet of the upland goose may be said to have become rudimentary in

  function, though not in structure. In the frigate-bird, the deeply-scooped

  membrane between the toes shows that structure has begun to change.

  He who believes in separate and innumerable acts of creation will say, that

  in these cases it has pleased the Creator to cause a being of one type to

  take the place of one of another type; but this seems to me only restating

  the fact in dignified language. He who believes in the struggle for

  existence and in the principle of natural selection, will acknowledge that

  every organic being is constantly endeavouring to increase in numbers; and