CHAPTER VI - DIFFICULTIES OF THE THEORY
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 here be 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
connections 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 a 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
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 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 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 to 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 amongst 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 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, 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 vision of North America,
which has webbed feet, and which resembles an otter in its fur,
short legs, and form of tail. During the summer this animal dives
for and preys on fish, but during the long winter it leaves the frozen
waters, and preys, like other pole-cats, 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 only
give 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, 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
formerly was ranked amongst 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 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 the 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 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 woodpeckers, 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 observation, 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
waterouzel, the acutest observer by examining its dead body would
never have suspected its subaquatic 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 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 re-stating 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 frigatebirds 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.
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