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-^ut it may "be urged that when several closely allied species inhabit the
same territory, we surely ought to find at the present time many transitional
forms. Let us take a simple case: in travelling from north to south over a
continent we generally meet at successive intervals with closely allied or
representative species, evidently filling nearly the same place m the natural
economy of the land. These representative species often meet and interlock;
and as the one becomes rarer and rarer, the other becomes more and more
frequent till the one replaces the other. But if we compare these species
where they intermingle, they are generally as absolutely distinct from each
other in every detail of structure as are specimens taken from the metropolis
inhabited by each. By my theory these alHed species are descended from a
common parent; and during the process of modification, each has become
adapted to the conditions of life of its own region, and has supplanted and
exterminated its original parent-form and all the transitional varieties
between its past and present states. Hence we ought not to expect at the
present time to meet with numerous transitional varieties m each region
though they must have existed there, and may be imbedded there m a fossil
condition. But in the intermediate region, having intermediate conditions ot
life why do we not now find closely linking intermediate varieties.'' I his
difficulty for a long time quite confounded me. But I think it can be m large

part explained. , . . . r • i_

In the first place we should be extremely cautious m inferring, because an
area is now continuous, that it has been continuous during a long period.
Geology would lead us to believe that most continents have been broken up
into islands even during the later tertiary periods; and m such islands
distinct species might have been separately formed without the possibility ot
intermediate varieties existing in the intermediate zones. By changes m the
form of the land and of climate, marine areas now continuous must of ten have
existed within recent times in a far less continuous and umform condition
than at present. But I will pass over this way of escaping from the difficulty;
for I believe that many perfectly defined species have been formed on strictly
continuous areas; though I do not doubt that the formerly broken condi ion
of areas now continuous, has played an important part in the formation
of new species, more especially with freely crossing and wandering ammals.
In looking at species as they are now distributed over a wide area we gen-
erally find them tolerably numerous over a large territory then becoming
somewhat abruptly rarer and rarer on the confines, and finally disappearing.
Hence the neutral territory between two representative species is generally


narrow in comparison with the territory proper to each. We see the same
fact in ascending mountains, and sometimes it is quite remarkable how
abruptly, as Alph. de Candolle has observed, a common alpine species dis-
appears. The same fact has been noticed by E. Forbes in sounding the depths
of the sea with the dredge. To those who look at climate and the physical
conditions of life as the all-important elements of distribution, these facts
ought to cause surprise, as climate and height or depth graduate away in-
sensibly. But when we bear in mind that almost every species, even in its
metropolis, would increase immensely in numbers, were it not for other com-
peting species; that nearly all either prey on or serve as prey for others; in
short, that each organic being is either directly or indirectly related in the
most important manner to other organic beings — ^we see that the range of the
inhabitants of any country by no means exclusively depends on insensibly
changing physical conditions, but in a large part on the presence of other
species, on which it lives, or by which it is destroyed, or with which it comes
into competition; and as these species are already defined objects, not blend-
ing one into another by insensible gradations, the range of any one species,
depending as it does on the range of others, will tend to be sharply defined.
Moreover, each species on the confines of its range, where it exists in lessened
numbers, will, during fluctuations in the number of its enemies or of its prey,
or in the nature of the seasons, be extremely liable to utter extermination;
and thus its geographical range will come to be still more sharply defined.

As allied or representative species, when inhabiting a continuous area, are
generally distributed in such a manner that each has a wide range, with a
comparatively narrow neutral territory between them, in which they become
rather suddenly rarer and rarer; then, as varieties do not essentially differ
from species, the same rule will probably apply to both; and if we take a
varying species inhabiting a very large area, we shall have to adapt two varie-
ties to two large areas, and a third variety to a narrow intermediate zone. The
intermediate variety, consequently, will exist in lesser numbers from inhabit-
ing a narrow and lesser area; and practically, as far as I can make out, this
rule holds good with varieties in a state of nature. I have met with striking
instances of the rule in the case of varieties intermediate between well-marked
varieties in the genus Balanus. And it would appear from information given
me by Mr. Watson, Dr. Asa Gray and Mr. Wollaston, that generally, when
varieties intermediate between two other forms occur, they are much rarer
numerically than the forms which they connect. Now, if we may trust these
facts and inferences, and conclude that varieties linking two other varieties
together generally have existed in lesser numbers than the forms which they
connect, then we can understand 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 it is a


far more important consideration, that during the process of further modifi-
cation, by which two varieties are supposed to be converted and perfected
into two distinct species, the two which exist in larger numbers, from inhabit-
ing 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 have a better chance, within any given
period, of presenting further favorable 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 the wide plains at the base; and that the inhabit-
ants are all trying with equal steadiness and skill to improve their stocks by
selection ; the chances in this case will be strongly in favor of the great hold-
ers on the mountains or on the plains, improving their breeds more quickly
than the small holders on the intermediate narrow, hilly tract; and conse-
quently 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: first, because new varieties are very slowly formed, for
variation is a slow process, and natural selection can do nothing until favor-
able individual dififerences or variations occur, and until a place in the natu-
ral 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 inhabit-
ants becoming slowly modified, with the new forms thus produced and the
old one acting and reacting on each other. So that, in any one region and at
any one time, we ought to see only a few species presenting slight modifica-
tions of structure in some degree permanent; and this assuredly we do see.

Secondly, areas now continuous must often have existed within the recent
period as isolated portions, in which many forms, more especially among 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 within each isolated por-
tion of the land, but these links during the process of natural selection will
have been supplanted and exterminated, so that they will no longer be found
in a living state.


Thirdly, when two or more varieties have been formed in different por-
tions of a strictly continuous area, intermediate varieties will, it is probable
at first have been formed in the intermediate zones, but they will generall)
have had a short duration. For these intermediate varieties will, from reason*
already assigned (namely from what we know of the actual distribution oi
closely allied or representative species, and likewise of acknowledged varie-
ties), 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 varieties, and thus be
further improved through natural selection and gain further advantages.

Lastly, looking not to any one time, but at all time, if my theory be true,
numberless intermediate varieties, linking closely together all the species of
the same group, 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 among fossil remains, which are preserved,
as we shall attempt to show in a future chapter, in an extremely imperfect
and intermittent record.



It has been asked by the opponents of such views as I hold, how, for in-
stance, could a land carnivorous animal have been converted into. one with
aquatic habits; for how could the animal in its transitional state have sub-
sisted? It would be easy to show that there now exist carnivorous animals
presenting close intermediate grades from strictly terrestrial to aquatic habits ;
and as each exists by a struggle for life, it is clear that each must be well
adapted 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 the summer this animal dives for and preys on fish,
but during the long winter it leaves the frozen waters, and preys, like other
pole-cats, 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
to answer. Yet I think such difficulties have 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 allied species ; and of diversi-
fied 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
difficulty 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 squir-
rels ; 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, to collect food more
quickly, or, as there is reason to believe, to lessen 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 possible 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 become 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 proc-
ess of natural selection, a perfect so-called flying squirrel was produced.

Now look at the Galeopithecus or so-called flying lemur, which was for-
merly ranked among bats, but is now believed to belong to the Insectivora.
An extremely wide flank-membrane stretches from the corners of the jaw to
the tail, and includes the limbs with the elongated fingers. This flank-mem-
brane is furnished with an extensor muscle. Although no graduated links of
structure, fitted for gliding through the air, now connect the Galeopithecus
with the other Insectivora, yet there is no difficulty in supposing that such
links formerly existed, and that each was developed in the same manner as
with the less perfectly gliding squirrels; each grade of structure having been
useful to its possessor. Nor can I see any insuperable difficulty in further be-
lieving that the membrane connecting fingers and forearm of the Galeopithe-
cus might have been greatly lengthened by natural selection; and this, as far
as the organs of flight are concerned, would have converted the animal into
a bat. In certain bats in which the wing-membrane extends from the top of
the shoulder to the tail and includes the hind-legs, we perhaps see traces of
an apparatus originally fitted for gliding through the air rather than for

If about a dozen genera of birds were to become extinct, who would have
ventured to surmise 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 as front-legs on the land, like the penguin; as sails, like
the ostrich; and functionally for no purpose, like the apteryx? Yet the struc-
ture of each of these birds is good for it, under 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 per-


haps may all be the result of disuse, indicate the steps by which birds actually
acquired their perfect power of flight; but they serve to show what diversified
means of transition are at least possible.

Seeing that a few members of such water-breathing classes as the Crus-
tacea 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 Ijad 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 the inhabitants of the open ocean, and had used their
incipient organs of flight exclusively, so far as we know, to escape being de-
voured 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 display-
ing early transitional grades of the structure will seldom have survived to the
present day, for they will have been supplanted by their successors, which
were gradually rendered more perfect through natural selection. Further-
more, we may conclude that transitional states 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, both of diversified and of changed
habits, in the individuals of the same species. In either case it would be easy
for natural selection to adapt the structure of the animal to its changed
habits, or exclusively to one of its several habits. It is, however, difficult to
decide and immaterial for us, whether habits generally change first and struc-
ture afterward; or whether slight modifications of structure lead to changed
habits; both probably often occurring almost simultaneously. Of cases of
changed habits it will suffice merely to allude to that of the many British in-
sects 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
into it 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
sometimes, 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 Hke a nuthatch. In North America the black bear
was seen by Hearne swimming for hours with widely open mouth, thus
catching, almost hke a whale, insects in the water.

As we sometimes see individuals following habits different from those
proper to their species and to the other species of the same genus, we might
expect that such individuals would occasionally give rise to new species, hav-
ing anomalous habits, and with their structure either slightly or considerably
modified from that of their type. And such instances occur in nature. Can a
more striking instance of adaptation be given than that of a woodpecker for
climbing trees and 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. On the plains of La Plata,
where hardly a tree grows, there is a woodpecker (Colaptes campestris)
which has two toes before and two behind, a long-pointed tongue, pointed
tail-feathers, sufficiently stiff to support the bird in a vertical position on a
post, but not so stiff as in the typical woodpeckers, and a straight, strong beak.
The beak, however, is not so straight or so strong as in the typical woodpeck-
ers, but it is strong enough to bore into wood. Hence this Colaptes, in all the
essential parts of its structure, is a woodpecker. Even in such trifling charac-
ters as the coloring, the harsh tone of the voice, and undulatory flight, its
close blood-relationship to our common woodpecker is plainly declared; yet,
as I can assert, not only from my own observations, but from those of the
accurate Azara, in certain large districts it does not climb trees, and it makes
its nest in holes in banks ! In certain other districts, however, this same wood-
pecker, as Mr. Hudson states, frequents trees, and bores holes in the trunk
for its nest. I may mention as another illustration of the varied habits of this
genus, that a Mexican Colaptes has been described by De Saussure as boring
holes into hard wood in order to lay up a store of acorns.

Petrels are the most aerial and oceanic of birds, but, in the quiet sounds of
Tierra del Fuego, the Puffinuria berardi, in its general habits, in its astonish-
ing power of diving, in its manner of swimming and of flying when made to
take flight, would be mistaken by any one for an auk or a grebe; nevertheless
it is essentially a petrel, but with many parts of its organization profoundly
modified in relation to its new habits of life; whereas the woodpecker of La
Plata has had its structure only slightly modified. In the case of the water-
ouzel, the acutest observer, by examining its dead body, would never have
suspected its sub-aquatic habits; yet this bird, which is allied to the thrush
family, subsists by diving — using its wings under water, and grasping stones
with its feet. All the members of the great order of Hymenopterous insects
are terrestrial, excepting the genus Proctotrupes, which Sir John Lubbock
has discovered to be aquatic in its habits; it often enters the water and dives
about by the use not of its legs but of its wings, and remains as long as four
hours beneath the surface; yet it exhibits no modification in structure in ac-
cordance with its abnormal habits.

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 in agreement. What can be plainer than that the webbec
feet of ducks and geese are formed for swimming? Yet there are upland geesi
with webbed feet which rarely go near the water; and no one, except Audui
bon, has seen the frigate-bird, which has all its four toes webbed, alight on
the surface of the ocean. On the other hand, grebes and coots are eminenth
aquatic, although their toes are only bordered by membrane. What seemj
plainer than that the long toes, not furnished with membrane of the Gralla4
tores, are formed for walking over swamps and floating plants? The water
hen and landrail are members of this order, yet the first is nearly as aquatici
as the coot, and the second is nearly as terrestrial as the quail or partridges
In such cases, and many others could be given, habits have changed without ai
corresponding change of structure. The webbed feet of the upland goose may
be said to have become almost rudimentary in function, though not in struc-:
ture. In the frigate-bird, the deeply scooped membrane between the toes'

Online LibraryCharles DarwinThe origin of species → online text (page 16 of 50)