The Origin of Species
Chapter 6: Difficulties on Theory
by Charles Darwin
Difficulties on the theory of descent with modification - Transitions -
Absence or rarity of transitional varieties - Transitions in habits of life - Diversified
habits in the same species - Species with habits widely different from those of their
allies - Organs of extreme perfection - Means of transition - Cases of difficulty - Natura
non facit saltum - Organs of small importance - Organs not in all cases absolutely perfect
- The law of Unity of Type and of the Conditions of Existence embraced by the theory of
Natural Selection |
Long before having arrived at this part of my work, a crowd of
difficulties will have occurred to the reader. Some of them are so grave that to this day
I can never reflect on them without being staggered; but, to the best of my judgment, the
greater number are only apparent, and those that are real are not, I think, fatal to my
theory.
These difficulties and objections may be classed under the following heads:-Firstly,
why, if species have descended from other species by insensibly fine gradations, do we not
everywhere see innumerable transitional forms? Why is not all nature in confusion instead
of the species being, as we see them, well defined?
Secondly, is it possible that an animal having, for instance, the structure and habits
of a bat, could have been formed by the modification of some animal with wholly different
habits? Can we believe that natural selection could produce, on the one hand, organs of
trifling importance, such as the tail of a giraffe, which serves as a fly-flapper, and, on
the other hand, organs of such wonderful structure, as the eye, of which we hardly as yet
fully understand the inimitable perfection?
Thirdly, can instincts be acquired and modified through natural selection? What shall
we say to so marvellous an instinct as that which leads the bee to make cells, which have
practically anticipated the discoveries of profound mathematicians?
Fourthly, how can we account for species, when crossed, being sterile and producing
sterile offspring, whereas, when varieties are crossed, their fertility is unimpaired?
The two first heads shall be here discussed Instinct and Hybridism in separate
chapters.
On the absence or rarity of transitional varieties. As natural selection acts
solely by the preservation of profitable modifications, each new form will tend in a
fully-stocked country to take the place of, and finally to exterminate, its own less
improved parent or other less-favoured forms with which it comes into competition. Thus
extinction and natural selection will, as we have seen, go hand in hand. Hence, if we look
at each species as descended from some other unknown form, both the parent and all the
transitional varieties will generally have been exterminated by the very process of
formation and perfection of the new form.
But, as by this theory innumerable transitional forms must have existed, why do we not
find them embedded in countless numbers in the crust of the earth? It will be much more
convenient to discuss this question in the chapter on the Imperfection of the geological
record; and I will here only state that I believe the answer mainly lies in the record
being incomparably less perfect than is generally supposed; the imperfection of the record
being chiefly due to organic beings not inhabiting profound depths of the sea, and to
their remains being embedded and preserved to a future age only in masses of sediment
sufficiently thick and extensive to withstand an enormous amount of future degradation;
and such fossiliferous masses can be accumulated only where much sediment is deposited on
the shallow bed of the sea, whilst it slowly subsides. These contingencies will concur
only rarely, and after enormously long intervals. Whilst the bed of the sea is stationary
or is rising, or when very little sediment is being deposited, there will be blanks in our
geological history. The crust of the earth is a vast museum; but the natural collections
have been made only at intervals of time immensely remote.
But 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 in 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
allied species have 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 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 in each region, though they must have existed there, and
may be embedded there in a fossil condition. But in the intermediate region, having
intermediate conditions of life, why do we not now find closely-linking intermediate
varieties? This difficulty for a long time quite confounded me. But I think it can be in
large part explained.
In the first place we should be extremely cautious in inferring, because an area is now
continuous, that it has been continuous during a long period. Geology would lead us to
believe that almost every continent has been broken up into islands even during the later
tertiary periods; and in such islands distinct species might have been separately formed
without the possibility of intermediate varieties existing in the intermediate zones. By
changes in the form of the land and of climate, marine areas now continuous must often
have existed within recent times in a far less continuous and uniform 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 condition of areas now continuous has played an
important part in the formation of new species, more especially with freely-crossing and
wandering animals.
In looking at species as they are now distributed over a wide area, we generally 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 disappears. The
same fact has been noticed by 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 insensibly. But when we bear in mind that almost every species, even
in its metropolis, would increase immensely in numbers, were it not for other competing
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 must see that the range of the inhabitants of any country by no
means exclusively depends on insensibly changing physical conditions, but in large part on
the presence of other species, on which it depends, or by which it is destroyed, or with
which it comes into competition; and as these species are already defined objects (however
they may have become so), not blending 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 seasons, be extremely liable to utter extermination; and thus its geographical
range will come to be still more sharply defined.
If I am right in believing that allied or representative species, when inhabiting a
continuous area, are generally so distributed 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 in imagination adapt a varying species to a very
large area, we shall have to adapt two varieties to two large areas, and a third variety
to a narrow intermediate zone. The intermediate variety, consequently, will exist in
lesser numbers from inhabiting 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 therefore conclude that varieties
linking two other varieties together have generally existed in lesser numbers than the
forms which they connect, then, I think, 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 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 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 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
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 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 aërial 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 that if any one being vary ever so little, either in habits or structure, and thus
gain an advantage over some other inhabitant of the country, it will seize on the place of
that inhabitant, however different it may be from its own place. Hence it will cause him
no surprise that there should be geese and frigate-birds with webbed feet, either living
on the dry land or most rarely alighting on the water; that there should be long-toed
corncrakes living in meadows instead of in swamps; that there should be woodpeckers where
not a tree grows; that there should be diving thrushes, and petrels with the habits of
auks.
Organs of extreme perfection and complication. To suppose that the eye, with all
its inimitable contrivances for adjusting the focus to different distances, for admitting
different amounts of light, and for the correction of spherical and chromatic aberration,
could have been formed by natural selection, seems, I freely confess, absurd in the
highest possible degree. Yet reason tells me, that if numerous gradations from a perfect
and complex eye to one very imperfect and simple, each grade being useful to its
possessor, can be shown to exist; if further, the eye does vary ever so slightly, and the
variations be inherited, which is certainly the case; and if any variation or modification
in the organ be ever useful to an animal under changing conditions of life, then the
difficulty of believing that a perfect and complex eye could be formed by natural
selection, though insuperable by our imagination, can hardly be considered real. How a
nerve comes to be sensitive to light, hardly concerns us more than how life itself first
originated; but I may remark that several facts make me suspect that any sensitive nerve
may be rendered sensitive to light, and likewise to those coarser vibrations of the air
which produce sound.
In looking for the gradations by which an organ in any species has been perfected, we
ought to look exclusively to its lineal ancestors; but this is scarcely ever possible, and
we are forced in each case to look to species of the same group, that is to the collateral
descendants from the same original parent-form, in order to see what gradations are
possible, and for the chance of some gradations having been transmitted from the earlier
stages of descent, in an unaltered or little altered condition. Amongst existing
Vertebrata, we find but a small amount of gradation in the structure of the eye, and from
fossil species we can learn nothing on this head. In this great class we should probably
have to descend far beneath the lowest known fossiliferous stratum to discover the earlier
stages, by which the eye has been perfected.
In the Articulata we can commence a series with an optic nerve merely coated with
pigment, and without any other mechanism; and from this low stage, numerous gradations of
structure, branching off in two fundamentally different lines, can be shown to exist,
until we reach a moderately high stage of perfection. In certain crustaceans, for
instance, there is a double cornea, the inner one divided into facets, within each of
which there is a lens shaped swelling. In other crustaceans the transparent cones which
are coated by pigment, and which properly act only by excluding lateral pencils of light,
are convex at their upper ends and must act by convergence; and at their lower ends there
seems to be an imperfect vitreous substance. With these facts, here far too briefly and
imperfectly given, which show that there is much graduated diversity in the eyes of living
crustaceans, and bearing in mind how small the number of living animals is in proportion
to those which have become extinct, I can see no very great difficulty (not more than in
the case of many other structures) in believing that natural selection has converted the
simple apparatus of an optic nerve merely coated with pigment and invested by transparent
membrane, into an optical instrument as perfect as is possessed by any member of the great
Articulate class.
He who will go thus far, if he find on finishing this treatise that large bodies of
facts, otherwise inexplicable, can be explained by the theory of descent, ought not to
hesitate to go further, and to admit that a structure even as perfect as the eye of an
eagle might be formed by natural selection, although in this case he does not know any of
the transitional grades. His reason ought to conquer his imagination; though I have felt
the difficulty far too keenly to be surprised at any degree of hesitation in extending the
principle of natural selection to such startling lengths.
It is scarcely possible to avoid comparing the eye to a telescope. We know that this
instrument has been perfected by the long-continued efforts of the highest human
intellects; and we naturally infer that the eye has been formed by a somewhat analogous
process. But may not this inference be presumptuous? Have we any right to assume that the
Creator works by intellectual powers like those of man? If we must compare the eye to an
optical instrument, we ought in imagination to take a thick layer of transparent tissue,
with a nerve sensitive to light beneath, and then suppose every part of this layer to be
continually changing slowly in density, so as to separate into layers of different
densities and thicknesses, placed at different distances from each other, and with the
surfaces of each layer slowly changing in form. Further we must suppose that there is a
power always intently watching each slight accidental alteration in the transparent
layers; and carefully selecting each alteration which, under varied circumstances, may in
any way, or in any degree, tend to produce a distincter image. We must suppose each new
state of the instrument to be multiplied by the million; and each to be preserved till a
better be produced, and then the old ones to be destroyed. In living bodies, variation
will cause the slight alterations, generation will multiply them almost infinitely, and
natural selection will pick out with unerring skill each improvement. Let this process go
on for millions on millions of years; and during each year on millions of individuals of
many kinds; and may we not believe that a living optical instrument might thus be formed
as superior to one of glass, as the works of the Creator are to those of man?
If it could be demonstrated that any complex organ existed, which could not possibly
have been formed by numerous, successive, slight modifications, my theory would absolutely
break down. But I can find out no such case. No doubt many organs exist of which we do not
know the transitional grades, more especially if we look to much-isolated species, round
which, according to my theory, there has been much extinction. Or again, if we look to an
organ common to all the members of a large class, for in this latter case the organ must
have been first formed at an extremely remote period, since which all the many members of
the class have been developed; and in order to discover the early transitional grades
through which the organ has passed, we should have to look to very ancient ancestral
forms, long since become extinct.
We should be extremely cautious in concluding that an organ could not have been formed
by transitional gradations of some kind. Numerous cases could be given amongst the lower
animals of the same organ performing at the same time wholly distinct functions; thus the
alimentary canal respires, digests, and excretes in the larva of the dragon-fly and in the
fish Cobites. In the Hydra, the animal may be turned inside out, and the exterior surface
will then digest and the stomach respire. In such cases natural selection might easily
specialise, if any advantage were thus gained, a part or organ, which had performed two
functions, for one function alone, and thus wholly change its nature by insensible steps.
Two distinct organs sometimes perform simultaneously the same function in the same
individual; to give one instance, there are fish with gills or branchiae that breathe the
air dissolved in the water, at the same time that they breathe free air in their
swimbladders, this latter organ having a ductus pneumaticus for its supply, and being
divided by highly vascular partitions. In these cases, one of the two organs might with
ease be modified and perfected so as to perform all the work by itself, being aided during
the process of modification by the other organ; and then this other organ might be
modified for some other and quite distinct purpose, or be quite obliterated.
The illustration of the swimbladder in fishes is a good one, because it shows us
clearly the highly important fact that an organ originally constructed for one purpose,
namely flotation, may be converted into one for a wholly different purpose, namely
respiration. The swimbladder has, also, been worked in as an accessory to the auditory
organs of certain fish, or, for I do not know which view is now generally held, a part of
the auditory apparatus has been worked in as a complement to the swimbladder. All
physiologists admit that the swimbladder is homologous, or 'ideally similar,' in position
and structure with the lungs of the higher vertebrate animals: hence there seems to me to
be no great difficulty in believing that natural selection has actually converted a
swimbladder into a lung, or organ used exclusively for respiration.
I can, indeed, hardly doubt that all vertebrate animals having true lungs have
descended by ordinary generation from an ancient prototype, of which we know nothing,
furnished with a floating apparatus or swimbladder. We can thus, as I infer from Professor
Owen's interesting description of these parts, understand the strange fact that every
particle of food and drink which we swallow has to pass over the orifice of the trachea,
with some risk of falling into the lungs, notwithstanding the beautiful contrivance by
which the glottis is closed. In the higher Vertebrata the branchiae have wholly
disappeared the slits on the sides of the neck and the loop-like course of the arteries
still marking in the embryo their former position. But it is conceivable that the now
utterly lost branchiae might have been gradually worked in by natural selection for some
quite distinct purpose: in the same manner as, on the view entertained by some naturalists
that the branchiae and dorsal scales of Annelids are homologous with the wings and
wing-covers of insects, it is probable that organs which at a very ancient period served
for respiration have been actually converted into organs of flight.
In considering transitions of organs, it is so important to bear in mind the
probability of conversion from one function to another, that I will give one more
instance. Pedunculated cirripedes have two minute folds of skin, called by me the
ovigerous frena, which serve, through the means of a sticky secretion, to retain the eggs
until they are hatched within the sack. These cirripedes have no branchiae, the whole
surface of the body and sack, including the small frena, serving for respiration. The
Balanidae or sessile cirripedes, on the other hand, have no ovigerous frena, the eggs
lying loose at the bottom of the sack, in the well-enclosed shell; but they have large
folded branchiae. Now I think no one will dispute that the ovigerous frena in the one
family are strictly homologous with the branchiae of the other family; indeed, they
graduate into each other. Therefore I do not doubt that little folds of skin, which
originally served as ovigerous frena, but which, likewise, very slightly aided the act of
respiration, have been gradually converted by natural selection into branchiae, simply
through an increase in their size and the obliteration of their adhesive glands. If all
pedunculated cirripedes had become extinct, and they have already suffered far more
extinction than have sessile cirripedes, who would ever have imagined that the branchiae
in this latter family had originally existed as organs for preventing the ova from being
washed out of the sack?
Although we must be extremely cautious in concluding that any organ could not possibly
have been produced by successive transitional gradations, yet, undoubtedly, grave cases of
difficulty occur, some of which will be discussed in my future work.
One of the gravest is that of neuter insects, which are often very differently
constructed from either the males or fertile females; but this case will be treated of in
the next chapter. The electric organs of fishes offer another case of special difficulty;
it is impossible to conceive by what steps these wondrous organs have been produced; but,
as Owen and others have remarked, their intimate structure closely resembles that of
common muscle; and as it has lately been shown that Rays have an organ closely analogous
to the electric apparatus, and yet do not, as Matteuchi asserts, discharge any
electricity, we must own that we are far too ignorant to argue that no transition of any
kind is possible.
The electric organs offer another and even more serious difficulty; for they occur in
only about a dozen fishes, of which several are widely remote in their affinities.
Generally when the same organ appears in several members of the same class, especially if
in members having very different habits of life, we may attribute its presence to
inheritance from a common ancestor; and its absence in some of the members to its loss
through disuse or natural selection. But if the electric organs had been inherited from
one ancient progenitor thus provided, we might have expected that all electric fishes
would have been specially related to each other. Nor does geology at all lead to the
belief that formerly most fishes had electric organs, which most of their modified
descendants have lost. The presence of luminous organs in a few insects, belonging to
different families and orders, offers a parallel case of difficulty. Other cases could be
given; for instance in plants, the very curious contrivance of a mass of pollen-grains,
borne on a foot-stalk with a sticky gland at the end, is the same in Orchis and Asclepias,
genera almost as remote as possible amongst flowering plants. In all these cases of two
very distinct species furnished with apparently the same anomalous organ, it should be
observed that, although the general appearance and function of the organ may be the same,
yet some fundamental difference can generally be detected. I am inclined to believe that
in nearly the same way as two men have sometimes independently hit on the very same
invention, so natural selection, working for the good of each being and taking advantage
of analogous variations, has sometimes modified in very nearly the same manner two parts
in two organic beings, which owe but little of their structure in common to inheritance
from the same ancestor.
Although in many cases it is most difficult to conjecture by what transitions an organ
could have arrived at its present state; yet, considering that the proportion of living
and known forms to the extinct and unknown is very small, I have been astonished how
rarely an organ can be named, towards which no transitional grade is known to lead. The
truth of this remark is indeed shown by that old canon in natural history of 'Natura non
facit saltum.' We meet with this admission in the writings of almost every experienced
naturalist; or, as Milne Edwards has well expressed it, nature is prodigal in variety, but
niggard in innovation. Why, on the theory of Creation, should this be so? Why should all
the parts and organs of many independent beings, each supposed to have been separately
created for its proper place in nature, be so invariably linked together by graduated
steps? Why should not Nature have taken a leap from structure to structure? On the theory
of natural selection, we can clearly understand why she should not; for natural selection
can act only by taking advantage of slight successive variations; she can never take a
leap, but must advance by the shortest and slowest steps.
Organs of little apparent importance. As natural selection acts by life and
death, by the preservation of individuals with any favourable variation, and by the
destruction of those with any unfavourable deviation of structure, I have sometimes felt
much difficulty in understanding the origin of simple parts, of which the importance does
not seem sufficient to cause the preservation of successively varying individuals. I have
sometimes felt as much difficulty, though of a very different kind, on this head, as in
the case of an organ as perfect and complex as the eye.
In the first place, we are much too ignorant in regard to the whole economy of any one
organic being, to say what slight modifications would be of importance or not. In a former
chapter I have given instances of most trifling characters, such as the down on fruit and
the colour of the flesh, which, from determining the attacks of insects or from being
correlated with constitutional differences, might assuredly be acted on by natural
selection. The tail of the giraffe looks like an artificially constructed fly-flapper; and
it seems at first incredible that this could have been adapted for its present purpose by
successive slight modifications, each better and better, for so trifling an object as
driving away flies; yet we should pause before being too positive even in this case, for
we know that the distribution and existence of cattle and other animals in South America
absolutely depends on their power of resisting the attacks of insects: so that individuals
which could by any means defend themselves from these small enemies, would be able to
range into new pastures and thus gain a great advantage. It is not that the larger
quadrupeds are actually destroyed (except in some rare cases) by the flies, but they are
incessantly harassed and their strength reduced, so that they are more subject to disease,
or not so well enabled in a coming dearth to search for food, or to escape from beasts of
prey.
Organs now of trifling importance have probably in some cases been of high importance
to an early progenitor, and, after having been slowly perfected at a former period, have
been transmitted in nearly the same state, although now become of very slight use; and any
actually injurious deviations in their structure will always have been checked by natural
selection. Seeing how important an organ of locomotion the tail is in most aquatic
animals, its general presence and use for many purposes in so many land animals, which in
their lungs or modified swim-bladders betray their aquatic origin, may perhaps be thus
accounted for. A well-developed tail having been formed in an aquatic animal, it might
subsequently come to be worked in for all sorts of purposes, as a fly-flapper, an organ of
prehension, or as an aid in turning, as with the dog, though the aid must be slight, for
the hare, with hardly any tail, can double quickly enough.
In the second place, we may sometimes attribute importance to characters which are
really of very little importance, and which have originated from quite secondary causes,
independently of natural selection. We should remember that climate, food, &c.,
probably have some little direct influence on the organisation; that characters reappear
from the law of reversion;, that correlation of growth will have had a most important
influence in modifying various structures; and finally, that sexual selection will often
have largely modified the external characters of animals having a will, to give one male
an advantage in fighting with another or in charming the females. Moreover when a
modification of structure has primarily arisen from the above or other unknown causes, it
may at first have been of no advantage to the species, but may subsequently have been
taken advantage of by the descendants of the species under new conditions of life and with
newly acquired habits.
To give a few instances to illustrate these latter remarks. If green woodpeckers alone
had existed, and we did not know that there were many black and pied kinds, I dare say
that we should have thought that the green colour was a beautiful adaptation to hide this
tree-frequenting bird from its enemies; and consequently that it was a character of
importance and might have been acquired through natural selection; as it is, I have no
doubt that the colour is due to some quite distinct cause, probably to sexual selection. A
trailing bamboo in the Malay Archipelago climbs the loftiest trees by the aid of
exquisitely constructed hooks clustered around the ends of the branches, and this
contrivance, no doubt, is of the highest service to the plant; but as we see nearly
similar hooks on many trees which are not climbers the hooks on the bamboo may have arisen
from unknown laws of growth, and have been subsequently taken advantage of by the plant
undergoing further modification and becoming a climber. The naked skin on the head of a
vulture is generally looked at as a direct adaptation for wallowing in putridity; and so
it may be, or it may possibly be due to the direct action of putrid matter; but we should
be very cautious in drawing any such inference, when we see that the skin on the head of
the clean-feeding male turkey is likewise naked. The sutures in the skulls of young
mammals have been advanced as a beautiful adaptation for aiding parturition, and no doubt
they facilitate, or may be indispensable for this act; but as sutures occur in the skulls
of young birds and reptiles, which have only to escape from a broken egg, we may infer
that this structure has arisen from the laws of growth, and has been taken advantage of in
the parturition of the higher animals.
We are profoundly ignorant of the causes producing slight and unimportant variations;
and we are immediately made conscious of this by reflecting on the differences in the
breeds of our domesticated animals in different countries, more especially in the less
civilized countries where there has been but little artificial selection. Careful
observers are convinced that a damp climate affects the growth of the hair, and that with
the hair the horns are correlated. Mountain breeds always differ from lowland breeds; and
a mountainous country would probably affect the hind limbs from exercising them more, and
possibly even the form of the pelvis; and then by the law of homologous variation, the
front limbs and even the head would probably be affected. The shape, also, of the pelvis
might affect by pressure the shape of the head of the young in the womb. The laborious
breathing necessary in high regions would, we have some reason to believe, increase the
size of the chest; and again correlation would come into play. Animals kept by savages in
different countries often have to struggle for their own subsistence, and would be exposed
to a certain extent to natural selection, and individuals with slightly different
constitutions would succeed best under different climates; and there is reason to believe
that constitution and colour are correlated. A good observer, also, states that in cattle
susceptibility to the attacks of flies is correlated with colour, as is the liability to
be poisoned by certain plants; so that colour would be thus subjected to the action of
natural selection. But we are far too ignorant to speculate on the relative importance of
the several known and unknown laws of variation; and I have here alluded to them only to
show that, if we are unable to account for the characteristic differences of our domestic
breeds, which nevertheless we generally admit to have arisen through ordinary generation,
we ought not to lay too much stress on our ignorance of the precise cause of the slight
analogous differences between species. I might have adduced for this same purpose the
differences between the races of man, which are so strongly marked; I may add that some
little light can apparently be thrown on the origin of these differences, chiefly through
sexual selection of a particular kind, but without here entering on copious details my
reasoning would appear frivolous.
The foregoing remarks lead me to say a few words on the protest lately made by some
naturalists, against the utilitarian doctrine that every detail of structure has been
produced for the good of its possessor. They believe that very many structures have been
created for beauty in the eyes of man, or for mere variety. This doctrine, if true, would
be absolutely fatal to my theory. Yet I fully admit that many structures are of no direct
use to their possessors. Physical conditions probably have had some little effect on
structure, quite independently of any good thus gained. Correlation of growth has no doubt
played a most important part, and a useful modification of one part will often have
entailed on other parts diversified changes of no direct use. So again characters which
formerly were useful, or which formerly had arisen from correlation of growth, or from
other unknown cause, may reappear from the law of reversion, though now of no direct use.
The effects of sexual selection, when displayed in beauty to charm the females, can be
called useful only in rather a forced sense. But by far the most important consideration
is that the chief part of the organisation of every being is simply due to inheritance;
and consequently, though each being assuredly is well fitted for its place in nature, many
structures now have no direct relation to the habits of life of each species. Thus, we can
hardly believe that the webbed feet of the upland goose or of the frigate-bird are of
special use to these birds; we cannot believe that the same bones in the arm of the
monkey, in the fore leg of the horse, in the wing of the bat, and in the flipper of the
seal, are of special use to these animals. We may safely attribute these structures to
inheritance. But to the progenitor of the upland goose and of the frigate-bird, webbed
feet no doubt were as useful as they now are to the most aquatic of existing birds. So we
may believe that the progenitor of the seal had not a flipper, but a foot with five toes
fitted for walking or grasping; and we may further venture to believe that the several
bones in the limbs of the monkey, horse, and bat, which have been inherited from a common
progenitor, were formerly of more special use to that progenitor, or its progenitors, than
they now are to these animals having such widely diversified habits. Therefore we may
infer that these several bones might have been acquired through natural selection,
subjected formerly, as now, to the several laws of inheritance, reversion, correlation of
growth, &c. Hence every detail of structure in every living creature (making some
little allowance for the direct action of physical conditions) may be viewed, either as
having been of special use to some ancestral form, or as being now of special use to the
descendants of this form either directly, or indirectly through the complex laws of
growth.
Natural selection cannot possibly produce any modification in any one species
exclusively for the good of another species; though throughout nature one species
incessantly takes advantage of, and profits by, the structure of another. But natural
selection can and does often produce structures for the direct injury of other species, as
we see in the fang of the adder, and in the ovipositor of the ichneumon, by which its eggs
are deposited in the living bodies of other insects. If it could be proved that any part
of the structure of any one species had been formed for the exclusive good of another
species, it would annihilate my theory, for such could not have been produced through
natural selection. Although many statements may be found in works on natural history to
this effect, I cannot find even one which seems to me of any weight. It is admitted that
the rattlesnake has a poison-fang for its own defence and for the destruction of its prey;
but some authors suppose that at the same time this snake is furnished with a rattle for
its own injury, namely, to warn its prey to escape. I would almost as soon believe that
the cat curls the end of its tail when preparing to spring, in order to warn the doomed
mouse. But I have not space here to enter on this and other such cases.
Natural selection will never produce in a being anything injurious to itself, for
natural selection acts solely by and for the good of each. No organ will be formed, as
Paley has remarked, for the purpose of causing pain or for doing an injury to its
possessor. If a fair balance be struck between the good and evil caused by each part, each
will be found on the whole advantageous. After the lapse of time, under changing
conditions of life, if any part comes to be injurious, it will be modified; or if it be
not so, the being will become extinct, as myriads have become extinct.
Natural selection tends only to make each organic being as perfect as, or slightly more
perfect than, the other inhabitants of the same country with which it has to struggle for
existence. And we see that this is the degree of perfection attained under nature. The
endemic productions of New Zealand, for instance, are perfect one compared with another;
but they are now rapidly yielding before the advancing legions of plants and animals
introduced from Europe. Natural selection will not produce absolute perfection, nor do we
always meet, as far as we can judge, with this high standard under nature. The correction
for the aberration of light is said, on high authority, not to be perfect even in that
most perfect organ, the eye. If our reason leads us to admire with enthusiasm a multitude
of inimitable contrivances in nature, this same reason tells us, though we may easily err
on both sides, that some other contrivances are less perfect. Can we consider the sting of
the wasp or of the bee as perfect, which, when used against many attacking animals, cannot
be withdrawn, owing to the backward serratures, and so inevitably causes the death of the
insect by tearing out its viscera?
If we look at the sting of the bee, as having originally existed in a remote progenitor
as a boring and serrated instrument, like that in so many members of the same great order,
and which has been modified but not perfected for its present purpose, with the poison
originally adapted to cause galls subsequently intensified, we can perhaps understand how
it is that the use of the sting should so often cause the insect's own death: for if on
the whole the power of stinging be useful to the community, it will fulfil all the
requirements of natural selection, though it may cause the death of some few members. If
we admire the truly wonderful power of scent by which the males of many insects find their
females, can we admire the production for this single purpose of thousands of drones,
which are utterly useless to the community for any other end, and which are ultimately
slaughtered by their industrious and sterile sisters? It may be difficult, but we ought to
admire the savage instinctive hatred of the queen-bee, which urges her instantly to
destroy the young queens her daughters as soon as born, or to perish herself in the
combat; for undoubtedly this is for the good of the community; and maternal love or
maternal hatred, though the latter fortunately is most rare, is all the same to the
inexorable principle of natural selection. If we admire the several ingenious
contrivances, by which the flowers of the orchis and of many other plants are fertilised
through insect agency, can we consider as equally perfect the elaboration by our fir-trees
of dense clouds of pollen, in order that a few granules may be wafted by a chance breeze
on to the ovules?
Summary of Chapter. We have in this chapter discussed some of the difficulties
and objections which may be urged against my theory. Many of them are very grave; but I
think that in the discussion light has been thrown on several facts, which on the theory
of independent acts of creation are utterly obscure. We have seen that species at any one
period are not indefinitely variable, and are not linked together by a multitude of
intermediate gradations, partly because the process of natural selection will always be
very slow, and will act, at any one time, only on a very few forms; and partly because the
very process of natural selection almost implies the continual supplanting and extinction
of preceding and intermediate gradations. Closely allied species, now living on a
continuous area, must often have been formed when the area was not continuous, and when
the conditions of life did not insensibly graduate away from one part to another. When two
varieties are formed in two districts of a continuous area, an intermediate variety will
often be formed, fitted for an intermediate zone; but from reasons assigned, the
intermediate variety will usually exist in lesser numbers than the two forms which it
connects; consequently the two latter, during the course of further modification, from
existing in greater numbers, will have a great advantage over the less numerous
intermediate variety, and will thus generally succeed in supplanting and exterminating it.
We have seen in this chapter how cautious we should be in concluding that the most
different habits of life could not graduate into each other; that a bat, for instance,
could not have been formed by natural selection from an animal which at first could only
glide through the air.
We have seen that a species may under new conditions of life change its habits, or have
diversified habits, with some habits very unlike those of its nearest congeners. Hence we
can understand bearing in mind that each organic being is trying to live wherever it can
live, how it has arisen that there are upland geese with webbed feet, ground woodpeckers,
diving thrushes, and petrels with the habits of auks.
Although the belief that an organ so perfect as the eye could have been formed by
natural selection, is more than enough to stagger any one; yet in the case of any organ,
if we know of a long series of gradations in complexity, each good for its possessor,
then, under changing conditions of life, there is no logical impossibility in the
acquirement of any conceivable degree of perfection through natural selection. In the
cases in which we know of no intermediate or transitional states, we should be very
cautious in concluding that none could have existed, for the homologies of many organs and
their intermediate states show that wonderful metamorphoses in function are at least
possible. For instance, a swim-bladder has apparently been converted into an air-breathing
lung. The same organ having performed simultaneously very different functions, and then
having been specialised for one function; and two very distinct organs having performed at
the same time the same function, the one having been perfected whilst aided by the other,
must often have largely facilitated transitions.
We are far too ignorant, in almost every case, to be enabled to assert that any part or
organ is so unimportant for the welfare of a species, that modifications in its structure
could not have been slowly accumulated by means of natural selection. But we may
confidently believe that many modifications, wholly due to the laws of growth, and at
first in no way advantageous to a species, have been subsequently taken advantage of by
the still further modified descendants of this species. We may, also, believe that a part
formerly of high importance has often been retained (as the tail of an aquatic animal by
its terrestrial descendants), though it has become of such small importance that it could
not, in its present state, have been acquired by natural selection, a power which acts
solely by the preservation of profitable variations in the struggle for life.
Natural selection will produce nothing in one species for the exclusive good or injury
of another; though it may well produce parts, organs, and excretions highly useful or even
indispensable, or highly injurious to another species, but in all cases at the same time
useful to the owner. Natural selection in each well-stocked country, must act chiefly
through the competition of the inhabitants one with another, and consequently will produce
perfection, or strength in the battle for life, only according to the standard of that
country. Hence the inhabitants of one country, generally the smaller one, will often
yield, as we see they do yield, to the inhabitants of another and generally larger
country. For in the larger country there will have existed more individuals, and more
diversified forms, and the competition will have been severer, and thus the standard of
perfection will have been rendered higher. Natural selection will not necessarily produce
absolute perfection; nor, as far as we can judge by our limited faculties, can absolute
perfection be everywhere found.
On the theory of natural selection we can clearly understand the full meaning of that
old canon in natural history, 'Natura non facit saltum.' This canon, if we look only to
the present inhabitants of the world, is not strictly correct, but if we include all those
of past times, it must by my theory be strictly true.
It is generally acknowledged that all organic beings have been formed on
two great laws Unity of Type, and the Conditions of Existence. By unity of type is meant
that fundamental agreement in structure, which we see in organic beings of the same class,
and which is quite independent of their habits of life. On my theory, unity of type is
explained by unity of descent. The expression of conditions of existence, so often
insisted on by the illustrious Cuvier, is fully embraced by the principle of natural
selection. For natural selection acts by either now adapting the varying parts of each
being to its organic and inorganic conditions of life; or by having adapted them during
long-past periods of time: the adaptations being aided in some cases by use and disuse,
being slightly affected by the direct action of the external conditions of life, and being
in all cases subjected to the several laws of growth. Hence, in fact, the law of the
Conditions of Existence is the higher law; as it includes, through the inheritance of
former adaptations, that of Unity of Type.