 Chapter
6 - Difficulties on Theory
*
Difficulties on the theory of descent with modification
*
Transitions *
Absence or rarity of transitional varieties *
Transitions in habits of life *
Diversified habits in the same species *
Species with habits widely different from those of their
allies *
Organs of extreme perfection *
Means of transition *
Cases of difficulty *
Natura non facit saltum *
Organs of small importance *
Organs not in all cases absolutely perfect *
The law of Unity of Type and of the Conditions of Existence
embraced by the theory of Natural Selection
LONG before having arrived at this part of my work, a
crowd of difficulties will have occurred to the reader.
Some of them are so grave that to this day I can never reflect
on them without being staggered; but, to the best of my
judgment, the greater number are only apparent, and those
that are real are not, I think, fatal to my theory.
These difficulties and objections may be classed under
the following heads:-Firstly, why, if species have descended
from other species by insensibly fine gradations, do we
not everywhere see innumerable transitional forms? Why is
not all nature in confusion instead of the species being,
as we see them, well defined?
Secondly, is it possible that an animal having, for instance,
the structure and habits of a bat, could have been formed
by the modification of some animal with wholly different
habits? Can we believe that natural selection could produce,
on the one hand, organs of trifling importance, such as
the tail of a giraffe, which serves as a fly-flapper, and,
on the other hand, organs of such wonderful structure, as
the eye, of which we hardly as yet fully understand the
inimitable perfection?
Thirdly, can instincts be acquired and modified through
natural selection? What shall we say to so marvellous an
instinct as that which leads the bee to make cells, which
have practically anticipated the discoveries of profound
mathematicians?
Fourthly, how can we account for species, when crossed,
being sterile and producing sterile offspring, whereas,
when varieties are crossed, their fertility is unimpaired?
The two first heads shall be here discussed Instinct and
Hybridism in separate chapters.
On the absence or rarity of transitional varieties.
As natural selection acts solely by the preservation of
profitable modifications, each new form will tend in a fully-stocked
country to take the place of, and finally to exterminate,
its own less improved parent or other less-favoured forms
with which it comes into competition. Thus extinction and
natural selection will, as we have seen, go hand in hand.
Hence, if we look at each species as descended from some
other unknown form, both the parent and all the transitional
varieties will generally have been exterminated by the very
process of formation and perfection of the new form.
But, as by this theory innumerable transitional forms
must have existed, why do we not find them embedded in countless
numbers in the crust of the earth? It will be much more
convenient to discuss this question in the chapter on the
Imperfection of the geological record; and I will here only
state that I believe the answer mainly lies in the record
being incomparably less perfect than is generally supposed;
the imperfection of the record being chiefly due to organic
beings not inhabiting profound depths of the sea, and to
their remains being embedded and preserved to a future age
only in masses of sediment sufficiently thick and extensive
to withstand an enormous amount of future degradation; and
such fossiliferous masses can be accumulated only where
much sediment is deposited on the shallow bed of the sea,
whilst it slowly subsides. These contingencies will concur
only rarely, and after enormously long intervals. Whilst
the bed of the sea is stationary or is rising, or when very
little sediment is being deposited, there will be blanks
in our geological history. The crust of the earth is a vast
museum; but the natural collections have been made only
at intervals of time immensely remote.
But it may be urged that when several closely-allied species
inhabit the same territory we surely ought to find at the
present time many transitional forms. Let us take a simple
case: in travelling from north to south over a continent,
we generally meet at successive intervals with closely allied
or representative species, evidently filling nearly the
same place in the natural economy of the land. These representative
species often meet and interlock; and as the one becomes
rarer and rarer, the other becomes more and more frequent,
till the one replaces the other. But if we compare these
species where they intermingle, they are generally as absolutely
distinct from each other in every detail of structure as
are specimens taken from the metropolis inhabited by each.
By my theory these allied species have descended from a
common parent; and during the process of modification, each
has become adapted to the conditions of life of its own
region, and has supplanted and exterminated its original
parent and all the transitional varieties between its past
and present states. Hence we ought not to expect at the
present time to meet with numerous transitional varieties
in each region, though they must have existed there, and
may be embedded there in a fossil condition. But in the
intermediate region, having intermediate conditions of life,
why do we not now find closely-linking intermediate varieties?
This difficulty for a long time quite confounded me. But
I think it can be in large part explained.
In the first place we should be extremely cautious in
inferring, because an area is now continuous, that it has
been continuous during a long period. Geology would lead
us to believe that almost every continent has been broken
up into islands even during the later tertiary periods;
and in such islands distinct species might have been separately
formed without the possibility of intermediate varieties
existing in the intermediate zones. By changes in the form
of the land and of climate, marine areas now continuous
must often have existed within recent times in a far less
continuous and uniform condition than at present. But I
will pass over this way of escaping from the difficulty;
for I believe that many perfectly defined species have been
formed on strictly continuous areas; though I do not doubt
that the formerly broken condition of areas now continuous
has played an important part in the formation of new species,
more especially with freely-crossing and wandering animals.
In looking at species as they are now distributed over
a wide area, we generally find them tolerably numerous over
a large territory, then becoming somewhat abruptly rarer
and rarer on the confines, and finally disappearing. Hence
the neutral territory between two representative species
is generally narrow in comparison with the territory proper
to each. We see the same fact in ascending mountains, and
sometimes it is quite remarkable how abruptly, as Alph.
De Candolle has observed, a common alpine species disappears.
The same fact has been noticed by Forbes in sounding the
depths of the sea with the dredge. To those who look at
climate and the physical conditions of life as the all-important
elements of distribution, these facts ought to cause surprise,
as climate and height or depth graduate away insensibly.
But when we bear in mind that almost every species, even
in its metropolis, would increase immensely in numbers,
were it not for other competing species; that nearly all
either prey on or serve as prey for others; in short, that
each organic being is either directly or indirectly related
in the most important manner to other organic beings, we
must see that the range of the inhabitants of any country
by no means exclusively depends on insensibly changing physical
conditions, but in large part on the presence of other species,
on which it depends, or by which it is destroyed, or with
which it comes into competition; and as these species are
already defined objects (however they may have become so),
not blending one into another by insensible gradations,
the range of any one species, depending as it does on the
range of others, will tend to be sharply defined. Moreover,
each species on the confines of its range, where it exists
in lessened numbers, will, during fluctuations in the number
of its enemies or of its prey, or in the seasons, be extremely
liable to utter extermination; and thus its geographical
range will come to be still more sharply defined.
If I am right in believing that allied or representative
species, when inhabiting a continuous area, are generally
so distributed that each has a wide range, with a comparatively
narrow neutral territory between them, in which they become
rather suddenly rarer and rarer; then, as varieties do not
essentially differ from species, the same rule will probably
apply to both; and if we in imagination adapt a varying
species to a very large area, we shall have to adapt two
varieties to two large areas, and a third variety to a narrow
intermediate zone. The intermediate variety, consequently,
will exist in lesser numbers from inhabiting a narrow and
lesser area; and practically, as far as I can make out,
this rule holds good with varieties in a state of nature.
I have met with striking instances of the rule in the case
of varieties intermediate between well-marked varieties
in the genus Balanus. And it would appear from information
given me by Mr Watson, Dr Asa Gray, and Mr Wollaston, that
generally when varieties intermediate between two other
forms occur, they are much rarer numerically than the forms
which they connect. Now, if we may trust these facts and
inferences, and therefore conclude that varieties linking
two other varieties together have generally existed in lesser
numbers than the forms which they connect, then, I think,
we can understand why intermediate varieties should not
endure for very long periods; why as a general rule they
should be exterminated and disappear, sooner than the forms
which they originally linked together.
For any form existing in lesser numbers would, as already
remarked, run a greater chance of being exterminated than
one existing in large numbers; and in this particular case
the intermediate form would be eminently liable to the inroads
of closely allied forms existing on both sides of it. But
a far more important consideration, as I believe, is that,
during the process of further modification, by which two
varieties are supposed on my theory to be converted and
perfected into two distinct species, the two which exist
in larger numbers from inhabiting larger areas, will have
a great advantage over the intermediate variety, which exists
in smaller numbers in a narrow and intermediate zone. For
forms existing in larger numbers will always have a better
chance, within any given period, of presenting further favourable
variations for natural selection to seize on, than will
the rarer forms which exist in lesser numbers. Hence, the
more common forms, in the race for life, will tend to beat
and supplant the less common forms, for these will be more
slowly modified and improved. It is the same principle which,
as I believe, accounts for the common species in each country,
as shown in the second chapter, presenting on an average
a greater number of well-marked varieties than do the rarer
species. I may illustrate what I mean by supposing three
varieties of sheep to be kept, one adapted to an extensive
mountainous region; a second to a comparatively narrow,
hilly tract; and a third to wide plains at the base; and
that the inhabitants are all trying with equal steadiness
and skill to improve their stocks by selection; the chances
in this case will be strongly in favour of the great holders
on the mountains or on the plains improving their breeds
more quickly than the small holders on the intermediate
narrow, hilly tract; and consequently the improved mountain
or plain breed will soon take the place of the less improved
hill breed; and thus the two breeds, which originally existed
in greater numbers, will come into close contact with each
other, without the interposition of the supplanted, intermediate
hill-variety.
To sum up, I believe that species come to be tolerably
well-defined objects, and do not at any one period present
an inextricable chaos of varying and intermediate links:
firstly, because new varieties are very slowly formed, for
variation is a very slow process, and natural selection
can do nothing until favourable variations chance to occur,
and until a place in the natural polity of the country can
be better filled by some modification of some one or more
of its inhabitants. And such new places will depend on slow
changes of climate, or on the occasional immigration of
new inhabitants, and, probably, in a still more important
degree, on some of the old inhabitants becoming slowly modified,
with the new forms thus produced and the old ones acting
and reacting on each other. So that, in any one region and
at any one time, we ought only to see a few species presenting
slight modifications of structure in some degree permanent;
and this assuredly we do see.
Secondly, areas now continuous must often have existed
within the recent period in isolated portions, in which
many forms, more especially amongst the classes which unite
for each birth and wander much, may have separately been
rendered sufficiently distinct to rank as representative
species. In this case, intermediate varieties between the
several representative species and their common parent,
must formerly have existed in each broken portion of the
land, but these links will have been supplanted and exterminated
during the process of natural selection, so that they will
no longer exist in a living state.
Thirdly, when two or more varieties have been formed in
different portions of a strictly continuous area, intermediate
varieties will, it is probable, at first have been formed
in the intermediate zones, but they will generally have
had a short duration. For these intermediate varieties will,
from reasons already assigned (namely from what we know
of the actual distribution of closely allied or representative
species, and likewise of acknowledged varieties), exist
in the intermediate zones in lesser numbers than the varieties
which they tend to connect. From this cause alone the intermediate
varieties will be liable to accidental extermination; and
during the process of further modification through natural
selection, they will almost certainly be beaten and supplanted
by the forms which they connect; for these from existing
in greater numbers will, in the aggregate, present more
variation, and thus be further improved through natural
selection and gain further advantages.
Lastly, looking not to any one time, but to all time,
if my theory be true, numberless intermediate varieties,
linking most closely all the species of the same group together,
must assuredly have existed; but the very process of natural
selection constantly tends, as has been so often remarked,
to exterminate the parent forms and the intermediate links.
Consequently evidence of their former existence could be
found only amongst fossil remains, which are preserved,
as we shall in a future chapter attempt to show, in an extremely
imperfect and intermittent record.
On the origin and transitions of organic beings with
peculiar habits and structure. It has been asked by
the opponents of such views as I hold, how, for instance,
a land carnivorous animal could have been converted into
one with aquatic habits; for how could the animal in its
transitional state have subsisted? It would be easy to show
that within the same group carnivorous animals exist having
every intermediate grade between truly aquatic and strictly
terrestrial habits; and as each exists by a struggle for
life, it is clear that each is well adapted in its habits
to its place in nature. Look at the Mustela vison of North
America, which has webbed feet and which resembles an otter
in its fur, short legs, and form of tail; during summer
this animal dives for and preys on fish, but during the
long winter it leaves the frozen waters, and preys like
other polecats on mice and land animals. If a different
case had been taken, and it had been asked how an insectivorous
quadruped could possibly have been converted into a flying
bat, the question would have been far more difficult, and
I could have given no answer. Yet I think such difficulties
have very little weight.
Here, as on other occasions, I lie under a heavy disadvantage,
for out of the many striking cases which I have collected,
I can give only one or two instances of transitional habits
and structures in closely allied species of the same genus;
and of diversified habits, either constant or occasional,
in the same species. And it seems to me that nothing less
than a long list of such cases is sufficient to lessen the
difficulty in any particular case like that of the bat.
Look at the family of squirrels; here we have the finest
gradation from animals with their tails only slightly flattened,
and from others, as Sir J. Richardson has remarked, with
the posterior part of their bodies rather wide and with
the skin on their flanks rather full, to the so-called flying
squirrels; and flying squirrels have their limbs and even
the base of the tail united by a broad expanse of skin,
which serves as a parachute and allows them to glide through
the air to an astonishing distance from tree to tree. We
cannot doubt that each structure is of use to each kind
of squirrel in its own country, by enabling it to escape
birds or beasts of prey, or to collect food more quickly,
or, as there is reason to believe, by lessening the danger
from occasional falls. But it does not follow from this
fact that the structure of each squirrel is the best that
it is possible to conceive under all natural conditions.
Let the climate and vegetation change, let other competing
rodents or new beasts of prey immigrate, or old ones become
modified, and all analogy would lead us to believe that
some at least of the squirrels would decrease in numbers
or become exterminated, unless they also became modified
and improved in structure in a corresponding manner. Therefore,
I can see no difficulty, more especially under changing
conditions of life, in the continued preservation of individuals
with fuller and fuller flank-membranes, each modification
being useful, each being propagated, until by the accumulated
effects of this process of natural selection, a perfect
so-called flying squirrel was produced.
Now look at the Galeopithecus or flying lemur, which formerly
was falsely ranked amongst bats. It has an extremely wide
flank-membrane, stretching from the corners of the jaw to
the tail, and including the limbs and the elongated fingers:
the flank membrane is, also, furnished with an extensor
muscle. Although no graduated links of structure, fitted
for gliding through the air, now connect the Galeopithecus
with the other Lemuridae, yet I can see no difficulty in
supposing that such links formerly existed, and that each
had been formed by the same steps as in the case of the
less perfectly gliding squirrels; and that each grade of
structure had been useful to its possessor. Nor can I see
any insuperable difficulty in further believing it possible
that the membrane-connected fingers and fore-arm of the
Galeopithecus might be greatly lengthened by natural selection;
and this, as far as the organs of flight are concerned,
would convert it into a bat. In bats which have the wing-membrane
extended from the top of the shoulder to the tail, including
the hind-legs, we perhaps see traces of an apparatus originally
constructed for gliding through the air rather than for
flight.
If about a dozen genera of birds had become extinct or
were unknown, who would have ventured to have surmised that
birds might have existed which used their wings solely as
flappers, like the logger-headed duck (Micropterus of Eyton);
as fins in the water and front legs on the land, like the
penguin; as sails, like the ostrich; and functionally for
no purpose, like the Apteryx. Yet the structure of each
of these birds is good for it, under the conditions of life
to which it is exposed, for each has to live by a struggle;
but it is not necessarily the best possible under all possible
conditions. It must not be inferred from these remarks that
any of the grades of wing-structure here alluded to, which
perhaps may all have resulted from disuse, indicate the
natural steps by which birds have acquired their perfect
power of flight; but they serve, at least, to show what
diversified means of transition are possible.
Seeing that a few members of such water-breathing classes
as the Crustacea and Mollusca are adapted to live on the
land, and seeing that we have flying birds and mammals,
flying insects of the most diversified types, and formerly
had flying reptiles, it is conceivable that flying-fish,
which now glide far through the air, slightly rising and
turning by the aid of their fluttering fins, might have
been modified into perfectly winged animals. If early transitional
state they had been inhabitants of the open ocean, and had
used their incipient organs of flight exclusively, as far
as we know, to escape being devoured by other fish?
When we see any structure highly perfected for any particular
habit, as the wings of a bird for flight, we should bear
in mind that animals displaying early transitional grades
of the structure will seldom continue to exist to the present
day, for they will have been supplanted by the very process
of perfection through natural selection. Furthermore, we
may conclude that transitional grades between structures
fitted for very different habits of life will rarely have
been developed at an early period in great numbers and under
many subordinate forms. Thus, to return to our imaginary
illustration of the flying-fish, it does not seem probable
that fishes capable of true flight would have been developed
under many subordinate forms, for taking prey of many kinds
in many ways, on the land and in the water, until their
organs of flight had come to a high stage of perfection,
so as to have given them a decided advantage over other
animals in the battle for life. Hence the chance of discovering
species with transitional grades of structure in a fossil
condition will always be less, from their having existed
in lesser numbers, than in the case of species with fully
developed structures.
I will now give two or three instances of diversified
and of changed habits in the individuals of the same species.
When either case occurs, it would be easy for natural selection
to fit the animal, by some modification of its structure,
for its changed habits, or exclusively for one of its several
different habits. But it is difficult to tell, and immaterial
for us, whether habits generally change first and structure
afterwards; or whether slight modifications of structure
lead to changed habits; both probably often change almost
simultaneously. Of cases of changed habits it will suffice
merely to allude to that of the many British insects which
now feed on exotic plants, or exclusively on artificial
substances. Of diversified habits innumerable instances
could be given: I have often watched a tyrant flycatcher
(Saurophagus sulphuratus) in South America, hovering over
one spot and then proceeding to another, like a kestrel,
and at other times standing stationary on the margin of
water, and then dashing like a kingfisher at a fish. In
our own country the larger titmouse (Parus major) may be
seen climbing branches, almost like a creeper; it often,
like a shrike, kills small birds by blows on the head; and
I have many times seen and heard it hammering the seeds
of the yew on a branch, and thus breaking them like a nuthatch.
In North America the black bear was seen by Hearne swimming
for hours with widely open mouth, thus catching, like a
whale, insects in the water. Even in so extreme a case as
this, if the supply of insects were constant, and if better
adapted competitors did not already exist in the country,
I can see no difficulty in a race of bears being rendered,
by natural selection, more and more aquatic in their structure
and habits, with larger and larger mouths, till a creature
was produced as monstrous as a whale.
As we sometimes see individuals of a species following
habits widely different from those both of their own species
and of the other species of the same genus, we might expect,
on my theory, that such individuals would occasionally have
given rise to new species, having anomalous habits, and
with their structure either slightly or considerably modified
from that of their proper type. And such instances do occur
in nature. Can a more striking instance of adaptation be
given than that of a woodpecker for climbing trees and for
seizing insects in the chinks of the bark? Yet in North
America there are woodpeckers which feed largely on fruit,
and others with elongated wings which chase insects on the
wing; and on the plains of La Plata, where not a tree grows,
there is a woodpecker, which in every essential part of
its organisation, even in its colouring, in the harsh tone
of its voice, and undulatory flight, told me plainly of
its close blood-relationship to our common species; yet
it is a woodpecker which never climbs a tree!
Petrels are the most aërial and oceanic of birds,
yet in the quiet Sounds of Tierra del Fuego, the Puffinuria
berardi, in its general habits, in its astonishing power
of diving, its manner of swimming, and of flying when unwillingly
it takes flight, would be mistaken by any one for an auk
or grebe; nevertheless, it is essentially a petrel, but
with many parts of its organisation profoundly modified.
On the other hand, the acutest observer by examining the
dead body of the water-ouzel would never have suspected
its sub-aquatic habits; yet this anomalous member of the
strictly terrestrial thrush family wholly subsists by diving,
grasping the stones with its feet and using its wings under
water.
He who believes that each being has been created as we
now see it, must occasionally have felt surprise when he
has met with an animal having habits and structure not at
all in agreement. What can be plainer than that the webbed
feet of ducks and geese are formed for swimming; yet there
are upland geese with webbed feet which rarely or never
go near the water; and no one except Audubon has seen the
frigate-bird, which has all its four toes webbed, alight
on the surface of the sea. On the other hand, grebes and
coots are eminently aquatic, although their toes are only
bordered by membrane. What seems plainer than that the long
toes of grallatores are formed for walking over swamps and
floating plants, yet the water-hen is nearly as aquatic
as the coot; and the landrail nearly as terrestrial as the
quail or partridge. In such cases, and many others could
be given, habits have changed without a corresponding change
of structure. The webbed feet of the upland goose may be
said to have become rudimentary in function, though not
in structure. In the frigate-bird, the deeply-scooped membrane
between the toes shows that structure has begun to change.
He who believes in separate and innumerable acts of creation
will say, that in these cases it has pleased the Creator
to cause a being of one type to take the place of one of
another type; but this seems to me only restating the fact
in dignified language. He who believes in the struggle for
existence and in the principle of natural selection, will
acknowledge that every organic being is constantly endeavouring
to increase in numbers; and that if any one being vary ever
so little, either in habits or structure, and thus gain
an advantage over some other inhabitant of the country,
it will seize on the place of that inhabitant, however different
it may be from its own place. Hence it will cause him no
surprise that there should be geese and frigate-birds with
webbed feet, either living on the dry land or most rarely
alighting on the water; that there should be long-toed corncrakes
living in meadows instead of in swamps; that there should
be woodpeckers where not a tree grows; that there should
be diving thrushes, and petrels with the habits of auks.
Organs of extreme perfection and complication.
To suppose that the eye, with all its inimitable contrivances
for adjusting the focus to different distances, for admitting
different amounts of light, and for the correction of spherical
and chromatic aberration, could have been formed by natural
selection, seems, I freely confess, absurd in the highest
possible degree. Yet reason tells me, that if numerous gradations
from a perfect and complex eye to one very imperfect and
simple, each grade being useful to its possessor, can be
shown to exist; if further, the eye does vary ever so slightly,
and the variations be inherited, which is certainly the
case; and if any variation or modification in the organ
be ever useful to an animal under changing conditions of
life, then the difficulty of believing that a perfect and
complex eye could be formed by natural selection, though
insuperable by our imagination, can hardly be considered
real. How a nerve comes to be sensitive to light, hardly
concerns us more than how life itself first originated;
but I may remark that several facts make me suspect that
any sensitive nerve may be rendered sensitive to light,
and likewise to those coarser vibrations of the air which
produce sound.
In looking for the gradations by which an organ in any
species has been perfected, we ought to look exclusively
to its lineal ancestors; but this is scarcely ever possible,
and we are forced in each case to look to species of the
same group, that is to the collateral descendants from the
same original parent-form, in order to see what gradations
are possible, and for the chance of some gradations having
been transmitted from the earlier stages of descent, in
an unaltered or little altered condition. Amongst existing
Vertebrata, we find but a small amount of gradation in the
structure of the eye, and from fossil species we can learn
nothing on this head. In this great class we should probably
have to descend far beneath the lowest known fossiliferous
stratum to discover the earlier stages, by which the eye
has been perfected.
In the Articulata we can commence a series with an optic
nerve merely coated with pigment, and without any other
mechanism; and from this low stage, numerous gradations
of structure, branching off in two fundamentally different
lines, can be shown to exist, until we reach a moderately
high stage of perfection. In certain crustaceans, for instance,
there is a double cornea, the inner one divided into facets,
within each of which there is a lens shaped swelling. In
other crustaceans the transparent cones which are coated
by pigment, and which properly act only by excluding lateral
pencils of light, are convex at their upper ends and must
act by convergence; and at their lower ends there seems
to be an imperfect vitreous substance. With these facts,
here far too briefly and imperfectly given, which show that
there is much graduated diversity in the eyes of living
crustaceans, and bearing in mind how small the number of
living animals is in proportion to those which have become
extinct, I can see no very great difficulty (not more than
in the case of many other structures) in believing that
natural selection has converted the simple apparatus of
an optic nerve merely coated with pigment and invested by
transparent membrane, into an optical instrument as perfect
as is possessed by any member of the great Articulate class.
He who will go thus far, if he find on finishing this
treatise that large bodies of facts, otherwise inexplicable,
can be explained by the theory of descent, ought not to
hesitate to go further, and to admit that a structure even
as perfect as the eye of an eagle might be formed by natural
selection, although in this case he does not know any of
the transitional grades. His reason ought to conquer his
imagination; though I have felt the difficulty far too keenly
to be surprised at any degree of hesitation in extending
the principle of natural selection to such startling lengths.
It is scarcely possible to avoid comparing the eye to
a telescope. We know that this instrument has been perfected
by the long-continued efforts of the highest human intellects;
and we naturally infer that the eye has been formed by a
somewhat analogous process. But may not this inference be
presumptuous? Have we any right to assume that the Creator
works by intellectual powers like those of man? If we must
compare the eye to an optical instrument, we ought in imagination
to take a thick layer of transparent tissue, with a nerve
sensitive to light beneath, and then suppose every part
of this layer to be continually changing slowly in density,
so as to separate into layers of different densities and
thicknesses, placed at different distances from each other,
and with the surfaces of each layer slowly changing in form.
Further we must suppose that there is a power always intently
watching each slight accidental alteration in the transparent
layers; and carefully selecting each alteration which, under
varied circumstances, may in any way, or in any degree,
tend to produce a distincter image. We must suppose each
new state of the instrument to be multiplied by the million;
and each to be preserved till a better be produced, and
then the old ones to be destroyed. In living bodies, variation
will cause the slight alterations, generation will multiply
them almost infinitely, and natural selection will pick
out with unerring skill each improvement. Let this process
go on for millions on millions of years; and during each
year on millions of individuals of many kinds; and may we
not believe that a living optical instrument might thus
be formed as superior to one of glass, as the works of the
Creator are to those of man?
If it could be demonstrated that any complex organ existed,
which could not possibly have been formed by numerous, successive,
slight modifications, my theory would absolutely break down.
But I can find out no such case. No doubt many organs exist
of which we do not know the transitional grades, more especially
if we look to much-isolated species, round which, according
to my theory, there has been much extinction. Or again,
if we look to an organ common to all the members of a large
class, for in this latter case the organ must have been
first formed at an extremely remote period, since which
all the many members of the class have been developed; and
in order to discover the early transitional grades through
which the organ has passed, we should have to look to very
ancient ancestral forms, long since become extinct.
We should be extremely cautious in concluding that an
organ could not have been formed by transitional gradations
of some kind. Numerous cases could be given amongst the
lower animals of the same organ performing at the same time
wholly distinct functions; thus the alimentary canal respires,
digests, and excretes in the larva of the dragon-fly and
in the fish Cobites. In the Hydra, the animal may be turned
inside out, and the exterior surface will then digest and
the stomach respire. In such cases natural selection might
easily specialise, if any advantage were thus gained, a
part or organ, which had performed two functions, for one
function alone, and thus wholly change its nature by insensible
steps. Two distinct organs sometimes perform simultaneously
the same function in the same individual; to give one instance,
there are fish with gills or branchiae that breathe the
air dissolved in the water, at the same time that they breathe
free air in their swimbladders, this latter organ having
a ductus pneumaticus for its supply, and being divided by
highly vascular partitions. In these cases, one of the two
organs might with ease be modified and perfected so as to
perform all the work by itself, being aided during the process
of modification by the other organ; and then this other
organ might be modified for some other and quite distinct
purpose, or be quite obliterated.
The illustration of the swimbladder in fishes is a good
one, because it shows us clearly the highly important fact
that an organ originally constructed for one purpose, namely
flotation, may be converted into one for a wholly different
purpose, namely respiration. The swimbladder has, also,
been worked in as an accessory to the auditory organs of
certain fish, or, for I do not know which view is now generally
held, a part of the auditory apparatus has been worked in
as a complement to the swimbladder. All physiologists admit
that the swimbladder is homologous, or 'ideally similar,'
in position and structure with the lungs of the higher vertebrate
animals: hence there seems to me to be no great difficulty
in believing that natural selection has actually converted
a swimbladder into a lung, or organ used exclusively for
respiration.
I can, indeed, hardly doubt that all vertebrate animals
having true lungs have descended by ordinary generation
from an ancient prototype, of which we know nothing, furnished
with a floating apparatus or swimbladder. We can thus, as
I infer from Professor Owen's interesting description of
these parts, understand the strange fact that every particle
of food and drink which we swallow has to pass over the
orifice of the trachea, with some risk of falling into the
lungs, notwithstanding the beautiful contrivance by which
the glottis is closed. In the higher Vertebrata the branchiae
have wholly disappeared the slits on the sides of the neck
and the loop-like course of the arteries still marking in
the embryo their former position. But it is conceivable
that the now utterly lost branchiae might have been gradually
worked in by natural selection for some quite distinct purpose:
in the same manner as, on the view entertained by some naturalists
that the branchiae and dorsal scales of Annelids are homologous
with the wings and wing-covers of insects, it is probable
that organs which at a very ancient period served for respiration
have been actually converted into organs of flight.
In considering transitions of organs, it is so important
to bear in mind the probability of conversion from one function
to another, that I will give one more instance. Pedunculated
cirripedes have two minute folds of skin, called by me the
ovigerous frena, which serve, through the means of a sticky
secretion, to retain the eggs until they are hatched within
the sack. These cirripedes have no branchiae, the whole
surface of the body and sack, including the small frena,
serving for respiration. The Balanidae or sessile cirripedes,
on the other hand, have no ovigerous frena, the eggs lying
loose at the bottom of the sack, in the well-enclosed shell;
but they have large folded branchiae. Now I think no one
will dispute that the ovigerous frena in the one family
are strictly homologous with the branchiae of the other
family; indeed, they graduate into each other. Therefore
I do not doubt that little folds of skin, which originally
served as ovigerous frena, but which, likewise, very slightly
aided the act of respiration, have been gradually converted
by natural selection into branchiae, simply through an increase
in their size and the obliteration of their adhesive glands.
If all pedunculated cirripedes had become extinct, and they
have already suffered far more extinction than have sessile
cirripedes, who would ever have imagined that the branchiae
in this latter family had originally existed as organs for
preventing the ova from being washed out of the sack?
Although we must be extremely cautious in concluding that
any organ could not possibly have been produced by successive
transitional gradations, yet, undoubtedly, grave cases of
difficulty occur, some of which will be discussed in my
future work.
One of the gravest is that of neuter insects, which are
often very differently constructed from either the males
or fertile females; but this case will be treated of in
the next chapter. The electric organs of fishes offer another
case of special difficulty; it is impossible to conceive
by what steps these wondrous organs have been produced;
but, as Owen and others have remarked, their intimate structure
closely resembles that of common muscle; and as it has lately
been shown that Rays have an organ closely analogous to
the electric apparatus, and yet do not, as Matteuchi asserts,
discharge any electricity, we must own that we are far too
ignorant to argue that no transition of any kind is possible.
The electric organs offer another and even more serious
difficulty; for they occur in only about a dozen fishes,
of which several are widely remote in their affinities.
Generally when the same organ appears in several members
of the same class, especially if in members having very
different habits of life, we may attribute its presence
to inheritance from a common ancestor; and its absence in
some of the members to its loss through disuse or natural
selection. But if the electric organs had been inherited
from one ancient progenitor thus provided, we might have
expected that all electric fishes would have been specially
related to each other. Nor does geology at all lead to the
belief that formerly most fishes had electric organs, which
most of their modified descendants have lost. The presence
of luminous organs in a few insects, belonging to different
families and orders, offers a parallel case of difficulty.
Other cases could be given; for instance in plants, the
very curious contrivance of a mass of pollen-grains, borne
on a foot-stalk with a sticky gland at the end, is the same
in Orchis and Asclepias, genera almost as remote as possible
amongst flowering plants. In all these cases of two very
distinct species furnished with apparently the same anomalous
organ, it should be observed that, although the general
appearance and function of the organ may be the same, yet
some fundamental difference can generally be detected. I
am inclined to believe that in nearly the same way as two
men have sometimes independently hit on the very same invention,
so natural selection, working for the good of each being
and taking advantage of analogous variations, has sometimes
modified in very nearly the same manner two parts in two
organic beings, which owe but little of their structure
in common to inheritance from the same ancestor.
Although in many cases it is most difficult to conjecture
by what transitions an organ could have arrived at its present
state; yet, considering that the proportion of living and
known forms to the extinct and unknown is very small, I
have been astonished how rarely an organ can be named, towards
which no transitional grade is known to lead. The truth
of this remark is indeed shown by that old canon in natural
history of 'Natura non facit saltum.' We meet with this
admission in the writings of almost every experienced naturalist;
or, as Milne Edwards has well expressed it, nature is prodigal
in variety, but niggard in innovation. Why, on the theory
of Creation, should this be so? Why should all the parts
and organs of many independent beings, each supposed to
have been separately created for its proper place in nature,
be so invariably linked together by graduated steps? Why
should not Nature have taken a leap from structure to structure?
On the theory of natural selection, we can clearly understand
why she should not; for natural selection can act only by
taking advantage of slight successive variations; she can
never take a leap, but must advance by the shortest and
slowest steps.
Organs of little apparent importance. As natural
selection acts by life and death, by the preservation of
individuals with any favourable variation, and by the destruction
of those with any unfavourable deviation of structure, I
have sometimes felt much difficulty in understanding the
origin of simple parts, of which the importance does not
seem sufficient to cause the preservation of successively
varying individuals. I have sometimes felt as much difficulty,
though of a very different kind, on this head, as in the
case of an organ as perfect and complex as the eye.
In the first place, we are much too ignorant in regard
to the whole economy of any one organic being, to say what
slight modifications would be of importance or not. In a
former chapter I have given instances of most trifling characters,
such as the down on fruit and the colour of the flesh, which,
from determining the attacks of insects or from being correlated
with constitutional differences, might assuredly be acted
on by natural selection. The tail of the giraffe looks like
an artificially constructed fly-flapper; and it seems at
first incredible that this could have been adapted for its
present purpose by successive slight modifications, each
better and better, for so trifling an object as driving
away flies; yet we should pause before being too positive
even in this case, for we know that the distribution and
existence of cattle and other animals in South America absolutely
depends on their power of resisting the attacks of insects:
so that individuals which could by any means defend themselves
from these small enemies, would be able to range into new
pastures and thus gain a great advantage. It is not that
the larger quadrupeds are actually destroyed (except in
some rare cases) by the flies, but they are incessantly
harassed and their strength reduced, so that they are more
subject to disease, or not so well enabled in a coming dearth
to search for food, or to escape from beasts of prey.
Organs now of trifling importance have probably in some
cases been of high importance to an early progenitor, and,
after having been slowly perfected at a former period, have
been transmitted in nearly the same state, although now
become of very slight use; and any actually injurious deviations
in their structure will always have been checked by natural
selection. Seeing how important an organ of locomotion the
tail is in most aquatic animals, its general presence and
use for many purposes in so many land animals, which in
their lungs or modified swim-bladders betray their aquatic
origin, may perhaps be thus accounted for. A well-developed
tail having been formed in an aquatic animal, it might subsequently
come to be worked in for all sorts of purposes, as a fly-flapper,
an organ of prehension, or as an aid in turning, as with
the dog, though the aid must be slight, for the hare, with
hardly any tail, can double quickly enough.
In the second place, we may sometimes attribute importance
to characters which are really of very little importance,
and which have originated from quite secondary causes, independently
of natural selection. We should remember that climate, food,
&c., probably have some little direct influence on the
organisation; that characters reappear from the law of reversion;,
that correlation of growth will have had a most important
influence in modifying various structures; and finally,
that sexual selection will often have largely modified the
external characters of animals having a will, to give one
male an advantage in fighting with another or in charming
the females. Moreover when a modification of structure has
primarily arisen from the above or other unknown causes,
it may at first have been of no advantage to the species,
but may subsequently have been taken advantage of by the
descendants of the species under new conditions of life
and with newly acquired habits.
To give a few instances to illustrate these latter remarks.
If green woodpeckers alone had existed, and we did not know
that there were many black and pied kinds, I dare say that
we should have thought that the green colour was a beautiful
adaptation to hide this tree-frequenting bird from its enemies;
and consequently that it was a character of importance and
might have been acquired through natural selection; as it
is, I have no doubt that the colour is due to some quite
distinct cause, probably to sexual selection. A trailing
bamboo in the Malay Archipelago climbs the loftiest trees
by the aid of exquisitely constructed hooks clustered around
the ends of the branches, and this contrivance, no doubt,
is of the highest service to the plant; but as we see nearly
similar hooks on many trees which are not climbers the hooks
on the bamboo may have arisen from unknown laws of growth,
and have been subsequently taken advantage of by the plant
undergoing further modification and becoming a climber.
The naked skin on the head of a vulture is generally looked
at as a direct adaptation for wallowing in putridity; and
so it may be, or it may possibly be due to the direct action
of putrid matter; but we should be very cautious in drawing
any such inference, when we see that the skin on the head
of the clean-feeding male turkey is likewise naked. The
sutures in the skulls of young mammals have been advanced
as a beautiful adaptation for aiding parturition, and no
doubt they facilitate, or may be indispensable for this
act; but as sutures occur in the skulls of young birds and
reptiles, which have only to escape from a broken egg, we
may infer that this structure has arisen from the laws of
growth, and has been taken advantage of in the parturition
of the higher animals.
We are profoundly ignorant of the causes producing slight
and unimportant variations; and we are immediately made
conscious of this by reflecting on the differences in the
breeds of our domesticated animals in different countries,
more especially in the less civilized countries where there
has been but little artificial selection. Careful observers
are convinced that a damp climate affects the growth of
the hair, and that with the hair the horns are correlated.
Mountain breeds always differ from lowland breeds; and a
mountainous country would probably affect the hind limbs
from exercising them more, and possibly even the form of
the pelvis; and then by the law of homologous variation,
the front limbs and even the head would probably be affected.
The shape, also, of the pelvis might affect by pressure
the shape of the head of the young in the womb. The laborious
breathing necessary in high regions would, we have some
reason to believe, increase the size of the chest; and again
correlation would come into play. Animals kept by savages
in different countries often have to struggle for their
own subsistence, and would be exposed to a certain extent
to natural selection, and individuals with slightly different
constitutions would succeed best under different climates;
and there is reason to believe that constitution and colour
are correlated. A good observer, also, states that in cattle
susceptibility to the attacks of flies is correlated with
colour, as is the liability to be poisoned by certain plants;
so that colour would be thus subjected to the action of
natural selection. But we are far too ignorant to speculate
on the relative importance of the several known and unknown
laws of variation; and I have here alluded to them only
to show that, if we are unable to account for the characteristic
differences of our domestic breeds, which nevertheless we
generally admit to have arisen through ordinary generation,
we ought not to lay too much stress on our ignorance of
the precise cause of the slight analogous differences between
species. I might have adduced for this same purpose the
differences between the races of man, which are so strongly
marked; I may add that some little light can apparently
be thrown on the origin of these differences, chiefly through
sexual selection of a particular kind, but without here
entering on copious details my reasoning would appear frivolous.
The foregoing remarks lead me to say a few words on the
protest lately made by some naturalists, against the utilitarian
doctrine that every detail of structure has been produced
for the good of its possessor. They believe that very many
structures have been created for beauty in the eyes of man,
or for mere variety. This doctrine, if true, would be absolutely
fatal to my theory. Yet I fully admit that many structures
are of no direct use to their possessors. Physical conditions
probably have had some little effect on structure, quite
independently of any good thus gained. Correlation of growth
has no doubt played a most important part, and a useful
modification of one part will often have entailed on other
parts diversified changes of no direct use. So again characters
which formerly were useful, or which formerly had arisen
from correlation of growth, or from other unknown cause,
may reappear from the law of reversion, though now of no
direct use. The effects of sexual selection, when displayed
in beauty to charm the females, can be called useful only
in rather a forced sense. But by far the most important
consideration is that the chief part of the organisation
of every being is simply due to inheritance; and consequently,
though each being assuredly is well fitted for its place
in nature, many structures now have no direct relation to
the habits of life of each species. Thus, we can hardly
believe that the webbed feet of the upland goose or of the
frigate-bird are of special use to these birds; we cannot
believe that the same bones in the arm of the monkey, in
the fore leg of the horse, in the wing of the bat, and in
the flipper of the seal, are of special use to these animals.
We may safely attribute these structures to inheritance.
But to the progenitor of the upland goose and of the frigate-bird,
webbed feet no doubt were as useful as they now are to the
most aquatic of existing birds. So we may believe that the
progenitor of the seal had not a flipper, but a foot with
five toes fitted for walking or grasping; and we may further
venture to believe that the several bones in the limbs of
the monkey, horse, and bat, which have been inherited from
a common progenitor, were formerly of more special use to
that progenitor, or its progenitors, than they now are to
these animals having such widely diversified habits. Therefore
we may infer that these several bones might have been acquired
through natural selection, subjected formerly, as now, to
the several laws of inheritance, reversion, correlation
of growth, &c. Hence every detail of structure in every
living creature (making some little allowance for the direct
action of physical conditions) may be viewed, either as
having been of special use to some ancestral form, or as
being now of special use to the descendants of this form
either directly, or indirectly through the complex laws
of growth.
Natural selection cannot possibly produce any modification
in any one species exclusively for the good of another species;
though throughout nature one species incessantly takes advantage
of, and profits by, the structure of another. But natural
selection can and does often produce structures for the
direct injury of other species, as we see in the fang of
the adder, and in the ovipositor of the ichneumon, by which
its eggs are deposited in the living bodies of other insects.
If it could be proved that any part of the structure of
any one species had been formed for the exclusive good of
another species, it would annihilate my theory, for such
could not have been produced through natural selection.
Although many statements may be found in works on natural
history to this effect, I cannot find even one which seems
to me of any weight. It is admitted that the rattlesnake
has a poison-fang for its own defence and for the destruction
of its prey; but some authors suppose that at the same time
this snake is furnished with a rattle for its own injury,
namely, to warn its prey to escape. I would almost as soon
believe that the cat curls the end of its tail when preparing
to spring, in order to warn the doomed mouse. But I have
not space here to enter on this and other such cases.
Natural selection will never produce in a being anything
injurious to itself, for natural selection acts solely by
and for the good of each. No organ will be formed, as Paley
has remarked, for the purpose of causing pain or for doing
an injury to its possessor. If a fair balance be struck
between the good and evil caused by each part, each will
be found on the whole advantageous. After the lapse of time,
under changing conditions of life, if any part comes to
be injurious, it will be modified; or if it be not so, the
being will become extinct, as myriads have become extinct.
Natural selection tends only to make each organic being
as perfect as, or slightly more perfect than, the other
inhabitants of the same country with which it has to struggle
for existence. And we see that this is the degree of perfection
attained under nature. The endemic productions of New Zealand,
for instance, are perfect one compared with another; but
they are now rapidly yielding before the advancing legions
of plants and animals introduced from Europe. Natural selection
will not produce absolute perfection, nor do we always meet,
as far as we can judge, with this high standard under nature.
The correction for the aberration of light is said, on high
authority, not to be perfect even in that most perfect organ,
the eye. If our reason leads us to admire with enthusiasm
a multitude of inimitable contrivances in nature, this same
reason tells us, though we may easily err on both sides,
that some other contrivances are less perfect. Can we consider
the sting of the wasp or of the bee as perfect, which, when
used against many attacking animals, cannot be withdrawn,
owing to the backward serratures, and so inevitably causes
the death of the insect by tearing out its viscera?
If we look at the sting of the bee, as having originally
existed in a remote progenitor as a boring and serrated
instrument, like that in so many members of the same great
order, and which has been modified but not perfected for
its present purpose, with the poison originally adapted
to cause galls subsequently intensified, we can perhaps
understand how it is that the use of the sting should so
often cause the insect's own death: for if on the whole
the power of stinging be useful to the community, it will
fulfil all the requirements of natural selection, though
it may cause the death of some few members. If we admire
the truly wonderful power of scent by which the males of
many insects find their females, can we admire the production
for this single purpose of thousands of drones, which are
utterly useless to the community for any other end, and
which are ultimately slaughtered by their industrious and
sterile sisters? It may be difficult, but we ought to admire
the savage instinctive hatred of the queen-bee, which urges
her instantly to destroy the young queens her daughters
as soon as born, or to perish herself in the combat; for
undoubtedly this is for the good of the community; and maternal
love or maternal hatred, though the latter fortunately is
most rare, is all the same to the inexorable principle of
natural selection. If we admire the several ingenious contrivances,
by which the flowers of the orchis and of many other plants
are fertilised through insect agency, can we consider as
equally perfect the elaboration by our fir-trees of dense
clouds of pollen, in order that a few granules may be wafted
by a chance breeze on to the ovules?
Summary of Chapter. We have in this chapter discussed
some of the difficulties and objections which may be urged
against my theory. Many of them are very grave; but I think
that in the discussion light has been thrown on several
facts, which on the theory of independent acts of creation
are utterly obscure. We have seen that species at any one
period are not indefinitely variable, and are not linked
together by a multitude of intermediate gradations, partly
because the process of natural selection will always be
very slow, and will act, at any one time, only on a very
few forms; and partly because the very process of natural
selection almost implies the continual supplanting and extinction
of preceding and intermediate gradations. Closely allied
species, now living on a continuous area, must often have
been formed when the area was not continuous, and when the
conditions of life did not insensibly graduate away from
one part to another. When two varieties are formed in two
districts of a continuous area, an intermediate variety
will often be formed, fitted for an intermediate zone; but
from reasons assigned, the intermediate variety will usually
exist in lesser numbers than the two forms which it connects;
consequently the two latter, during the course of further
modification, from existing in greater numbers, will have
a great advantage over the less numerous intermediate variety,
and will thus generally succeed in supplanting and exterminating
it.
We have seen in this chapter how cautious we should be
in concluding that the most different habits of life could
not graduate into each other; that a bat, for instance,
could not have been formed by natural selection from an
animal which at first could only glide through the air.
We have seen that a species may under new conditions of
life change its habits, or have diversified habits, with
some habits very unlike those of its nearest congeners.
Hence we can understand bearing in mind that each organic
being is trying to live wherever it can live, how it has
arisen that there are upland geese with webbed feet, ground
woodpeckers, diving thrushes, and petrels with the habits
of auks.
Although the belief that an organ so perfect as the eye
could have been formed by natural selection, is more than
enough to stagger any one; yet in the case of any organ,
if we know of a long series of gradations in complexity,
each good for its possessor, then, under changing conditions
of life, there is no logical impossibility in the acquirement
of any conceivable degree of perfection through natural
selection. In the cases in which we know of no intermediate
or transitional states, we should be very cautious in concluding
that none could have existed, for the homologies of many
organs and their intermediate states show that wonderful
metamorphoses in function are at least possible. For instance,
a swim-bladder has apparently been converted into an air-breathing
lung. The same organ having performed simultaneously very
different functions, and then having been specialised for
one function; and two very distinct organs having performed
at the same time the same function, the one having been
perfected whilst aided by the other, must often have largely
facilitated transitions.
We are far too ignorant, in almost every case, to be enabled
to assert that any part or organ is so unimportant for the
welfare of a species, that modifications in its structure
could not have been slowly accumulated by means of natural
selection. But we may confidently believe that many modifications,
wholly due to the laws of growth, and at first in no way
advantageous to a species, have been subsequently taken
advantage of by the still further modified descendants of
this species. We may, also, believe that a part formerly
of high importance has often been retained (as the tail
of an aquatic animal by its terrestrial descendants), though
it has become of such small importance that it could not,
in its present state, have been acquired by natural selection,
a power which acts solely by the preservation of profitable
variations in the struggle for life.
Natural selection will produce nothing in one species
for the exclusive good or injury of another; though it may
well produce parts, organs, and excretions highly useful
or even indispensable, or highly injurious to another species,
but in all cases at the same time useful to the owner. Natural
selection in each well-stocked country, must act chiefly
through the competition of the inhabitants one with another,
and consequently will produce perfection, or strength in
the battle for life, only according to the standard of that
country. Hence the inhabitants of one country, generally
the smaller one, will often yield, as we see they do yield,
to the inhabitants of another and generally larger country.
For in the larger country there will have existed more individuals,
and more diversified forms, and the competition will have
been severer, and thus the standard of perfection will have
been rendered higher. Natural selection will not necessarily
produce absolute perfection; nor, as far as we can judge
by our limited faculties, can absolute perfection be everywhere
found.
On the theory of natural selection we can clearly understand
the full meaning of that old canon in natural history, 'Natura
non facit saltum.' This canon, if we look only to the present
inhabitants of the world, is not strictly correct, but if
we include all those of past times, it must by my theory
be strictly true.
It is generally acknowledged that all organic beings have
been formed on two great laws Unity of Type, and the Conditions
of Existence. By unity of type is meant that fundamental
agreement in structure, which we see in organic beings of
the same class, and which is quite independent of their
habits of life. On my theory, unity of type is explained
by unity of descent. The expression of conditions of existence,
so often insisted on by the illustrious Cuvier, is fully
embraced by the principle of natural selection. For natural
selection acts by either now adapting the varying parts
of each being to its organic and inorganic conditions of
life; or by having adapted them during long-past periods
of time: the adaptations being aided in some cases by use
and disuse, being slightly affected by the direct action
of the external conditions of life, and being in all cases
subjected to the several laws of growth. Hence, in fact,
the law of the Conditions of Existence is the higher law;
as it includes, through the inheritance of former adaptations,
that of Unity of Type.
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