Chapter VI

Difficulties on Theory

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

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

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

escape being devoured by other fish?

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

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

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

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

perfection through natural selection. Furthermore, we may conclude that

transitional grades between structures fitted for very different habits of

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

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

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

capable of true flight would have been developed under many subordinate

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

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

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

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

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

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

developed structures.

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

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

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

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

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

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

whether slight modifications of structure lead to changed habits; both

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

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

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

diversified habits innumerable instances could be given: I have often

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

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

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

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

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

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

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

breaking them like a nuthatch. In North America the black bear was seen by

Hearne swimming for hours with widely open mouth, thus catching, like a

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

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

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

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

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

produced as monstrous as a whale.

As we sometimes see individuals of a species following habits widely

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

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

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

with their structure either slightly or considerably modified from that of

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

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

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

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

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

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

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

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

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

never climbs a tree!

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

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

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

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

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

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

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

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

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

with its feet and using its wings under water.

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

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

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

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

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

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

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

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

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

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

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

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

habits have changed without a corresponding change of structure. The

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

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

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

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

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

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

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

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

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

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.