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 Lemuridæ, yet I can see no difficulty in supposing that such
links formerly existed, and that each had been formed by the same steps
as in the case of the less perfectly gliding squirrels; and that each
grade of structure had been useful to its possessor. Nor can I see any
insuperable difficulty in further believing it possible that the
membrane-connected fingers and fore-arm of the Galeopithecus might be
greatly lengthened by natural selection; and this, as far as the organs
of flight are concerned, would convert it into a bat. In bats which
have the wing-membrane extended from the top of the shoulder to the
tail, including the hind-legs, we perhaps see traces of an apparatus
originally constructed for gliding through the air rather than for
flight.
If about a dozen genera of birds had become extinct or were unknown,
who would have ventured to have
surmised that birds might have existed which used their wings solely as
flappers, like the logger-headed duck (Micropterus of Eyton); as fins
in the water and front legs on the land, like the penguin; as sails,
like the ostrich; and functionally for no purpose, like the Apteryx.
Yet the structure of each of these birds is good for it, under the
conditions of life to which it is exposed, for each has to live by a
struggle; but it is not necessarily the best possible under all
possible conditions. It must not be inferred from these remarks that
any of the grades of wing-structure here alluded to, which perhaps may
all have resulted from disuse, indicate the natural steps by which
birds have acquired their perfect power of flight; but they serve, at
least, to show what diversified means of transition are possible.
Seeing that a few members of such water-breathing classes as the
Crustacea and Mollusca are adapted to live on the land, and seeing that
we have flying birds and mammals, flying insects of the most
diversified types, and formerly had flying reptiles, it is conceivable
that flying-fish, which now glide far through the air, slightly rising
and turning by the aid of their fluttering fins, might have been
modified into perfectly winged animals. If this had been effected, who
would have ever imagined that in an early transitional state they had
been inhabitants of the open ocean, and had used their incipient organs
of flight exclusively, as far as we know, to escape being devoured by
other fish?
When we see any structure highly perfected for any particular habit, as
the wings of a bird for flight, we should bear in mind that animals
displaying early transitional grades of the structure will seldom
continue to exist to the present day, for they will have been
supplanted by the very process of perfection through natural selection.
Furthermore, we may conclude that transitional
grades between structures fitted for very different habits of life will
rarely have been developed at an early period in great numbers and
under many subordinate forms. Thus, to return to our imaginary
illustration of the flying-fish, it does not seem probable that fishes
capable of true flight would have been developed under many subordinate
forms, for taking prey of many kinds in many ways, on the land and in
the water, until their organs of flight had come to a high stage of
perfection, so as to have given them a decided advantage over other
animals in the battle for life. Hence the chance of discovering species
with transitional grades of structure in a fossil condition will always
be less, from their having existed in lesser numbers, than in the case
of species with fully developed structures.
I will now give two or three instances of diversified and of changed
habits in the individuals of the same species. When either case occurs,
it would be easy for natural selection to fit the animal, by some
modification of its structure, for its changed habits, or exclusively
for one of its several different habits. But it is difficult to tell,
and immaterial for us, whether habits generally change first and
structure afterwards; or whether slight modifications of structure lead
to changed habits; both probably often change almost simultaneously. Of
cases of changed habits it will suffice merely to allude to that of the
many British insects which now feed on exotic plants, or exclusively on
artificial substances. Of diversified habits innumerable instances
could be given: I have often watched a tyrant flycatcher (Saurophagus
sulphuratus) in South America, hovering over one spot and then
proceeding to another, like a kestrel, and at other times standing
stationary on the margin of water, and then dashing like a kingfisher
at a fish. In our own country the larger titmouse (Parus major) may be
seen climbing branches, almost like a creeper; it often, like a shrike,
kills small birds by blows on the head; and I have many times seen and
heard it hammering the seeds of the yew on a branch, and thus breaking
them like a nuthatch. In North America the black bear was seen by
Hearne swimming for hours with widely open mouth, thus catching, like a
whale, insects in the water. Even in so extreme a case as this, if the
supply of insects were constant, and if better adapted competitors did
not already exist in the country, I can see no difficulty in a race of
bears being rendered, by natural selection, more and more aquatic in
their structure and habits, with larger and larger mouths, till a
creature was produced as monstrous as a whale.
As we sometimes see individuals of a species following habits widely
different from those both of their own species and of the other species
of the same genus, we might expect, on my theory, that such individuals
would occasionally have given rise to new species, having anomalous
habits, and with their structure either slightly or considerably
modified from that of their proper type. And such instances do occur in
nature. Can a more striking instance of adaptation be given than that
of a woodpecker for climbing trees and for seizing insects in the
chinks of the bark? Yet in North America there are woodpeckers which
feed largely on fruit, and others with elongated wings which chase
insects on the wing; and on the plains of La Plata, where not a tree
grows, there is a woodpecker, which in every essential part of its
organisation, even in its colouring, in the harsh tone of its voice,
and undulatory flight, told me plainly of its close blood-relationship
to our common species; yet it is a woodpecker which never climbs a
tree!
Petrels are the most 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 branchiæ 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 branchiæ 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 branchiæ 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 branchiæ 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 branchiæ, the whole surface of the body
and sack, including 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, in the
well-enclosed shell; but they have large folded 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 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 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
already 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?
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, etc., 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, etc.
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.