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.
2 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?
3 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?
4 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?
5 Fourthly, how can we account for species, when
crossed, being sterile and producing sterile offspring, whereas, when
varieties are crossed, their fertility is unimpaired?
6 The two first heads shall be here discussed--Instinct
and Hybridism in separate chapters.
7 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.
8 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.
9 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.
10 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.
11 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.
12 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.
13 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.
14 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.
15 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.
16 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.
17 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.
18 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.
19 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.
20 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.
21 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.
22 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.
23 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?
24 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.
25 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.
26 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!
27 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.
28 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.
29 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.
30 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.
31 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.
32 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.
33 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.
34 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?
35 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.
36 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.
37 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.
38 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.
39 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?
40 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.
41 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.
42 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.
43 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.
44 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.
45 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.
46 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.
47 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.
48 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.
49 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.
50 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.
51 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.
52 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.
53 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?
54 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?
55 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.
56 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.
57 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.
58 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.
59 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.
60 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.
61 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.
62 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.