|
Chapter VI
DIFFICULTIES OF THE THEORY.
Difficulties of the theory of descent with modification -- Absence or
rarity of transitional varieties -- Transitions in habits of life --
Diversified habits in the same species -- Species with habits widely
different from those of their allies -- Organs of extreme perfection --
Modes of transition -- Cases of difficulty -- Natura non facit saltum --
Organs of small importance -- Organs not in all cases absolutely perfect --
The law of Unity of Type and of the Conditions of Existence embraced by the
theory of Natural Selection.
Long before the reader has arrived at this part of my work, a crowd of
difficulties will have occurred to him. Some of them are so serious that
to this day I can hardly reflect on them without being in some degree
staggered; but, to the best of my judgment, the greater number are only
apparent, and those that are real are not, I think, fatal to the theory.
These difficulties and objections may be classed under the following heads:
First, why, if species have descended from other species by fine
gradations, do we not everywhere see innumerable transitional forms? Why
is not all nature in confusion, instead of the species being, as we see
them, well defined?
Secondly, is it possible that an animal having, for instance, the structure
and habits of a bat, could have been formed by the modification of some
other animal with widely different habits and structure? Can we believe
that natural selection could produce, on the one hand, an organ of trifling
importance, such as the tail of a giraffe, which serves as a fly-flapper,
and, on the other hand, an organ so wonderful as the eye?
Thirdly, can instincts be acquired and modified through natural selection?
What shall we say to the instinct which leads the bee to make cells, and
which has practically anticipated the discoveries of profound
mathematicians?
Fourthly, how can we account for species, when crossed, being sterile and
producing sterile offspring, whereas, when varieties are crossed, their
fertility is unimpaired?
The two first heads will be here discussed; some miscellaneous objections
in the following chapter; Instinct and Hybridism in the two succeeding
chapters.
ON THE ABSENCE OR RARITY OF TRANSITIONAL VARIETIES.
As natural selection acts solely by the preservation of profitable
modifications, each new form will tend in a fully-stocked country to take
the place of, and finally to exterminate, its own less improved parent-form
and other less-favoured forms with which it comes into competition. Thus
extinction and natural selection go hand in hand. Hence, if we look at
each species as descended from some unknown form, both the parent and all
the transitional varieties will generally have been exterminated by the
very process of the formation and perfection of the new form.
But, as by this theory innumerable transitional forms must have existed,
why do we not find them embedded in countless numbers in the crust of the
earth? It will be more convenient to discuss this question in the chapter
on the imperfection of the geological record; and I will here only state
that I believe the answer mainly lies in the record being incomparably less
perfect than is generally supposed. The crust of the earth is a vast
museum; but the natural collections have been imperfectly made, and only at
long intervals of time.
But it may be urged that when several closely allied species inhabit the
same territory, we surely ought to find at the present time many
transitional forms. Let us take a simple case: in travelling from north
to south over a continent, we generally meet at successive intervals with
closely allied or representative species, evidently filling nearly the same
place in the natural economy of the land. These representative species
often meet and interlock; and as the one becomes rarer and rarer, the other
becomes more and more frequent, till the one replaces the other. But if we
compare these species where they intermingle, they are generally as
absolutely distinct from each other in every detail of structure as are
specimens taken from the metropolis inhabited by each. By my theory these
allied species are descended from a common parent; and during the process
of modification, each has become adapted to the conditions of life of its
own region, and has supplanted and exterminated its original parent-form
and all the transitional varieties between its past and present states.
Hence we ought not to expect at the present time to meet with numerous
transitional varieties in each region, though they must have existed there,
and may be embedded there in a fossil condition. But in the intermediate
region, having intermediate conditions of life, why do we not now find
closely-linking intermediate varieties? This difficulty for a long time
quite confounded me. But I think it can be in large part explained.
In the first place we should be extremely cautious in inferring, because an
area is now continuous, that it has been continuous during a long period.
Geology would lead us to believe that most continents have been broken up
into islands even during the later tertiary periods; and in such islands
distinct species might have been separately formed without the possibility
of intermediate varieties existing in the intermediate zones. By changes
in the form of the land and of climate, marine areas now continuous must
often have existed within recent times in a far less continuous and uniform
condition than at present. But I will pass over this way of escaping from
the difficulty; for I believe that many perfectly defined species have been
formed on strictly continuous areas; though I do not doubt that the
formerly broken condition of areas now continuous, has played an important
part in the formation of new species, more especially with freely-crossing
and wandering animals.
In looking at species as they are now distributed over a wide area, we
generally find them tolerably numerous over a large territory, then
becoming somewhat abruptly rarer and rarer on the confines, and finally
disappearing. Hence the neutral territory between two representative
species is generally narrow in comparison with the territory proper to
each. We see the same fact in ascending mountains, and sometimes it is
quite remarkable how abruptly, as Alph. De Candolle has observed, a common
alpine species disappears. The same fact has been noticed by E. Forbes in
sounding the depths of the sea with the dredge. To those who look at
climate and the physical conditions of life as the all-important elements
of distribution, these facts ought to cause surprise, as climate and height
or depth graduate away insensibly. But when we bear in mind that almost
every species, even in its metropolis, would increase immensely in numbers,
were it not for other competing species; that nearly all either prey on or
serve as prey for others; in short, that each organic being is either
directly or indirectly related in the most important manner to other
organic beings--we see that the range of the inhabitants of any country by
no means exclusively depends on insensibly changing physical conditions,
but in large part on the presence of other species, on which it lives, or
by which it is destroyed, or with which it comes into competition; and as
these species are already defined objects, not blending one into another by
insensible gradations, the range of any one species, depending as it does
on the range of others, will tend to be sharply defined. Moreover, each
species on the confines of its range, where it exists in lessened numbers,
will, during fluctuations in the number of its enemies or of its prey, or
in the nature of the seasons, be extremely liable to utter extermination;
and thus its geographical range will come to be still more sharply defined.
As allied or representative species, when inhabiting a continuous area, are
generally distributed in such a manner that each has a wide range, with a
comparatively narrow neutral territory between them, in which they become
rather suddenly rarer and rarer; then, as varieties do not essentially
differ from species, the same rule will probably apply to both; and if we
take a varying species inhabiting a very large area, we shall have to adapt
two varieties to two large areas, and a third variety to a narrow
intermediate zone. The intermediate variety, consequently, will exist in
lesser numbers from inhabiting a narrow and lesser area; and practically,
as far as I can make out, this rule holds good with varieties in a state of
nature. I have met with striking instances of the rule in the case of
varieties intermediate between well-marked varieties in the genus Balanus.
And it would appear from information given me by Mr. Watson, Dr. Asa Gray,
and Mr. Wollaston, that generally, when varieties intermediate between two
other forms occur, they are much rarer numerically than the forms which
they connect. Now, if we may trust these facts and inferences, and
conclude that varieties linking two other varieties together generally have
existed in lesser numbers than the forms which they connect, then we can
understand why intermediate varieties should not endure for very long
periods: why, as a general rule, they should be exterminated and
disappear, sooner than the forms which they originally linked together.
For any form existing in lesser numbers would, as already remarked, run a
greater chance of being exterminated than one existing in large numbers;
and in this particular case the intermediate form would be eminently liable
to the inroads of closely allied forms existing on both sides of it. But
it is a far more important consideration, that during the process of
further modification, by which two varieties are supposed to be converted
and perfected into two distinct species, the two which exist in larger
numbers, from inhabiting larger areas, will have a great advantage over the
intermediate variety, which exists in smaller numbers in a narrow and
intermediate zone. For forms existing in larger numbers will have a better
chance, within any given period, of presenting further favourable
variations for natural selection to seize on, than will the rarer forms
which exist in lesser numbers. Hence, the more common forms, in the race
for life, will tend to beat and supplant the less common forms, for these
will be more slowly modified and improved. It is the same principle which,
as I believe, accounts for the common species in each country, as shown in
the second chapter, presenting on an average a greater number of
well-marked varieties than do the rarer species. I may illustrate what I
mean by supposing three varieties of sheep to be kept, one adapted to an
extensive mountainous region; a second to a comparatively narrow, hilly
tract; and a third to the wide plains at the base; and that the inhabitants
are all trying with equal steadiness and skill to improve their stocks by
selection; the chances in this case will be strongly in favour of the great
holders on the mountains or on the plains improving their breeds more
quickly than the small holders on the intermediate narrow, hilly tract; and
consequently the improved mountain or plain breed will soon take the place
of the less improved hill breed; and thus the two breeds, which originally
existed in greater numbers, will come into close contact with each other,
without the interposition of the supplanted, intermediate hill variety.
To sum up, I believe that species come to be tolerably well-defined
objects, and do not at any one period present an inextricable chaos of
varying and intermediate links: first, because new varieties are very
slowly formed, for variation is a slow process, and natural selection can
do nothing until favourable individual differences or variations occur, and
until a place in the natural polity of the country can be better filled by
some modification of some one or more of its inhabitants. And such new
places will depend on slow changes of climate, or on the occasional
immigration of new inhabitants, and, probably, in a still more important
degree, on some of the old inhabitants becoming slowly modified, with the
new forms thus produced and the old ones acting and reacting on each other.
So that, in any one region and at any one time, we ought to see only a few
species presenting slight modifications of structure in some degree
permanent; and this assuredly we do see.
Secondly, areas now continuous must often have existed within the recent
period as isolated portions, in which many forms, more especially among the
classes which unite for each birth and wander much, may have separately
been rendered sufficiently distinct to rank as representative species. In
this case, intermediate varieties between the several representative
species and their common parent, must formerly have existed within each
isolated portion of the land, but these links during the process of natural
selection will have been supplanted and exterminated, so that they will no
longer be found in a living state.
Thirdly, when two or more varieties have been formed in different portions
of a strictly continuous area, intermediate varieties will, it is probable,
at first have been formed in the intermediate zones, but they will
generally have had a short duration. For these intermediate varieties
will, from reasons already assigned (namely from what we know of the actual
distribution of closely allied or representative species, and likewise of
acknowledged varieties), exist in the intermediate zones in lesser numbers
than the varieties which they tend to connect. From this cause alone the
intermediate varieties will be liable to accidental extermination; and
during the process of further modification through natural selection, they
will almost certainly be beaten and supplanted by the forms which they
connect; for these, from existing in greater numbers will, in the
aggregate, present more varieties, and thus be further improved through
natural selection and gain further advantages.
Lastly, looking not to any one time, but at all time, if my theory be true,
numberless intermediate varieties, linking closely together all the species
of the same group, must assuredly have existed; but the very process of
natural selection constantly tends, as has been so often remarked, to
exterminate the parent forms and the intermediate links. Consequently
evidence of their former existence could be found only among fossil remains,
which are preserved, as we shall attempt to show in a future chapter, in an
extremely imperfect and intermittent record.
ON THE ORIGIN AND TRANSITION OF ORGANIC BEINGS WITH PECULIAR HABITS AND
STRUCTURE.
It has been asked by the opponents of such views as I hold, how, for
instance, could a land carnivorous animal have been converted into one with
aquatic habits; for how could the animal in its transitional state have
subsisted? It would be easy to show that there now exist carnivorous
animals presenting close intermediate grades from strictly terrestrial to
aquatic habits; and as each exists by a struggle for life, it is clear that
each must be well adapted to its place in nature. Look at the Mustela
vison of North America, which has webbed feet, and which resembles an otter
in its fur, short legs, and form of tail; during summer this animal dives
for and preys on fish, but during the long winter it leaves the frozen
waters, and preys, like other polecats on mice and land animals. If a
different case had been taken, and it had been asked how an insectivorous
quadruped could possibly have been converted into a flying bat, the
question would have been far more difficult to answer. Yet I think such
difficulties have little weight.
Here, as on other occasions, I lie under a heavy disadvantage, for, out of
the many striking cases which I have collected, I can give only one or two
instances of transitional habits and structures in allied species; and of
diversified habits, either constant or occasional, in the same species.
And it seems to me that nothing less than a long list of such cases is
sufficient to lessen the difficulty in any particular case like that of the
bat.
Look at the family of squirrels; here we have the finest gradation from
animals with their tails only slightly flattened, and from others, as Sir
J. Richardson has remarked, with the posterior part of their bodies rather
wide and with the skin on their flanks rather full, to the so-called flying
squirrels; and flying squirrels have their limbs and even the base of the
tail united by a broad expanse of skin, which serves as a parachute and
allows them to glide through the air to an astonishing distance from tree
to tree. We cannot doubt that each structure is of use to each kind of
squirrel in its own country, by enabling it to escape birds or beasts of
prey, or to collect food more quickly, or, as there is reason to believe,
to lessen the danger from occasional falls. But it does not follow from
this fact that the structure of each squirrel is the best that it is
possible to conceive under all possible conditions. Let the climate and
vegetation change, let other competing rodents or new beasts of prey
immigrate, or old ones become modified, and all analogy would lead us to
believe that some, at least, of the squirrels would decrease in numbers or
become exterminated, unless they also become modified and improved in
structure in a corresponding manner. Therefore, I can see no difficulty,
more especially under changing conditions of life, in the continued
preservation of individuals with fuller and fuller flank-membranes, each
modification being useful, each being propagated, until, by the accumulated
effects of this process of natural selection, a perfect so-called flying
squirrel was produced.
Now look at the Galeopithecus or so-called flying lemur, which was formerly
ranked among bats, but is now believed to belong to the Insectivora. An
extremely wide flank-membrane stretches from the corners of the jaw to the
tail, and includes the limbs with the elongated fingers. This flank-
membrane is furnished with an extensor muscle. Although no graduated links
of structure, fitted for gliding through the air, now connect the
Galeopithecus with the other Insectivora, yet there is no difficulty in
supposing that such links formerly existed, and that each was developed in
the same manner as with the less perfectly gliding squirrels; each grade of
structure having been useful to its possessor. Nor can I see any
insuperable difficulty in further believing it possible that the
membrane-connected fingers and fore-arm of the Galeopithecus might have
been greatly lengthened by natural selection; and this, as far as the
organs of flight are concerned, would have converted the animal into a bat.
In certain bats in which the wing-membrane extends from the top of the
shoulder to the tail and includes the hind-legs, we perhaps see traces of
an apparatus originally fitted for gliding through the air rather than for
flight.
If about a dozen genera of birds were to become extinct, who would have
ventured to surmise that birds might have existed which used their wings
solely as flappers, like the logger headed duck (Micropterus of Eyton); as
fins in the water and as front legs on the land, like the penguin; as
sails, like the ostrich; and functionally for no purpose, like the apteryx?
Yet the structure of each of these birds is good for it, under the
conditions of life to which it is exposed, for each has to live by a
struggle: but it is not necessarily the best possible under all possible
conditions. It must not be inferred from these remarks that any of the
grades of wing-structure here alluded to, which perhaps may all be the
result of disuse, indicate the steps by which birds actually acquired their
perfect power of flight; but they serve to show what diversified means of
transition are at least possible.
Seeing that a few members of such water-breathing classes as the Crustacea
and Mollusca are adapted to live on the land; and seeing that we have
flying birds and mammals, flying insects of the most diversified types, and
formerly had flying reptiles, it is conceivable that flying-fish, which now
glide far through the air, slightly rising and turning by the aid of their
fluttering fins, might have been modified into perfectly winged animals.
If this had been effected, who would have ever imagined that in an early
transitional state they had been inhabitants of the open ocean, and had
used their incipient organs of flight exclusively, so far as we know, to
escape being devoured by other fish?
When we see any structure highly perfected for any particular habit, as the
wings of a bird for flight, we should bear in mind that animals displaying
early transitional grades of the structure will seldom have survived to the
present day, for they will have been supplanted by their successors, which
were gradually rendered more perfect through natural selection.
Furthermore, we may conclude that transitional states between structures
fitted for very different habits of life will rarely have been developed at
an early period in great numbers and under many subordinate forms. Thus,
to return to our imaginary illustration of the flying-fish, it does not
seem probable that fishes capable of true flight would have been developed
under many subordinate forms, for taking prey of many kinds in many ways,
on the land and in the water, until their organs of flight had come to a
high stage of perfection, so as to have given them a decided advantage over
other animals in the battle for life. Hence the chance of discovering
species with transitional grades of structure in a fossil condition will
always be less, from their having existed in lesser numbers, than in the
case of species with fully developed structures.
I will now give two or three instances, both of diversified and of changed
habits, in the individuals of the same species. In either case it would be
easy for natural selection to adapt the structure of the animal to its
changed habits, or exclusively to one of its several habits. It is,
however, difficult to decide and immaterial for us, whether habits
generally change first and structure afterwards; or whether slight
modifications of structure lead to changed habits; both probably often
occurring almost simultaneously. Of cases of changed habits it will
suffice merely to allude to that of the many British insects which now feed
on exotic plants, or exclusively on artificial substances. Of diversified
habits innumerable instances could be given: I have often watched a tyrant
flycatcher (Saurophagus sulphuratus) in South America, hovering over one
spot and then proceeding to another, like a kestrel, and at other times
standing stationary on the margin of water, and then dashing into it like a
kingfisher at a fish. In our own country the larger titmouse (Parus major)
may be seen climbing branches, almost like a creeper; it sometimes, like a
shrike, kills small birds by blows on the head; and I have many times seen
and heard it hammering the seeds of the yew on a branch, and thus breaking
them like a nuthatch. In North America the black bear was seen by Hearne
swimming for hours with widely open mouth, thus catching, almost like a
whale, insects in the water.
As we sometimes see individuals following habits different from those
proper to their species and to the other species of the same genus, we
might expect that such individuals would occasionally give rise to new
species, having anomalous habits, and with their structure either slightly
or considerably modified from that of their type. And such instances occur
in nature. Can a more striking instance of adaptation be given than that
of a woodpecker for climbing trees and seizing insects in the chinks of the
bark? Yet in North America there are woodpeckers which feed largely on
fruit, and others with elongated wings which chase insects on the wing. On
the plains of La Plata, where hardly a tree grows, there is a woodpecker
(Colaptes campestris) which has two toes before and two behind, a long-
pointed tongue, pointed tail-feathers, sufficiently stiff to support the
bird in a vertical position on a post, but not so stiff as in the typical
wood-peckers, and a straight, strong beak. The beak, however, is not so
straight or so strong as in the typical woodpeckers but it is strong enough
to bore into wood. Hence this Colaptes, in all the essential parts of its
structure, is a woodpecker. Even in such trifling characters as the
colouring, the harsh tone of the voice, and undulatory flight, its close
blood-relationship to our common woodpecker is plainly declared; yet, as I
can assert, not only from my own observations, but from those of the
accurate Azara, in certain large districts it does not climb trees, and it
makes its nest in holes in banks! In certain other districts, however,
this same woodpecker, as Mr. Hudson states, frequents trees, and bores
holes in the trunk for its nest. I may mention as another illustration of
the varied habits of this genus, that a Mexican Colaptes has been described
by De Saussure as boring holes into hard wood in order to lay up a store of
acorns.
Petrels are the most aerial and oceanic of birds, but, in the quiet sounds
of Tierra del Fuego, the Puffinuria berardi, in its general habits, in its
astonishing power of diving, in its manner of swimming and of flying when
made to take flight, would be mistaken by any one for an auk or a grebe;
nevertheless, it is essentially a petrel, but with many parts of its
organisation profoundly modified in relation to its new habits of life;
whereas the woodpecker of La Plata has had its structure only slightly
modified. In the case of the water-ouzel, the acutest observer, by
examining its dead body, would never have suspected its sub-aquatic habits;
yet this bird, which is allied to the thrush family, subsists by
diving,--using its wings under water and grasping stones with its feet.
All the members of the great order of Hymenopterous insects are
terrestrial, excepting the genus Proctotrupes, which Sir John Lubbock has
discovered to be aquatic in its habits; it often enters the water and dives
about by the use not of its legs but of its wings, and remains as long as
four hours beneath the surface; yet it exhibits no modification in
structure in accordance with its abnormal habits.
He who believes that each being has been created as we now see it, must
occasionally have felt surprise when he has met with an animal having
habits and structure not in agreement. What can be plainer than that the
webbed feet of ducks and geese are formed for swimming? Yet there are
upland geese with webbed feet which rarely go near the water; and no one
except Audubon, has seen the frigate-bird, which has all its four toes
webbed, alight on the surface of the ocean. On the other hand, grebes and
coots are eminently aquatic, although their toes are only bordered by
membrane. What seems plainer than that the long toes, not furnished with
membrane, of the Grallatores, are formed for walking over swamps and
floating plants. The water-hen and landrail are members of this order, yet
the first is nearly as aquatic as the coot, and the second is nearly as
terrestrial as the quail or partridge. In such cases, and many others
could be given, habits have changed without a corresponding change of
structure. The webbed feet of the upland goose may be said to have become
almost rudimentary in function, though not in structure. In the
frigate-bird, the deeply scooped membrane between the toes shows that
structure has begun to change.
He who believes in separate and innumerable acts of creation may say, that
in these cases it has pleased the Creator to cause a being of one type to
take the place of one belonging to another type; but this seems to me only
restating the fact in dignified language. He who believes in the struggle
for existence and in the principle of natural selection, will acknowledge
that every organic being is constantly endeavouring to increase in numbers;
and that if any one being varies ever so little, either in habits or
structure, and thus gains an advantage over some other inhabitant of the
same country, it will seize on the place of that inhabitant, however
different that may be from its own place. Hence it will cause him no
surprise that there should be geese and frigate-birds with webbed feet,
living on the dry land and rarely alighting on the water, that there should
be long-toed corncrakes, living in meadows instead of in swamps; that there
should be woodpeckers where hardly a tree grows; that there should be
diving thrushes and diving Hymenoptera, and petrels with the habits of
auks.
ORGANS OF EXTREME PERFECTION AND COMPLICATION.
To suppose that the eye with all its inimitable contrivances for adjusting
the focus to different distances, for admitting different amounts of light,
and for the correction of spherical and chromatic aberration, could have
been formed by natural selection, seems, I freely confess, absurd in the
highest degree. When it was first said that the sun stood still and the
world turned round, the common sense of mankind declared the doctrine
false; but the old saying of Vox populi, vox Dei, as every philosopher
knows, cannot be trusted in science. Reason tells me, that if numerous
gradations from a simple and imperfect eye to one complex and perfect can
be shown to exist, each grade being useful to its possessor, as is
certainly the case; if further, the eye ever varies and the variations be
inherited, as is likewise certainly the case; and if such variations should
be useful to any animal under changing conditions of life, then the
difficulty of believing that a perfect and complex eye could be formed by
natural selection, though insuperable by our imagination, should not be
considered as subversive of the theory. How a nerve comes to be sensitive
to light, hardly concerns us more than how life itself originated; but I
may remark that, as some of the lowest organisms in which nerves cannot be
detected, are capable of perceiving light, it does not seem impossible that
certain sensitive elements in their sarcode should become aggregated and
developed into nerves, endowed with this special sensibility.
In searching for the gradations through which an organ in any species has
been perfected, we ought to look exclusively to its lineal progenitors; but
this is scarcely ever possible, and we are forced to look to other species
and genera of the same group, that is to the collateral descendants from
the same parent-form, in order to see what gradations are possible, and for
the chance of some gradations having been transmitted in an unaltered or
little altered condition. But the state of the same organ in distinct
classes may incidentally throw light on the steps by which it has been
perfected.
The simplest organ which can be called an eye consists of an optic nerve,
surrounded by pigment-cells and covered by translucent skin, but without
any lens or other refractive body. We may, however, according to M.
Jourdain, descend even a step lower and find aggregates of pigment-cells,
apparently serving as organs of vision, without any nerves, and resting
merely on sarcodic tissue. Eyes of the above simple nature are not capable
of distinct vision, and serve only to distinguish light from darkness. In
certain star-fishes, small depressions in the layer of pigment which
surrounds the nerve are filled, as described by the author just quoted,
with transparent gelatinous matter, projecting with a convex surface, like
the cornea in the higher animals. He suggests that this serves not to form
an image, but only to concentrate the luminous rays and render their
perception more easy. In this concentration of the rays we gain the first
and by far the most important step towards the formation of a true,
picture-forming eye; for we have only to place the naked extremity of the
optic nerve, which in some of the lower animals lies deeply buried in the
body, and in some near the surface, at the right distance from the
concentrating apparatus, and an image will be formed on it.
In the great class of the Articulata, we may start from an optic nerve
simply coated with pigment, the latter sometimes forming a sort of pupil,
but destitute of lens or other optical contrivance. With insects it is now
known that the numerous facets on the cornea of their great compound eyes
form true lenses, and that the cones include curiously modified nervous
filaments. But these organs in the Articulata are so much diversified that
Muller formerly made three main classes with seven subdivisions, besides a
fourth main class of aggregated simple eyes.
When we reflect on these facts, here given much too briefly, with respect
to the wide, diversified, and graduated range of structure in the eyes of
the lower animals; and when we bear in mind how small the number of all
living forms must be in comparison with those which have become extinct,
the difficulty ceases to be very great in believing that natural selection
may have converted the simple apparatus of an optic nerve, coated with
pigment and invested by transparent membrane, into an optical instrument as
perfect as is possessed by any member of the Articulata class.
He who will go thus far, ought not to hesitate to go one step further, if
he finds on finishing this volume that large bodies of facts, otherwise
inexplicable, can be explained by the theory of modification through
natural selection; he ought to admit that a structure even as perfect as an
eagle's eye might thus be formed, although in this case he does not know
the transitional states. It has been objected that in order to modify the
eye and still preserve it as a perfect instrument, many changes would have
to be effected simultaneously, which, it is assumed, could not be done
through natural selection; but as I have attempted to show in my work on
the variation of domestic animals, it is not necessary to suppose that the
modifications were all simultaneous, if they were extremely slight and
gradual. Different kinds of modification would, also, serve for the same
general purpose: as Mr. Wallace has remarked, "If a lens has too short or
too long a focus, it may be amended either by an alteration of curvature,
or an alteration of density; if the curvature be irregular, and the rays do
not converge to a point, then any increased regularity of curvature will be
an improvement. So the contraction of the iris and the muscular movements
of the eye are neither of them essential to vision, but only improvements
which might have been added and perfected at any stage of the construction
of the instrument." Within the highest division of the animal kingdom,
namely, the Vertebrata, we can start from an eye so simple, that it
consists, as in the lancelet, of a little sack of transparent skin,
furnished with a nerve and lined with pigment, but destitute of any other
apparatus. In fishes and reptiles, as Owen has remarked, "The range of
gradation of dioptric structures is very great." It is a significant fact
that even in man, according to the high authority of Virchow, the beautiful
crystalline lens is formed in the embryo by an accumulation of epidermic
cells, lying in a sack-like fold of the skin; and the vitreous body is
formed from embryonic subcutaneous tissue. To arrive, however, at a just
conclusion regarding the formation of the eye, with all its marvellous yet
not absolutely perfect characters, it is indispensable that the reason
should conquer the imagination; but I have felt the difficulty far to
keenly to be surprised at others hesitating to extend the principle of
natural selection to so startling a length.
It is scarcely possible to avoid comparing the eye with a telescope. We
know that this instrument has been perfected by the long-continued efforts
of the highest human intellects; and we naturally infer that the eye has
been formed by a somewhat analogous process. But may not this inference be
presumptuous? Have we any right to assume that the Creator works by
intellectual powers like those of man? If we must compare the eye to an
optical instrument, we ought in imagination to take a thick layer of
transparent tissue, with spaces filled with fluid, and with a nerve
sensitive to light beneath, and then suppose every part of this layer to be
continually changing slowly in density, so as to separate into layers of
different densities and thicknesses, placed at different distances from
each other, and with the surfaces of each layer slowly changing in form.
Further we must suppose that there is a power, represented by natural
selection or the survival of the fittest, always intently watching each
slight alteration in the transparent layers; and carefully preserving each
which, under varied circumstances, in any way or degree, tends to produce a
distincter image. We must suppose each new state of the instrument to be
multiplied by the million; each to be preserved until a better is produced,
and then the old ones to be all destroyed. In living bodies, variation
will cause the slight alteration, generation will multiply them almost
infinitely, and natural selection will pick out with unerring skill each
improvement. Let this process go on for millions of years; and during each
year on millions of individuals of many kinds; and may we not believe that
a living optical instrument might thus be formed as superior to one of
glass, as the works of the Creator are to those of man?
MODES Of TRANSITION.
If it could be demonstrated that any complex organ existed, which could not
possibly have been formed by numerous, successive, slight modifications, my
theory would absolutely break down. But I can find out no such case. No
doubt many organs exist of which we do not know the transitional grades,
more especially if we look to much-isolated species, around which,
according to the theory, there has been much extinction. Or again, if we
take an organ common to all the members of a class, for in this latter case
the organ must have been originally formed at a remote period, since which
all the many members of the class have been developed; and in order to
discover the early transitional grades through which the organ has passed,
we should have to look to very ancient ancestral forms, long since become
extinct.
We should be extremely cautious in concluding that an organ could not have
been formed by transitional gradations of some kind. Numerous cases could
be given among the lower animals of the same organ performing at the same
time wholly distinct functions; thus in the larva of the dragon-fly and in
the fish Cobites the alimentary canal respires, digests, and excretes. In
the Hydra, the animal may be turned inside out, and the exterior surface
will then digest and the stomach respire. In such cases natural selection
might specialise, if any advantage were thus gained, the whole or part of
an organ, which had previously performed two functions, for one function
alone, and thus by insensible steps greatly change its nature. Many plants
are known which regularly produce at the same time differently constructed
flowers; and if such plants were to produce one kind alone, a great change
would be effected with comparative suddenness in the character of the
species. It is, however, probable that the two sorts of flowers borne by
the same plant were originally differentiated by finely graduated steps,
which may still be followed in some few cases.
Again, two distinct organs, or the same organ under two very different
forms, may simultaneously perform in the same individual the same function,
and this is an extremely important means of transition: to give one
instance--there are fish with gills or branchiae that breathe the air
dissolved in the water, at the same time that they breathe free air in
their swim-bladders, this latter organ being divided by highly vascular
partitions and having a ductus pneumaticus for the supply of air. To give
another instance from the vegetable kingdom: plants climb by three
distinct means, by spirally twining, by clasping a support with their
sensitive tendrils, and by the emission of aerial rootlets; these three
means are usually found in distinct groups, but some few species exhibit
two of the means, or even all three, combined in the same individual. In
all such cases one of the two organs might readily be modified and
perfected so as to perform all the work, being aided during the progress of
modification by the other organ; and then this other organ might be
modified for some other and quite distinct purpose, or be wholly
obliterated.
The illustration of the swim-bladder in fishes is a good one, because it
shows us clearly the highly important fact that an organ originally
constructed for one purpose, namely flotation, may be converted into one
for a widely different purpose, namely respiration. The swim-bladder has,
also, been worked in as an accessory to the auditory organs of certain
fishes. All physiologists admit that the swim-bladder is homologous, or
"ideally similar" in position and structure with the lungs of the higher
vertebrate animals: hence there is no reason to doubt that the swim-
bladder has actually been converted into lungs, or an organ used
exclusively for respiration.
According to this view it may be inferred that all vertebrate animals with
true lungs are descended by ordinary generation from an ancient and unknown
prototype which was furnished with a floating apparatus or swim-bladder.
We can thus, as I infer from Professor Owen's interesting description of
these parts, understand the strange fact that every particle of food and
drink which we swallow has to pass over the orifice of the trachea, with
some risk of falling into the lungs, notwithstanding the beautiful
contrivance by which the glottis is closed. In the higher Vertebrata the
branchiae have wholly disappeared--but in the embryo the slits on the sides
of the neck and the loop-like course of the arteries still mark their
former position. But it is conceivable that the now utterly lost branchiae
might have been gradually worked in by natural selection for some distinct
purpose: for instance, Landois has shown that the wings of insects are
developed from the trachea; it is therefore highly probable that in this
great class organs which once served for respiration have been actually
converted into organs for flight.
In considering transitions of organs, it is so important to bear in mind
the probability of conversion from one function to another, that I will
give another instance. Pedunculated cirripedes have two minute folds of
skin, called by me the ovigerous frena, which serve, through the means of a
sticky secretion, to retain the eggs until they are hatched within the
sack. These cirripedes have no branchiae, the whole surface of the body
and of the sack, together with 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, within the
well-enclosed shell; but they have, in the same relative position with the
frena, large, much-folded membranes, which freely communicate with the
circulatory lacunae of the sack and body, and which have been considered by
all naturalists to act as 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 it need not be doubted that the two little folds of skin, which
originally served as ovigerous frena, but which, likewise, very slightly
aided in the act of respiration, have been gradually converted by natural
selection into 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 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?
There is another possible mode of transition, namely, through the
acceleration or retardation of the period of reproduction. This has lately
been insisted on by Professor Cope and others in the United States. It is
now known that some animals are capable of reproduction at a very early
age, before they have acquired their perfect characters; and if this power
became thoroughly well developed in a species, it seems probable that the
adult stage of development would sooner or later be lost; and in this case,
especially if the larva differed much from the mature form, the character
of the species would be greatly changed and degraded. Again, not a few
animals, after arriving at maturity, go on changing in character during
nearly their whole lives. With mammals, for instance, the form of the
skull is often much altered with age, of which Dr. Murie has given some
striking instances with seals. Every one knows how the horns of stags
become more and more branched, and the plumes of some birds become more
finely developed, as they grow older. Professor Cope states that the teeth
of certain lizards change much in shape with advancing years. With
crustaceans not only many trivial, but some important parts assume a new
character, as recorded by Fritz Muller, after maturity. In all such cases-
-and many could be given--if the age for reproduction were retarded, the
character of the species, at least in its adult state, would be modified;
nor is it improbable that the previous and earlier stages of development
would in some cases be hurried through and finally lost. Whether species
have often or ever been modified through this comparatively sudden mode of
transition, I can form no opinion; but if this has occurred, it is probable
that the differences between the young and the mature, and between the
mature and the old, were primordially acquired by graduated steps.
SPECIAL DIFFICULTIES OF THE THEORY OF NATURAL SELECTION.
Although we must be extremely cautious in concluding that any organ could
not have been produced by successive, small, transitional gradations, yet
undoubtedly serious cases of difficulty occur.
One of the most serious is that of neuter insects, which are often
differently constructed from either the males or fertile females; but this
case will be treated of in the next chapter. The electric organs of fishes
offer another case of special difficulty; for it is impossible to conceive
by what steps these wondrous organs have been produced. But this is not
surprising, for we do not even know of what use they are. In the gymnotus
and torpedo they no doubt serve as powerful means of defence, and perhaps
for securing prey; yet in the ray, as observed by Matteucci, an analogous
organ in the tail manifests but little electricity, even when the animal is
greatly irritated; so little that it can hardly be of any use for the above
purposes. Moreover, in the ray, besides the organ just referred to, there
is, as Dr. R. McDonnell has shown, another organ near the head, not known
to be electrical, but which appears to be the real homologue of the
electric battery in the torpedo. It is generally admitted that there
exists between these organs and ordinary muscle a close analogy, in
intimate structure, in the distribution of the nerves, and in the manner in
which they are acted on by various reagents. It should, also, be
especially observed that muscular contraction is accompanied by an
electrical discharge; and, as Dr. Radcliffe insists, "in the electrical
apparatus of the torpedo during rest, there would seem to be a charge in
every respect like that which is met with in muscle and nerve during the
rest, and the discharge of the torpedo, instead of being peculiar, may be
only another form of the discharge which attends upon the action of muscle
and motor nerve." Beyond this we cannot at present go in the way of
explanation; but as we know so little about the uses of these organs, and
as we know nothing about the habits and structure of the progenitors of the
existing electric fishes, it would be extremely bold to maintain that no
serviceable transitions are possible by which these organs might have been
gradually developed.
These organs appear at first to offer another and far more serious
difficulty; for they occur in about a dozen kinds of fish, of which several
are widely remote in their affinities. When the same organ is found in
several members of the same class, especially if in members having very
different habits of life, we may generally attribute its presence to
inheritance from a common ancestor; and its absence in some of the members
to loss through disuse or natural selection. So that, if the electric
organs had been inherited from some one ancient progenitor, we might have
expected that all electric fishes would have been specially related to each
other; but this is far from the case. Nor does geology at all lead to the
belief that most fishes formerly possessed electric organs, which their
modified descendants have now lost. But when we look at the subject more
closely, we find in the several fishes provided with electric organs, that
these are situated in different parts of the body, that they differ in
construction, as in the arrangement of the plates, and, according to
Pacini, in the process or means by which the electricity is excited--and
lastly, in being supplied with nerves proceeding from different sources,
and this is perhaps the most important of all the differences. Hence in
the several fishes furnished with electric organs, these cannot be
considered as homologous, but only as analogous in function. Consequently
there is no reason to suppose that they have been inherited from a common
progenitor; for had this been the case they would have closely resembled
each other in all respects. Thus the difficulty of an organ, apparently
the same, arising in several remotely allied species, disappears, leaving
only the lesser yet still great difficulty: namely, by what graduated
steps these organs have been developed in each separate group of fishes.
The luminous organs which occur in a few insects, belonging to widely
different families, and which are situated in different parts of the body,
offer, under our present state of ignorance, a difficulty almost exactly
parallel with that of the electric organs. Other similar cases could be
given; for instance in plants, the very curious contrivance of a mass of
pollen-grains, borne on a foot-stalk with an adhesive gland, is apparently
the same in Orchis and Asclepias, genera almost as remote as is possible
among flowering plants; but here again the parts are not homologous. In
all cases of beings, far removed from each other in the scale of
organisation, which are furnished with similar and peculiar organs, it will
be found that although the general appearance and function of the organs
may be the same, yet fundamental differences between them can always be
detected. For instance, the eyes of Cephalopods or cuttle-fish and of
vertebrate animals appear wonderfully alike; and in such widely sundered
groups no part of this resemblance can be due to inheritance from a common
progenitor. Mr. Mivart has advanced this case as one of special
difficulty, but I am unable to see the force of his argument. An organ for
vision must be formed of transparent tissue, and must include some sort of
lens for throwing an image at the back of a darkened chamber. Beyond this
superficial resemblance, there is hardly any real similarity between the
eyes of cuttle-fish and vertebrates, as may be seen by consulting Hensen's
admirable memoir on these organs in the Cephalopoda. It is impossible for
me here to enter on details, but I may specify a few of the points of
difference. The crystalline lens in the higher cuttle-fish consists of two
parts, placed one behind the other like two lenses, both having a very
different structure and disposition to what occurs in the vertebrata. The
retina is wholly different, with an actual inversion of the elemental
parts, and with a large nervous ganglion included within the membranes of
the eye. The relations of the muscles are as different as it is possible
to conceive, and so in other points. Hence it is not a little difficult to
decide how far even the same terms ought to be employed in describing the
eyes of the Cephalopoda and Vertebrata. It is, of course, open to any one
to deny that the eye in either case could have been developed through the
natural selection of successive slight variations; but if this be admitted
in the one case it is clearly possible in the other; and fundamental
differences of structure in the visual organs of two groups might have been
anticipated, in accordance with this view of their manner of formation. As
two men have sometimes independently hit on the same invention, so in the
several foregoing cases it appears that natural selection, working for the
good of each being, and taking advantage of all favourable variations, has
produced similar organs, as far as function is concerned, in distinct
organic beings, which owe none of their structure in common to inheritance
from a common progenitor.
Fritz Muller, in order to test the conclusions arrived at in this volume,
has followed out with much care a nearly similar line of argument. Several
families of crustaceans include a few species, possessing an air-breathing
apparatus and fitted to live out of the water. In two of these families,
which were more especially examined by Muller, and which are nearly related
to each other, the species agree most closely in all important characters:
namely in their sense organs, circulating systems, in the position of the
tufts of hair within their complex stomachs, and lastly in the whole
structure of the water-breathing branchiae, even to the microscopical hooks
by which they are cleansed. Hence it might have been expected that in the
few species belonging to both families which live on the land, the equally
important air-breathing apparatus would have been the same; for why should
this one apparatus, given for the same purpose, have been made to differ,
while all the other important organs were closely similar, or rather,
identical.
Fritz Muller argues that this close similarity in so many points of
structure must, in accordance with the views advanced by me, be accounted
for by inheritance from a common progenitor. But as the vast majority of
the species in the above two families, as well as most other crustaceans,
are aquatic in their habits, it is improbable in the highest degree that
their common progenitor should have been adapted for breathing air. Muller
was thus led carefully to examine the apparatus in the air-breathing
species; and he found it to differ in each in several important points, as
in the position of the orifices, in the manner in which they are opened and
closed, and in some accessory details. Now such differences are
intelligible, and might even have been expected, on the supposition that
species belonging to distinct families had slowly become adapted to live
more and more out of water, and to breathe the air. For these species,
from belonging to distinct families, would have differed to a certain
extent, and in accordance with the principle that the nature of each
variation depends on two factors, viz., the nature of the organism and that
of the surrounding conditions, their variability assuredly would not have
been exactly the same. Consequently natural selection would have had
different materials or variations to work on, in order to arrive at the
same functional result; and the structures thus acquired would almost
necessarily have differed. On the hypothesis of separate acts of creation
the whole case remains unintelligible. This line of argument seems to have
had great weight in leading Fritz Muller to accept the views maintained by
me in this volume.
Another distinguished zoologist, the late Professor Claparede, has argued
in the same manner, and has arrived at the same result. He shows that
there are parasitic mites (Acaridae), belonging to distinct sub-families
and families, which are furnished with hair-claspers. These organs must
have been independently developed, as they could not have been inherited
from a common progenitor; and in the several groups they are formed by the
modification of the fore legs, of the hind legs, of the maxillae or lips,
and of appendages on the under side of the hind part of the body.
In the foregoing cases, we see the same end gained and the same function
performed, in beings not at all or only remotely allied, by organs in
appearance, though not in development, closely similar. On the other hand,
it is a common rule throughout nature that the same end should be gained,
even sometimes in the case of closely related beings, by the most
diversified means. How differently constructed is the feathered wing of a
bird and the membrane-covered wing of a bat; and still more so the four
wings of a butterfly, the two wings of a fly, and the two wings with the
elytra of a beetle. Bivalve shells are made to open and shut, but on what
a number of patterns is the hinge constructed, from the long row of neatly
interlocking teeth in a Nucula to the simple ligament of a Mussel! Seeds
are disseminated by their minuteness, by their capsule being converted into
a light balloon-like envelope, by being embedded in pulp or flesh, formed
of the most diverse parts, and rendered nutritious, as well as
conspicuously coloured, so as to attract and be devoured by birds, by
having hooks and grapnels of many kinds and serrated awns, so as to adhere
to the fur of quadrupeds, and by being furnished with wings and plumes, as
different in shape as they are elegant in structure, so as to be wafted by
every breeze. I will give one other instance: for this subject of the
same end being gained by the most diversified means well deserves
attention. Some authors maintain that organic beings have been formed in
many ways for the sake of mere variety, almost like toys in a shop, but
such a view of nature is incredible. With plants having separated sexes,
and with those in which, though hermaphrodites, the pollen does not
spontaneously fall on the stigma, some aid is necessary for their
fertilisation. With several kinds this is effected by the pollen-grains,
which are light and incoherent, being blown by the wind through mere chance
on to the stigma; and this is the simplest plan which can well be
conceived. An almost equally simple, though very different plan occurs in
many plants in which a symmetrical flower secretes a few drops of nectar,
and is consequently visited by insects; and these carry the pollen from the
anthers to the stigma.
>From this simple stage we may pass through an inexhaustible number of
contrivances, all for the same purpose and effected in essentially the same
manner, but entailing changes in every part of the flower. The nectar may
be stored in variously shaped receptacles, with the stamens and pistils
modified in many ways, sometimes forming trap-like contrivances, and
sometimes capable of neatly adapted movements through irritability or
elasticity. From such structures we may advance till we come to such a
case of extraordinary adaptation as that lately described by Dr. Cruger in
the Coryanthes. This orchid has part of its labellum or lower lip hollowed
out into a great bucket, into which drops of almost pure water continually
fall from two secreting horns which stand above it; and when the bucket is
half-full, the water overflows by a spout on one side. The basal part of
the labellum stands over the bucket, and is itself hollowed out into a sort
of chamber with two lateral entrances; within this chamber there are
curious fleshy ridges. The most ingenious man, if he had not witnessed
what takes place, could never have imagined what purpose all these parts
serve. But Dr. Cruger saw crowds of large humble-bees visiting the
gigantic flowers of this orchid, not in order to suck nectar, but to gnaw
off the ridges within the chamber above the bucket; in doing this they
frequently pushed each other into the bucket, and their wings being thus
wetted they could not fly away, but were compelled to crawl out through the
passage formed by the spout or overflow. Dr. Cruger saw a "continual
procession" of bees thus crawling out of their involuntary bath. The
passage is narrow, and is roofed over by the column, so that a bee, in
forcing its way out, first rubs its back against the viscid stigma and then
against the viscid glands of the pollen-masses. The pollen-masses are thus
glued to the back of the bee which first happens to crawl out through the
passage of a lately expanded flower, and are thus carried away. Dr. Cruger
sent me a flower in spirits of wine, with a bee which he had killed before
it had quite crawled out, with a pollen-mass still fastened to its back.
When the bee, thus provided, flies to another flower, or to the same flower
a second time, and is pushed by its comrades into the bucket and then
crawls out by the passage, the pollen-mass necessarily comes first into
contact with the viscid stigma, and adheres to it, and the flower is
fertilised. Now at last we see the full use of ever
< BackForward >
|
