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
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