CHAPTER VI.

DIFFICULTIES OF THE THEORY.

Difficulties of the theory of descent with modification—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—Modes

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 the reader has arrived at this part of my work, a crowd of

difficulties will have occurred to him. Some of them are so serious

that to this day I can hardly reflect on them without being in some

degree 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

the theory.

These difficulties and objections may be classed under the following

heads: First, why, if species have descended from other species by 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 other animal with widely different habits and

structure? Can we believe that natural selection could produce, on the

one hand, an organ of trifling importance, such as the tail of a

giraffe, which serves as a fly-flapper, and, on the other hand, an

organ so wonderful as the eye?

Thirdly, can instincts be acquired and modified through natural

selection? What shall we say to the instinct which leads the bee to

make cells, and which has 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 will be here discussed; some miscellaneous

objections in the following chapter; Instinct and Hybridism in the two

succeeding 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-form and

other less-favoured forms with which it comes into competition. Thus

extinction and natural selection go hand in hand. Hence, if we look at

each species as descended from some unknown form, both the parent and

all the transitional varieties will generally have been exterminated by

the very process of the 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 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 crust

of the earth is a vast museum; but the natural collections have been

imperfectly made, and only at long intervals of time.

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 are descended from a common parent;

and during the process of modification, each has become adapted to the

conditions of life of its own region, and has supplanted and

exterminated its original parent-form and all the transitional

varieties between its past and present states. Hence we ought not to

expect at the present time to meet with numerous transitional varieties

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 most continents have

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 E.

Forbes in sounding the depths of the sea with the dredge. To those who

look at climate and the physical conditions of life as the

all-important elements of distribution, these facts ought to cause

surprise, as climate and height or depth graduate away 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 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 lives, or by which it is

destroyed, or with which it comes into competition; and as these

species are already defined objects, 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 nature of the seasons, be extremely liable to utter

extermination; and thus its geographical range will come to be still

more sharply defined.

As allied or representative species, when inhabiting a continuous area,

are generally distributed in such a manner that each has a wide range,

with a comparatively narrow neutral territory between them, in which

they become rather suddenly rarer and rarer; then, as varieties do not

essentially differ from species, the same rule will probably apply to

both; and if we take a varying species inhabiting a very large area, we

shall have to adapt two 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 conclude

that varieties linking two other varieties together generally have

existed in lesser numbers than the forms which they connect, then we

can understand why intermediate varieties should not endure for very

long periods: why, as a general rule, they should be exterminated and

disappear, sooner than the forms which they originally linked together.

For any form existing in lesser numbers would, as already remarked, run

a greater chance of being exterminated than one existing in large

numbers; and in this particular case the intermediate form would be

eminently liable to the inroads of closely allied forms existing on

both sides of it. But it is a far more important consideration, that

during the process of further modification, by which two varieties are

supposed 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 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 the 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: first, because new varieties are very

slowly formed, for variation is a slow process, and natural selection

can do nothing until favourable individual differences or variations

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 to see only 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 as isolated portions, in which many forms, more

especially among the classes which unite for each birth and wander

much, may have separately been rendered sufficiently distinct to rank

as representative species. In this case, intermediate varieties between

the several representative species and their common parent, must

formerly have existed within each isolated portion of the land, but

these links during the process of natural selection will have been

supplanted and exterminated, so that they will no longer be found in a

living state.

Thirdly, when two or more varieties have been formed in different

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

varieties, and thus be further improved through natural selection and

gain further advantages.

Lastly, looking not to any one time, but at all time, if my theory be

true, numberless intermediate varieties, linking closely together all

the species of the same group, must assuredly have existed; but the

very process of natural selection constantly tends, as has been so

often remarked, to exterminate the parent forms and the intermediate

links. Consequently evidence of their former existence could be found

only among fossil remains, which are preserved, as we shall attempt to

show in a future chapter, in an extremely imperfect and intermittent

record.

_On the Origin and Transition of Organic Beings with peculiar Habits

and Structure._—It has been asked by the opponents of such views as I

hold, how, for instance, could a land carnivorous animal 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 there

now exist carnivorous animals presenting close intermediate grades from

strictly terrestrial to aquatic habits; and as each exists by a

struggle for life, it is clear that each must be well adapted to its

place in nature. Look at the Mustela vison of North America, which has

webbed feet, and which resembles an otter in its fur, short legs, and

form of tail; during 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 to answer. Yet I think such difficulties have

little weight.

Here, as on other occasions, I lie under a heavy disadvantage, for, out

of the many striking cases which I have collected, I can give only one

or two instances of transitional habits and structures in allied

species; and of 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, to lessen the danger from

occasional falls. But it does not follow from this fact that the

structure of each squirrel is the best that it is possible to conceive

under all possible conditions. Let the climate and vegetation change,

let other competing rodents or new beasts of prey immigrate, or old

ones become modified, and all analogy would lead us to believe that

some, at least, of the squirrels would decrease in numbers or become

exterminated, unless they also become modified and improved in

structure in a corresponding manner. Therefore, I can see no

difficulty, more especially under changing conditions of life, in the

continued preservation of individuals with fuller and fuller

flank-membranes, each modification being useful, each being propagated,

until, by the accumulated effects of this process of natural selection,

a perfect so-called flying squirrel was produced.

Now look at the Galeopithecus or so-called flying lemur, which was

formerly ranked among bats, but is now believed to belong to the

Insectivora. An extremely wide flank-membrane stretches from the

corners of the jaw to the tail, and includes the limbs with the

elongated fingers. This flank-membrane is furnished with an extensor

muscle. Although no graduated links of structure, fitted for gliding

through the air, now connect the Galeopithecus with the other

Insectivora, yet there is no difficulty in supposing that such links

formerly existed, and that each was developed in the same manner as

with the less perfectly gliding squirrels; each grade of structure

having been useful to its possessor. Nor can I see any insuperable

difficulty in further believing it possible that the membrane-connected

fingers and fore-arm of the Galeopithecus might have been greatly

lengthened by natural selection; and this, as far as the organs of

flight are concerned, would have converted the animal into a bat. In

certain bats in which the wing-membrane extends from the top of the

shoulder to the tail and includes the hind-legs, we perhaps see traces

of an apparatus originally fitted for gliding through the air rather

than for flight.

If about a dozen genera of birds were to become extinct, who would have

ventured to surmise that birds might have existed which used their

wings solely as flappers, like the logger headed duck (Micropterus of

Eyton); as fins in the water and as front legs on the land, like the

penguin; as sails, like the ostrich; and functionally for no purpose,

like the apteryx? Yet the 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 be the result of disuse, indicate the steps by which

birds actually acquired their perfect power of flight; but they serve

to show what diversified means of transition are at least possible.

Seeing that a few members of such water-breathing classes as the

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, so 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 have

survived to the present day, for they will have been supplanted by

their successors, which were gradually rendered more perfect through

natural selection. Furthermore, we may conclude that transitional

states between structures fitted for very different habits of life will

rarely have been developed at an early period in great numbers and

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

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

capable of true flight would have been developed under many subordinate

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

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

perfection, so as to have given them a decided advantage over other

animals in the battle for life. Hence the chance of discovering species

with transitional grades of structure in a fossil condition will always

be less, from their having existed in lesser numbers, than in the case

of species with fully developed structures.

I will now give two or three instances, both of diversified and of

changed habits, in the individuals of the same species. In either case

it would be easy for natural selection to adapt the structure of the

animal to its changed habits, or exclusively to one of its several

habits. It is, however, difficult to decide and immaterial for us,

whether habits generally change first and structure afterwards; or

whether slight modifications of structure lead to changed habits; both

probably often occurring almost simultaneously. Of cases of changed

habits it will suffice merely to allude to that of the many British

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 into it like a kingfisher at a fish. In our own

country the larger titmouse (Parus major) may be seen climbing

branches, almost like a creeper; it sometimes, like a shrike, kills

small birds by blows on the head; and I have many times seen and heard

it hammering the seeds of the yew on a branch, and thus breaking them

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

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

whale, insects in the water.

As we sometimes see individuals following habits different from those

proper to their species and to the other species of the same genus, we

might expect that such individuals would occasionally give rise to new

species, having anomalous habits, and with their structure either

slightly or considerably modified from that of their type. And such

instances occur in nature. Can a more striking instance of adaptation

be given than that of a woodpecker for climbing trees and seizing

insects in the chinks of the bark? Yet in North America there are

woodpeckers which feed largely on fruit, and others with elongated

wings which chase insects on the wing. On the plains of La Plata, where

hardly a tree grows, there is a woodpecker (Colaptes campestris) which

has two toes before and two behind, a long-pointed tongue, pointed

tail-feathers, sufficiently stiff to support the bird in a vertical

position on a post, but not so stiff as in the typical wood-peckers,

and a straight, strong beak. The beak, however, is not so straight or

so strong as in the typical woodpeckers but it is strong enough to bore

into wood. Hence this Colaptes, in all the essential parts of its

structure, is a woodpecker. Even in such trifling characters as the

colouring, the harsh tone of the voice, and undulatory flight, its

close blood-relationship to our common woodpecker is plainly declared;

yet, as I can assert, not only from my own observations, but from those

of the accurate Azara, in certain large districts it does not climb

trees, and it makes its nest in holes in banks! In certain other

districts, however, this same woodpecker, as Mr. Hudson states,

frequents trees, and bores holes in the trunk for its nest. I may

mention as another illustration of the varied habits of this genus,

that a Mexican Colaptes has been described by De Saussure as boring

holes into hard wood in order to lay up a store of acorns.

Petrels are the most aërial and oceanic of birds, but, in the quiet

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

habits, in its astonishing power of diving, in its manner of swimming

and of flying when made to take flight, would be mistaken by any one

for an auk or a grebe; nevertheless, it is essentially a petrel, but

with many parts of its organisation profoundly modified in relation to

its new habits of life; whereas the woodpecker of La Plata has had its

structure only slightly modified. In the case of the water-ouzel, the

acutest observer, by examining its dead body, would never have

suspected its sub-aquatic habits; yet this bird, which is allied to the

thrush family, subsists by diving,—using its wings under water and

grasping stones with its feet. All the members of the great order of

Hymenopterous insects are terrestrial, excepting the genus

Proctotrupes, which Sir John Lubbock has discovered to be aquatic in

its habits; it often enters the water and dives about by the use not of

its legs but of its wings, and remains as long as four hours beneath

the surface; yet it exhibits no modification in structure in accordance

with its abnormal habits.

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

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

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

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

are upland geese with webbed feet which rarely 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 ocean. 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,

not furnished with membrane, of the Grallatores, are formed for walking

over swamps and floating plants. The water-hen and landrail are members

of this order, yet the first is nearly as aquatic as the coot, and the

second is 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 almost 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 may say,

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

type to take the place of one belonging to 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 varies ever so little,

either in habits or structure, and thus gains an advantage over some

other inhabitant of the same country, it will seize on the place of

that inhabitant, however different that may be from its own place.

Hence it will cause him no surprise that there should be geese and

frigate-birds with webbed feet, living on the dry land and 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 hardly a tree grows; that there should be diving

thrushes and diving Hymenoptera, 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 degree. When it was first said

that the sun stood still and the world turned round, the common sense

of mankind declared the doctrine false; but the old saying of _Vox

populi, vox Dei_, as every philosopher knows, cannot be trusted in

science. Reason tells me, that if numerous gradations from a simple and

imperfect eye to one complex and perfect can be shown to exist, each

grade being useful to its possessor, as is certainly the case; if

further, the eye ever varies and the variations be inherited, as is

likewise certainly the case; and if such variations should be useful to

any 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, should not be

considered as subversive of the theory. How a nerve comes to be

sensitive to light, hardly concerns us more than how life itself

originated; but I may remark that, as some of the lowest organisms in

which nerves cannot be detected, are capable of perceiving light, it

does not seem impossible that certain sensitive elements in their

sarcode should become aggregated and developed into nerves, endowed

with this special sensibility.

In searching for the gradations through which an organ in any species

has been perfected, we ought to look exclusively to its lineal

progenitors; but this is scarcely ever possible, and we are forced to

look to other species and genera of the same group, that is to the

collateral descendants from the same parent-form, in order to see what

gradations are possible, and for the chance of some gradations having

been transmitted in an unaltered or little altered condition. But the

state of the same organ in distinct classes may incidentally throw

light on the steps by which it has been perfected.

The simplest organ which can be called an eye consists of an optic

nerve, surrounded by pigment-cells and covered by translucent skin, but

without any lens or other refractive body. We may, however, according

to M. Jourdain, descend even a step lower and find aggregates of

pigment-cells, apparently serving as organs of vision, without any

nerves, and resting merely on sarcodic tissue. Eyes of the above simple

nature are not capable of distinct vision, and serve only to

distinguish light from darkness. In certain star-fishes, small

depressions in the layer of pigment which surrounds the nerve are

filled, as described by the author just quoted, with transparent

gelatinous matter, projecting with a convex surface, like the cornea in

the higher animals. He suggests that this serves not to form an image,

but only to concentrate the luminous rays and render their perception

more easy. In this concentration of the rays we gain the first and by

far the most important step towards the formation of a true,

picture-forming eye; for we have only to place the naked extremity of

the optic nerve, which in some of the lower animals lies deeply buried

in the body, and in some near the surface, at the right distance from

the concentrating apparatus, and an image will be formed on it.

In the great class of the Articulata, we may start from an optic nerve

simply coated with pigment, the latter sometimes forming a sort of

pupil, but destitute of lens or other optical contrivance. With insects

it is now known that the numerous facets on the cornea of their great

compound eyes form true lenses, and that the cones include curiously

modified nervous filaments. But these organs in the Articulata are so

much diversified that Müller formerly made three main classes with

seven subdivisions, besides a fourth main class of aggregated simple

eyes.

When we reflect on these facts, here given much too briefly, with

respect to the wide, diversified, and graduated range of structure in

the eyes of the lower animals; and when we bear in mind how small the

number of all living forms must be in comparison with those which have

become extinct, the difficulty ceases to be very great in believing

that natural selection may have converted the simple apparatus of an

optic nerve, coated with pigment and invested by transparent membrane,

into an optical instrument as perfect as is possessed by any member of

the Articulata class.

He who will go thus far, ought not to hesitate to go one step further,

if he finds on finishing this volume that large bodies of facts,

otherwise inexplicable, can be explained by the theory of modification

through natural selection; he ought to admit that a structure even as

perfect as an eagle’s eye might thus be formed, although in this case

he does not know the transitional states. It has been objected that in

order to modify the eye and still preserve it as a perfect instrument,

many changes would have to be effected simultaneously, which, it is

assumed, could not be done through natural selection; but as I have

attempted to show in my work on the variation of domestic animals, it

is not necessary to suppose that the modifications were all

simultaneous, if they were extremely slight and gradual. Different

kinds of modification would, also, serve for the same general purpose:

as Mr. Wallace has remarked, “If a lens has too short or too long a

focus, it may be amended either by an alteration of curvature, or an

alteration of density; if the curvature be irregular, and the rays do

not converge to a point, then any increased regularity of curvature

will be an improvement. So the contraction of the iris and the muscular

movements of the eye are neither of them essential to vision, but only

improvements which might have been added and perfected at any stage of

the construction of the instrument.” Within the highest division of the

animal kingdom, namely, the Vertebrata, we can start from an eye so

simple, that it consists, as in the lancelet, of a little sack of

transparent skin, furnished with a nerve and lined with pigment, but

destitute of any other apparatus. In fishes and reptiles, as Owen has

remarked, “The range of gradation of dioptric structures is very

great.” It is a significant fact that even in man, according to the

high authority of Virchow, the beautiful crystalline lens is formed in

the embryo by an accumulation of epidermic cells, lying in a sack-like

fold of the skin; and the vitreous body is formed from embryonic

subcutaneous tissue. To arrive, however, at a just conclusion regarding

the formation of the eye, with all its marvellous yet not absolutely

perfect characters, it is indispensable that the reason should conquer

the imagination; but I have felt the difficulty far to keenly to be

surprised at others hesitating to extend the principle of natural

selection to so startling a length.

It is scarcely possible to avoid comparing the eye with 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 spaces filled with

fluid, and 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, represented by natural selection or the survival of the

fittest, always intently watching each slight alteration in the

transparent layers; and carefully preserving each which, under varied

circumstances, in any way or degree, tends to produce a distincter

image. We must suppose each new state of the instrument to be

multiplied by the million; each to be preserved until a better is

produced, and then the old ones to be all destroyed. In living bodies,

variation will cause the slight alteration, generation will multiply

them almost infinitely, and natural selection will pick out with

unerring skill each improvement. Let this process go on for 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?

_Modes of Transition._

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, around which, according to the theory, there has been much

extinction. Or again, if we take an organ common to all the members of

a class, for in this latter case the organ must have been originally

formed at a 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 among the lower animals of the same organ

performing at the same time wholly distinct functions; thus in the

larva of the dragon-fly and in the fish Cobites the alimentary canal

respires, digests, and excretes. 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 specialise, if any

advantage were thus gained, the whole or part of an organ, which had

previously performed two functions, for one function alone, and thus by

insensible steps greatly change its nature. Many plants are known which

regularly produce at the same time differently constructed flowers; and

if such plants were to produce one kind alone, a great change would be

effected with comparative suddenness in the character of the species.

It is, however, probable that the two sorts of flowers borne by the

same plant were originally differentiated by finely graduated steps,

which may still be followed in some few cases.

Again, two distinct organs, or the same organ under two very different

forms, may simultaneously perform in the same individual the same

function, and this is an extremely important means of transition: to

give one instance—there are fish with gills or branchiæ that breathe

the air dissolved in the water, at the same time that they breathe free

air in their swim-bladders, this latter organ being divided by highly

vascular partitions and having a ductus pneumaticus for the supply of

air. To give another instance from the vegetable kingdom: plants climb

by three distinct means, by spirally twining, by clasping a support

with their sensitive tendrils, and by the emission of aërial rootlets;

these three means are usually found in distinct groups, but some few

species exhibit two of the means, or even all three, combined in the

same individual. In all such cases one of the two organs might readily

be modified and perfected so as to perform all the work, being aided

during the progress of modification by the other organ; and then this

other organ might be modified for some other and quite distinct

purpose, or be wholly obliterated.

The illustration of the swim-bladder 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 widely different purpose, namely respiration. The

swim-bladder has, also, been worked in as an accessory to the auditory

organs of certain fishes. All physiologists admit that the swim-bladder

is homologous, or “ideally similar” in position and structure with the

lungs of the higher vertebrate animals: hence there is no reason to

doubt that the swim-bladder has actually been converted into lungs, or

an organ used exclusively for respiration.

According to this view it may be inferred that all vertebrate animals

with true lungs are descended by ordinary generation from an ancient

and unknown prototype which was furnished with a floating apparatus or

swim-bladder. 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 branchiæ have wholly

disappeared—but in the embryo the slits on the sides of the neck and

the loop-like course of the arteries still mark their former position.

But it is conceivable that the now utterly lost branchiæ might have

been gradually worked in by natural selection for some distinct

purpose: for instance, Landois has shown that the wings of insects are

developed from the trachea; it is therefore highly probable that in

this great class organs which once served for respiration have been

actually converted into organs for 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 another 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 branchiæ, the whole

surface of the body and of the sack, together with the small frena,

serving for respiration. The Balanidæ or sessile cirripedes, on the

other hand, have no ovigerous frena, the eggs lying loose at the bottom

of the sack, within the well-enclosed shell; but they have, in the same

relative position with the frena, large, much-folded membranes, which

freely communicate with the circulatory lacunæ of the sack and body,

and which have been considered by all naturalists to act as branchiæ.

Now I think no one will dispute that the ovigerous frena in the one

family are strictly homologous with the branchiæ of the other family;

indeed, they graduate into each other. Therefore it need not be doubted

that the two little folds of skin, which originally served as ovigerous

frena, but which, likewise, very slightly aided in the act of

respiration, have been gradually converted by natural selection into

branchiæ, simply through an increase in their size and the obliteration

of their adhesive glands. If all pedunculated cirripedes had become

extinct, and they have suffered far more extinction than have sessile

cirripedes, who would ever have imagined that the branchiæ in this

latter family had originally existed as organs for preventing the ova

from being washed out of the sack?

There is another possible mode of transition, namely, through the

acceleration or retardation of the period of reproduction. This has

lately been insisted on by Professor Cope and others in the United

States. It is now known that some animals are capable of reproduction

at a very early age, before they have acquired their perfect

characters; and if this power became thoroughly well developed in a

species, it seems probable that the adult stage of development would

sooner or later be lost; and in this case, especially if the larva

differed much from the mature form, the character of the species would

be greatly changed and degraded. Again, not a few animals, after

arriving at maturity, go on changing in character during nearly their

whole lives. With mammals, for instance, the form of the skull is often

much altered with age, of which Dr. Murie has given some striking

instances with seals. Every one knows how the horns of stags become

more and more branched, and the plumes of some birds become more finely

developed, as they grow older. Professor Cope states that the teeth of

certain lizards change much in shape with advancing years. With

crustaceans not only many trivial, but some important parts assume a

new character, as recorded by Fritz Müller, after maturity. In all such

cases—and many could be given—if the age for reproduction were

retarded, the character of the species, at least in its adult state,

would be modified; nor is it improbable that the previous and earlier

stages of development would in some cases be hurried through and

finally lost. Whether species have often or ever been modified through

this comparatively sudden mode of transition, I can form no opinion;

but if this has occurred, it is probable that the differences between

the young and the mature, and between the mature and the old, were

primordially acquired by graduated steps.

_Special Diffculties of the Theory of Natural Selection._

Although we must be extremely cautious in concluding that any organ

could not have been produced by successive, small, transitional

gradations, yet undoubtedly serious cases of difficulty occur.

One of the most serious is that of neuter insects, which are often

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; for it is

impossible to conceive by what steps these wondrous organs have been

produced. But this is not surprising, for we do not even know of what

use they are. In the gymnotus and torpedo they no doubt serve as

powerful means of defence, and perhaps for securing prey; yet in the

ray, as observed by Matteucci, an analogous organ in the tail manifests

but little electricity, even when the animal is greatly irritated; so

little that it can hardly be of any use for the above purposes.

Moreover, in the ray, besides the organ just referred to, there is, as

Dr. R. McDonnell has shown, another organ near the head, not known to

be electrical, but which appears to be the real homologue of the

electric battery in the torpedo. It is generally admitted that there

exists between these organs and ordinary muscle a close analogy, in

intimate structure, in the distribution of the nerves, and in the

manner in which they are acted on by various reagents. It should, also,

be especially observed that muscular contraction is accompanied by an

electrical discharge; and, as Dr. Radcliffe insists, “in the electrical

apparatus of the torpedo during rest, there would seem to be a charge

in every respect like that which is met with in muscle and nerve during

the rest, and the discharge of the torpedo, instead of being peculiar,

may be only another form of the discharge which attends upon the action

of muscle and motor nerve.” Beyond this we cannot at present go in the

way of explanation; but as we know so little about the uses of these

organs, and as we know nothing about the habits and structure of the

progenitors of the existing electric fishes, it would be extremely bold

to maintain that no serviceable transitions are possible by which these

organs might have been gradually developed.

These organs appear at first to offer another and far more serious

difficulty; for they occur in about a dozen kinds of fish, of which

several are widely remote in their affinities. When the same organ is

found in several members of the same class, especially if in members

having very different habits of life, we may generally attribute its

presence to inheritance from a common ancestor; and its absence in some

of the members to loss through disuse or natural selection. So that, if

the electric organs had been inherited from some one ancient

progenitor, we might have expected that all electric fishes would have

been specially related to each other; but this is far from the case.

Nor does geology at all lead to the belief that most fishes formerly

possessed electric organs, which their modified descendants have now

lost. But when we look at the subject more closely, we find in the

several fishes provided with electric organs, that these are situated

in different parts of the body, that they differ in construction, as in

the arrangement of the plates, and, according to Pacini, in the process

or means by which the electricity is excited—and lastly, in being

supplied with nerves proceeding from different sources, and this is

perhaps the most important of all the differences. Hence in the several

fishes furnished with electric organs, these cannot be considered as

homologous, but only as analogous in function. Consequently there is no

reason to suppose that they have been inherited from a common

progenitor; for had this been the case they would have closely

resembled each other in all respects. Thus the difficulty of an organ,

apparently the same, arising in several remotely allied species,

disappears, leaving only the lesser yet still great difficulty: namely,

by what graduated steps these organs have been developed in each

separate group of fishes.

The luminous organs which occur in a few insects, belonging to widely

different families, and which are situated in different parts of the

body, offer, under our present state of ignorance, a difficulty almost

exactly parallel with that of the electric organs. Other similar cases

could be given; for instance in plants, the very curious contrivance of

a mass of pollen-grains, borne on a foot-stalk with an adhesive gland,

is apparently the same in Orchis and Asclepias, genera almost as remote

as is possible among flowering plants; but here again the parts are not

homologous. In all cases of beings, far removed from each other in the

scale of organisation, which are furnished with similar and peculiar

organs, it will be found that although the general appearance and

function of the organs may be the same, yet fundamental differences

between them can always be detected. For instance, the eyes of

Cephalopods or cuttle-fish and of vertebrate animals appear wonderfully

alike; and in such widely sundered groups no part of this resemblance

can be due to inheritance from a common progenitor. Mr. Mivart has

advanced this case as one of special difficulty, but I am unable to see

the force of his argument. An organ for vision must be formed of

transparent tissue, and must include some sort of lens for throwing an

image at the back of a darkened chamber. Beyond this superficial

resemblance, there is hardly any real similarity between the eyes of

cuttle-fish and vertebrates, as may be seen by consulting Hensen’s

admirable memoir on these organs in the Cephalopoda. It is impossible

for me here to enter on details, but I may specify a few of the points

of difference. The crystalline lens in the higher cuttle-fish consists

of two parts, placed one behind the other like two lenses, both having

a very different structure and disposition to what occurs in the

vertebrata. The retina is wholly different, with an actual inversion of

the elemental parts, and with a large nervous ganglion included within

the membranes of the eye. The relations of the muscles are as different

as it is possible to conceive, and so in other points. Hence it is not

a little difficult to decide how far even the same terms ought to be

employed in describing the eyes of the Cephalopoda and Vertebrata. It

is, of course, open to any one to deny that the eye in either case

could have been developed through the natural selection of successive

slight variations; but if this be admitted in the one case it is

clearly possible in the other; and fundamental differences of structure

in the visual organs of two groups might have been anticipated, in

accordance with this view of their manner of formation. As two men have

sometimes independently hit on the same invention, so in the several

foregoing cases it appears that natural selection, working for the good

of each being, and taking advantage of all favourable variations, has

produced similar organs, as far as function is concerned, in distinct

organic beings, which owe none of their structure in common to

inheritance from a common progenitor.

Fritz Müller, in order to test the conclusions arrived at in this

volume, has followed out with much care a nearly similar line of

argument. Several families of crustaceans include a few species,

possessing an air-breathing apparatus and fitted to live out of the

water. In two of these families, which were more especially examined by

Müller, and which are nearly related to each other, the species agree

most closely in all important characters: namely in their sense organs,

circulating systems, in the position of the tufts of hair within their

complex stomachs, and lastly in the whole structure of the

water-breathing branchiæ, even to the microscopical hooks by which they

are cleansed. Hence it might have been expected that in the few species

belonging to both families which live on the land, the equally

important air-breathing apparatus would have been the same; for why

should this one apparatus, given for the same purpose, have been made

to differ, whilst all the other important organs were closely similar,

or rather, identical.

Fritz Müller argues that this close similarity in so many points of

structure must, in accordance with the views advanced by me, be

accounted for by inheritance from a common progenitor. But as the vast

majority of the species in the above two families, as well as most

other crustaceans, are aquatic in their habits, it is improbable in the

highest degree that their common progenitor should have been adapted

for breathing air. Müller was thus led carefully to examine the

apparatus in the air-breathing species; and he found it to differ in

each in several important points, as in the position of the orifices,

in the manner in which they are opened and closed, and in some

accessory details. Now such differences are intelligible, and might

even have been expected, on the supposition that species belonging to

distinct families had slowly become adapted to live more and more out

of water, and to breathe the air. For these species, from belonging to

distinct families, would have differed to a certain extent, and in

accordance with the principle that the nature of each variation depends

on two factors, viz., the nature of the organism and that of the

surrounding conditions, their variability assuredly would not have been

exactly the same. Consequently natural selection would have had

different materials or variations to work on, in order to arrive at the

same functional result; and the structures thus acquired would almost

necessarily have differed. On the hypothesis of separate acts of

creation the whole case remains unintelligible. This line of argument

seems to have had great weight in leading Fritz Müller to accept the

views maintained by me in this volume.

Another distinguished zoologist, the late Professor Claparède, has

argued in the same manner, and has arrived at the same result. He shows

that there are parasitic mites (Acaridæ), belonging to distinct

sub-families and families, which are furnished with hair-claspers.

These organs must have been independently developed, as they could not

have been inherited from a common progenitor; and in the several groups

they are formed by the modification of the fore legs, of the hind legs,

of the maxillæ or lips, and of appendages on the under side of the hind

part of the body.

In the foregoing cases, we see the same end gained and the same

function performed, in beings not at all or only remotely allied, by

organs in appearance, though not in development, closely similar. On

the other hand, it is a common rule throughout nature that the same end

should be gained, even sometimes in the case of closely related beings,

by the most diversified means. How differently constructed is the

feathered wing of a bird and the membrane-covered wing of a bat; and

still more so the four wings of a butterfly, the two wings of a fly,

and the two wings with the elytra of a beetle. Bivalve shells are made

to open and shut, but on what a number of patterns is the hinge

constructed, from the long row of neatly interlocking teeth in a Nucula

to the simple ligament of a Mussel! Seeds are disseminated by their

minuteness, by their capsule being converted into a light balloon-like

envelope, by being embedded in pulp or flesh, formed of the most

diverse parts, and rendered nutritious, as well as conspicuously

coloured, so as to attract and be devoured by birds, by having hooks

and grapnels of many kinds and serrated awns, so as to adhere to the

fur of quadrupeds, and by being furnished with wings and plumes, as

different in shape as they are elegant in structure, so as to be wafted

by every breeze. I will give one other instance: for this subject of

the same end being gained by the most diversified means well deserves

attention. Some authors maintain that organic beings have been formed

in many ways for the sake of mere variety, almost like toys in a shop,

but such a view of nature is incredible. With plants having separated

sexes, and with those in which, though hermaphrodites, the pollen does

not spontaneously fall on the stigma, some aid is necessary for their

fertilisation. With several kinds this is effected by the

pollen-grains, which are light and incoherent, being blown by the wind

through mere chance on to the stigma; and this is the simplest plan

which can well be conceived. An almost equally simple, though very

different plan occurs in many plants in which a symmetrical flower

secretes a few drops of nectar, and is consequently visited by insects;

and these carry the pollen from the anthers to the stigma.

From this simple stage we may pass through an inexhaustible number of

contrivances, all for the same purpose and effected in essentially the

same manner, but entailing changes in every part of the flower. The

nectar may be stored in variously shaped receptacles, with the stamens

and pistils modified in many ways, sometimes forming trap-like

contrivances, and sometimes capable of neatly adapted movements through

irritability or elasticity. From such structures we may advance till we

come to such a case of extraordinary adaptation as that lately

described by Dr. Crüger in the Coryanthes. This orchid has part of its

labellum or lower lip hollowed out into a great bucket, into which

drops of almost pure water continually fall from two secreting horns

which stand above it; and when the bucket is half-full, the water

overflows by a spout on one side. The basal part of the labellum stands

over the bucket, and is itself hollowed out into a sort of chamber with

two lateral entrances; within this chamber there are curious fleshy

ridges. The most ingenious man, if he had not witnessed what takes

place, could never have imagined what purpose all these parts serve.

But Dr. Crüger saw crowds of large humble-bees visiting the gigantic

flowers of this orchid, not in order to suck nectar, but to gnaw off

the ridges within the chamber above the bucket; in doing this they

frequently pushed each other into the bucket, and their wings being

thus wetted they could not fly away, but were compelled to crawl out

through the passage formed by the spout or overflow. Dr. Crüger saw a

“continual procession” of bees thus crawling out of their involuntary

bath. The passage is narrow, and is roofed over by the column, so that

a bee, in forcing its way out, first rubs its back against the viscid

stigma and then against the viscid glands of the pollen-masses. The

pollen-masses are thus glued to the back of the bee which first happens

to crawl out through the passage of a lately expanded flower, and are

thus carried away. Dr. Crüger sent me a flower in spirits of wine, with

a bee which he had killed before it had quite crawled out, with a

pollen-mass still fastened to its back. When the bee, thus provided,

flies to another flower, or to the same flower a second time, and is

pushed by its comrades into the bucket and then crawls out by the

passage, the pollen-mass necessarily comes first into contact with the

viscid stigma, and adheres to it, and the flower is fertilised. Now at

last we see the full use of every part of the flower, of the

water-secreting horns of the bucket half-full of water, which prevents

the bees from flying away, and forces them to crawl out through the

spout, and rub against the properly placed viscid pollen-masses and the

viscid stigma.

The construction of the flower in another closely allied orchid,

namely, the Catasetum, is widely different, though serving the same

end; and is equally curious. Bees visit these flowers, like those of

the Coryanthes, in order to gnaw the labellum; in doing this they

inevitably touch a long, tapering, sensitive projection, or, as I have

called it, the antenna. This antenna, when touched, transmits a

sensation or vibration to a certain membrane which is instantly

ruptured; this sets free a spring by which the pollen-mass is shot

forth, like an arrow, in the right direction, and adheres by its viscid

extremity to the back of the bee. The pollen-mass of the male plant

(for the sexes are separate in this orchid) is thus carried to the

flower of the female plant, where it is brought into contact with the

stigma, which is viscid enough to break certain elastic threads, and

retain the pollen, thus effecting fertilisation.

How, it may be asked, in the foregoing and in innumerable other

instances, can we understand the graduated scale of complexity and the

multifarious means for gaining the same end. The answer no doubt is, as

already remarked, that when two forms vary, which already differ from

each other in some slight degree, the variability will not be of the

same exact nature, and consequently the results obtained through

natural selection for the same general purpose will not be the same. We

should also bear in mind that every highly developed organism has

passed through many changes; and that each modified structure tends to

be inherited, so that each modification will not readily be quite lost,

but may be again and again further altered. Hence, the structure of

each part of each species, for whatever purpose it may serve, is the

sum of many inherited changes, through which the species has passed

during its successive adaptations to changed habits and conditions of

life.

Finally, then, although in many cases it is most difficult even to

conjecture by what transitions organs could have arrived at their

present state; yet, considering how small the proportion of living and

known forms is to the extinct and unknown, I have been astonished how

rarely an organ can be named, towards which no transitional grade is

known to lead. It is certainly true, that new organs appearing as if

created for some special purpose rarely or never appear in any being;

as indeed is shown by that old, but somewhat exaggerated, 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

there be so much variety and so little real novelty? Why should all the

parts and organs of many independent beings, each supposed to have been

separately created for its own proper place in nature, be so commonly

linked together by graduated steps? Why should not Nature take a sudden

leap from structure to structure? On the theory of natural selection,

we can clearly understand why she should not; for natural selection

acts only by taking advantage of slight successive variations; she can

never take a great and sudden leap, but must advance by the short and

sure, though slow steps.

_Organs of little apparent Importance, as affected by Natural

Selection._

As natural selection acts by life and death, by the survival of the

fittest, and by the destruction of the less well-fitted individuals, I

have sometimes felt great difficulty in understanding the origin or

formation of parts of little importance; almost as great, though of a

very different kind, as in the case of the most perfect and complex

organs.

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

very trifling characters, such as the down on fruit and the colour of

its flesh, the colour of the skin and hair of quadrupeds, which, from

being correlated with constitutional differences, or from determining

the attacks of insects, 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 fitted, for so trifling an object

as to drive 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 depend 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 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 to existing species

in nearly the same state, although now of very slight use; but any

actually injurious deviations in their structure would of course 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 in the case of the dog, though the aid in this

latter respect must be slight, for the hare, with hardly any tail, can

double still more quickly.

In the second place, we may easily err in attributing importance to

characters, and in believing that they have been developed through

natural selection. We must by no means overlook the effects of the

definite action of changed conditions of life, of so-called spontaneous

variations, which seem to depend in a quite subordinate degree on the

nature of the conditions, of the tendency to reversion to long-lost

characters, of the complex laws of growth, such as of correlation,

comprehension, of the pressure of one part on another, &c., and finally

of sexual selection, by which characters of use to one sex are often

gained and then transmitted more or less perfectly to the other sex,

though of no use to the sex. But structures thus indirectly gained,

although at first of no advantage to a species, may subsequently have

been taken advantage of by its modified descendants, under new

conditions of life and newly acquired habits.

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 conceal this

tree-frequenting bird from its enemies; and consequently that it was a

character of importance, and had been acquired through natural

selection; as it is, the colour is probably in chief part due to sexual

selection. A trailing palm 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, and which, as there is reason to believe from

the distribution of the thorn-bearing species in Africa and South

America, serve as a defence against browsing quadrupeds, so the spikes

on the palm may at first have been developed for this object, and

subsequently have been improved and taken advantage of by the plant, as

it underwent further modification and became a climber. The naked skin

on the head of a vulture is generally considered 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 cause of each slight variation or

individual difference; and we are immediately made conscious of this by

reflecting on the differences between the breeds of our domesticated

animals in different countries, more especially in the less civilized

countries, where there has been but little methodical selection.

Animals kept by savages in different countries often have to struggle

for their own subsistence, and are exposed to a certain extent to

natural selection, and individuals with slightly different

constitutions would succeed best under different climates. With cattle

susceptibility to the attacks of flies is correlated with colour, as is

the liability to be poisoned by certain plants; so that even colour

would be thus subjected to the action of natural selection. Some

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 the head would probably be affected. The

shape, also, of the pelvis might affect by pressure the shape of

certain parts of the young in the womb. The laborious breathing

necessary in high regions tends, as we have good reason to believe, to

increase the size of the chest; and again correlation would come into

play. The effects of lessened exercise, together with abundant food, on

the whole organisation is probably still more important, and this, as

H. von Nathusius has lately shown in his excellent Treatise, is

apparently one chief cause of the great modification which the breeds

of swine have undergone. But we are far too ignorant to speculate on

the relative importance of the several known and unknown causes of

variation; and I have made these remarks only to show that, if we are

unable to account for the characteristic differences of our several

domestic breeds, which nevertheless are generally admitted to have

arisen through ordinary generation from one or a few parent-stocks, we

ought not to lay too much stress on our ignorance of the precise cause

of the slight analogous differences between true species.

_Utilitarian Doctrine, how far true: Beauty, how acquired._

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 many structures have been created for the sake of

beauty, to delight man or the Creator (but this latter point is beyond

the scope of scientific discussion), or for the sake of mere variety, a

view already discussed. Such doctrines, if true, would be absolutely

fatal to my theory. I fully admit that many structures are now of no

direct use to their possessors, and may never have been of any use to

their progenitors; but this does not prove that they were formed solely

for beauty or variety. No doubt the definite action of changed

conditions, and the various causes of modifications, lately specified,

have all produced an effect, probably a great effect, independently of

any advantage thus gained. But a still more important consideration is

that the chief part of the organisation of every living creature is due

to inheritance; and consequently, though each being assuredly is well

fitted for its place in nature, many structures have now no very close

and direct relation to present habits of life. 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 similar 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 webbed feet no doubt were as useful to the

progenitor of the upland goose and of the frigate-bird, as they now are

to the most aquatic of living birds. So we may believe that the

progenitor of the seal did not possess 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, were originally developed, on the principle of utility, probably

through the reduction of more numerous bones in the fin of some ancient

fish-like progenitor of the whole class. It is scarcely possible to

decide how much allowance ought to be made for such causes of change,

as the definite action of external conditions, so-called spontaneous

variations, and the complex laws of growth; but with these important

exceptions, we may conclude that the structure of every living creature

either now is, or was formerly, of some direct or indirect use to its

possessor.

With respect to the belief that organic beings have been created

beautiful for the delight of man—a belief which it has been pronounced

is subversive of my whole theory—I may first remark that the sense of

beauty obviously depends on the nature of the mind, irrespective of any

real quality in the admired object; and that the idea of what is

beautiful, is not innate or unalterable. We see this, for instance, in

the men of different races admiring an entirely different standard of

beauty in their women. If beautiful objects had been created solely for

man’s gratification, it ought to be shown that before man appeared

there was less beauty on the face of the earth than since he came on

the stage. Were the beautiful volute and cone shells of the Eocene

epoch, and the gracefully sculptured ammonites of the Secondary period,

created that man might ages afterwards admire them in his cabinet? Few

objects are more beautiful than the minute siliceous cases of the

diatomaceæ: were these created that they might be examined and admired

under the higher powers of the microscope? The beauty in this latter

case, and in many others, is apparently wholly due to symmetry of

growth. Flowers rank among the most beautiful productions of nature;

but they have been rendered conspicuous in contrast with the green

leaves, and in consequence at the same time beautiful, so that they may

be easily observed by insects. I have come to this conclusion from

finding it an invariable rule that when a flower is fertilised by the

wind it never has a gaily-coloured corolla. Several plants habitually

produce two kinds of flowers; one kind open and coloured so as to

attract insects; the other closed, not coloured, destitute of nectar,

and never visited by insects. Hence, we may conclude that, if insects

had not been developed on the face of the earth, our plants would not

have been decked with beautiful flowers, but would have produced only

such poor flowers as we see on our fir, oak, nut and ash trees, on

grasses, spinach, docks and nettles, which are all fertilised through

the agency of the wind. A similar line of argument holds good with

fruits; that a ripe strawberry or cherry is as pleasing to the eye as

to the palate—that the gaily-coloured fruit of the spindle-wood tree

and the scarlet berries of the holly are beautiful objects—will be

admitted by everyone. But this beauty serves merely as a guide to birds

and beasts, in order that the fruit may be devoured and the matured

seeds disseminated. I infer that this is the case from having as yet

found no exception to the rule that seeds are always thus disseminated

when embedded within a fruit of any kind (that is within a fleshy or

pulpy envelope), if it be coloured of any brilliant tint, or rendered

conspicuous by being white or black.

On the other hand, I willingly admit that a great number of male

animals, as all our most gorgeous birds, some fishes, reptiles, and

mammals, and a host of magnificently coloured butterflies, have been

rendered beautiful for beauty’s sake. But this has been effected

through sexual selection, that is, by the more beautiful males having

been continually preferred by the females, and not for the delight of

man. So it is with the music of birds. We may infer from all this that

a nearly similar taste for beautiful colours and for musical sounds

runs through a large part of the animal kingdom. When the female is as

beautifully coloured as the male, which is not rarely the case with

birds and butterflies, the cause apparently lies in the colours

acquired through sexual selection having been transmitted to both

sexes, instead of to the males alone. How the sense of beauty in its

simplest form—that is, the reception of a peculiar kind of pleasure

from certain colours, forms and sounds—was first developed in the mind

of man and of the lower animals, is a very obscure subject. The same

sort of difficulty is presented if we enquire how it is that certain

flavours and odours give pleasure, and others displeasure. Habit in all

these cases appears to have come to a certain extent into play; but

there must be some fundamental cause in the constitution of the nervous

system in each species.

Natural selection cannot possibly produce any modification in a species

exclusively for the good of another species; though throughout nature

one species incessantly takes advantage of, and profits by the

structures of others. But natural selection can and does often produce

structures for the direct injury of other animals, 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 it is furnished with a rattle for its own

injury, namely, to warn its prey. 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. It is a much more probable view that the

rattlesnake uses its rattle, the cobra expands its frill and the

puff-adder swells while hissing so loudly and harshly, in order to

alarm the many birds and beasts which are known to attack even the most

venomous species. Snakes act on the same principle which makes the hen

ruffle her feathers and expand her wings when a dog approaches her

chickens. But I have not space here to enlarge on the many ways by

which animals endeavour to frighten away their enemies.

Natural selection will never produce in a being any structure more

injurious than beneficial to that being, 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 comes into competition. And we see that this is the

standard 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 by Müller not to be perfect even in that

most perfect organ, the human eye. Helmholtz, whose judgment no one

will dispute, after describing in the strongest terms the wonderful

powers of the human eye, adds these remarkable words: “That which we

have discovered in the way of inexactness and imperfection in the

optical machine and in the image on the retina, is as nothing in

comparison with the incongruities which we have just come across in the

domain of the sensations. One might say that nature has taken delight

in accumulating contradictions in order to remove all foundation from

the theory of a pre-existing harmony between the external and internal

worlds.” 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 bee as perfect, which, when

used against many kinds of enemies, cannot be withdrawn, owing to the

backward serratures, and thus inevitably causes the death of the insect

by tearing out its viscera?

If we look at the sting of the bee, as having existed in a remote

progenitor, as a boring and serrated instrument, like that in so many

members of the same great order, and that it has since been modified

but not perfected for its present purpose, with the poison originally

adapted for some other object, such as to produce galls, since

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 social 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 purpose, 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 to destroy the

young queens, her daughters, as soon as they are 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 principles

of natural selection. If we admire the several ingenious contrivances

by which orchids and many other plants are fertilised through insect

agency, can we consider as equally perfect the elaboration of dense

clouds of pollen by our fir-trees, so that a few granules may be wafted

by chance on to the ovules?

_Summary: the Law of Unity of Type and of the Conditions of Existence

embraced by the Theory of Natural Selection._

We have in this chapter discussed some of the difficulties and

objections which may be urged against the theory. Many of them are

serious; but I think that in the discussion light has been thrown on

several facts, which on the belief 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 is always very slow, and at any one time acts only on a few

forms; and partly because the very process of natural selection 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 only glided through the air.

We have seen that a species under new conditions of life may change its

habits, or it may have diversified habits, with some 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 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 extremely cautious in concluding that none can have existed,

for the metamorphoses of many organs show what wonderful changes 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 in part or in whole specialised for one function; and two

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 have seen that in two beings widely remote from each other in the

natural scale, organs serving for the same purpose and in external

appearance closely similar may have been separately and independently

formed; but when such organs are closely examined, essential

differences in their structure can almost always be detected; and this

naturally follows from the principle of natural selection. On the other

hand, the common rule throughout nature is infinite diversity of

structure for gaining the same end; and this again naturally follows

from the same great principle.

In many cases we are far too ignorant to be enabled to assert that a

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. In many other cases, modifications are

probably the direct result of the laws of variation or of growth,

independently of any good having been thus gained. But even such

structures have often, as we may feel assured, been subsequently taken

advantage of, and still further modified, for the good of species under

new conditions of life. We may, also, believe that a part formerly of

high importance has frequently 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 means of natural selection.

Natural selection can 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

possessor. In each well-stocked country natural selection acts through

the competition of the inhabitants and consequently leads to success in

the battle for life, only in accordance with the standard of that

particular country. Hence the inhabitants of one country, generally the

smaller one, often yield to the inhabitants of another and generally

the 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 lead to

absolute perfection; nor, as far as we can judge by our limited

faculties, can absolute perfection be everywhere predicated.

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 to the present inhabitants alone of the

world, is not strictly correct; but if we include all those of past

times, whether known or unknown, it must on this 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 past

periods of time: the adaptations being aided in many cases by the

increased use or disuse of parts, being affected by the direct action

of external conditions of life, and subjected in all cases to the

several laws of growth and variation. Hence, in fact, the law of the

Conditions of Existence is the higher law; as it includes, through the

inheritance of former variations and adaptations, that of Unity of

Type.