On the Origin of Species by Means of Natural Selection
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Chapter VI
Difficulties on Theory
Difficulties on the theory of descent with modification -- Transitions --
Absence or rarity of transitional varieties -- Transitions in habits of
life -- Diversified habits in the same species -- Species with habits
widely different from those of their allies -- Organs of extreme perfection
-- Means of transition -- Cases of difficulty -- Natura non facit saltum --
Organs of small importance -- Organs not in all cases absolutely perfect --
The law of Unity of Type and of the Conditions of Existence embraced by the
theory of Natural Selection.
Long before having arrived at this part of my work, a crowd of difficulties
will have occurred to the reader. Some of them are so grave that to this
day I can never reflect on them without being staggered; but, to the best
of my judgment, the greater number are only apparent, and those that are
real are not, I think, fatal to my theory.
These difficulties and objections may be classed under the following
heads:- Firstly, why, if species have descended from other species by
insensibly fine gradations, do we not everywhere see innumerable
transitional forms? Why is not all nature in confusion instead of the
species being, as we see them, well defined?
Secondly, is it possible that an animal having, for instance, the structure
and habits of a bat, could have been formed by the modification of some
animal with wholly different habits? Can we believe that natural selection
could produce, on the one hand, organs of trifling importance, such as the
tail of a giraffe, which serves as a fly-flapper, and, on the other hand,
organs of such wonderful structure, as the eye, of which we hardly as yet
fully understand the inimitable perfection?
Thirdly, can instincts be acquired and modified through natural selection?
What shall we say to so marvellous an instinct as that which leads the bee
to make cells, which have practically anticipated the discoveries of
profound mathematicians?
Fourthly, how can we account for species, when crossed, being sterile and
producing sterile offspring, whereas, when varieties are crossed, their
fertility is unimpaired?
The two first heads shall be here discussed--Instinct and Hybridism in
separate chapters.
On the absence or rarity of transitional varieties. -- As natural selection
acts solely by the preservation of profitable modifications, each new form
will tend in a fully-stocked country to take the place of, and finally to
exterminate, its own less improved parent or other less-favoured forms with
which it comes into competition. Thus extinction and natural selection
will, as we have seen, go hand in hand. Hence, if we look at each species
as descended from some other unknown form, both the parent and all the
transitional varieties will generally have been exterminated by the very
process of formation and perfection of the new form.
But, as by this theory innumerable transitional forms must have existed,
why do we not find them embedded in countless numbers in the crust of the
earth? It will be much more convenient to discuss this question in the
chapter on the Imperfection of the geological record; and I will here only
state that I believe the answer mainly lies in the record being
incomparably less perfect than is generally supposed; the imperfection of
the record being chiefly due to organic beings not inhabiting profound
depths of the sea, and to their remains being embedded and preserved to a
future age only in masses of sediment sufficiently thick and extensive to
withstand an enormous amount of future degradation; and such fossiliferous
masses can be accumulated only where much sediment is deposited on the
shallow bed of the sea, whilst it slowly subsides. These contingencies
will concur only rarely, and after enormously long intervals. Whilst the
bed of the sea is stationary or is rising, or when very little sediment is
being deposited, there will be blanks in our geological history. The crust
of the earth is a vast museum; but the natural collections have been made
only at intervals of time immensely remote.
But it may be urged that when several closely-allied species inhabit the
same territory we surely ought to find at the present time many
transitional forms. Let us take a simple case: in travelling from north
to south over a continent, we generally meet at successive intervals with
closely allied or representative species, evidently filling nearly the same
place in the natural economy of the land. These representative species
often meet and interlock; and as the one becomes rarer and rarer, the other
becomes more and more frequent, till the one replaces the other. But if we
compare these species where they intermingle, they are generally as
absolutely distinct from each other in every detail of structure as are
specimens taken from the metropolis inhabited by each. By my theory these
allied species have descended from a common parent; and during the process
of modification, each has become adapted to the conditions of life of its
own region, and has supplanted and exterminated its original parent and all
the transitional varieties between its past and present states. Hence we
ought not to expect at the present time to meet with numerous transitional
varieties in each region, though they must have existed there, and may be
embedded there in a fossil condition. But in the intermediate region,
having intermediate conditions of life, why do we not now find
closely-linking intermediate varieties? This difficulty for a long time
quite confounded me. But I think it can be in large part explained.
In the first place we should be extremely cautious in inferring, because an
area is now continuous, that it has been continuous during a long period.
Geology would lead us to believe that almost every continent has been
broken up into islands even during the later tertiary periods; and in such
islands distinct species might have been separately formed without the
possibility of intermediate varieties existing in the intermediate zones.
By changes in the form of the land and of climate, marine areas now
continuous must often have existed within recent times in a far less
continuous and uniform condition than at present. But I will pass over
this way of escaping from the difficulty; for I believe that many perfectly
defined species have been formed on strictly continuous areas; though I do
not doubt that the formerly broken condition of areas now continuous has
played an important part in the formation of new species, more especially
with freely-crossing and wandering animals.
In looking at species as they are now distributed over a wide area, we
generally find them tolerably numerous over a large territory, then
becoming somewhat abruptly rarer and rarer on the confines, and finally
disappearing. Hence the neutral territory between two representative
species is generally narrow in comparison with the territory proper to
each. We see the same fact in ascending mountains, and sometimes it is
quite remarkable how abruptly, as Alph. De Candolle has observed, a common
alpine species disappears. The same fact has been noticed by Forbes in
sounding the depths of the sea with the dredge. To those who look at
climate and the physical conditions of life as the all-important elements
of distribution, these facts ought to cause surprise, as climate and height
or depth graduate away insensibly. But when we bear in mind that almost
every species, even in its metropolis, would increase immensely in numbers,
were it not for other competing species; that nearly all either prey on or
serve as prey for others; in short, that each organic being is either
directly or indirectly related in the most important manner to other
organic beings, we must see that the range of the inhabitants of any
country by no means exclusively depends on insensibly changing physical
conditions, but in large part on the presence of other species, on which it
depends, or by which it is destroyed, or with which it comes into
competition; and as these species are already defined objects (however they
may have become so), not blending one into another by insensible
gradations, the range of any one species, depending as it does on the range
of others, will tend to be sharply defined. Moreover, each species on the
confines of its range, where it exists in lessened numbers, will, during
fluctuations in the number of its enemies or of its prey, or in the
seasons, be extremely liable to utter extermination; and thus its
geographical range will come to be still more sharply defined.
If I am right in believing that allied or representative species, when
inhabiting a continuous area, are generally so distributed that each has a
wide range, with a comparatively narrow neutral territory between them, in
which they become rather suddenly rarer and rarer; then, as varieties do
not essentially differ from species, the same rule will probably apply to
both; and if we in imagination adapt a varying species to a very large
area, we shall have to adapt two varieties to two large areas, and a third
variety to a narrow intermediate zone. The intermediate variety,
consequently, will exist in lesser numbers from inhabiting a narrow and
lesser area; and practically, as far as I can make out, this rule holds
good with varieties in a state of nature. I have met with striking
instances of the rule in the case of varieties intermediate between
well-marked varieties in the genus Balanus. And it would appear from
information given me by Mr. Watson, Dr. Asa Gray, and Mr. Wollaston, that
generally when varieties intermediate between two other forms occur, they
are much rarer numerically than the forms which they connect. Now, if we
may trust these facts and inferences, and therefore conclude that varieties
linking two other varieties together have generally existed in lesser
numbers than the forms which they connect, then, I think, we can understand
why intermediate varieties should not endure for very long periods;--why as
a general rule they should be exterminated and disappear, sooner than the
forms which they originally linked together.
For any form existing in lesser numbers would, as already remarked, run a
greater chance of being exterminated than one existing in large numbers;
and in this particular case the intermediate form would be eminently liable
to the inroads of closely allied forms existing on both sides of it. But a
far more important consideration, as I believe, is that, during the process
of further modification, by which two varieties are supposed on my theory
to be converted and perfected into two distinct species, the two which
exist in larger numbers from inhabiting larger areas, will have a great
advantage over the intermediate variety, which exists in smaller numbers in
a narrow and intermediate zone. For forms existing in larger numbers will
always have a better chance, within any given period, of presenting further
favourable variations for natural selection to seize on, than will the
rarer forms which exist in lesser numbers. Hence, the more common forms,
in the race for life, will tend to beat and supplant the less common forms,
for these will be more slowly modified and improved. It is the same
principle which, as I believe, accounts for the common species in each
country, as shown in the second chapter, presenting on an average a greater
number of well-marked varieties than do the rarer species. I may
illustrate what I mean by supposing three varieties of sheep to be kept,
one adapted to an extensive mountainous region; a second to a comparatively
narrow, hilly tract; and a third to wide plains at the base; and that the
inhabitants are all trying with equal steadiness and skill to improve their
stocks by selection; the chances in this case will be strongly in favour of
the great holders on the mountains or on the plains improving their breeds
more quickly than the small holders on the intermediate narrow, hilly
tract; and consequently the improved mountain or plain breed will soon take
the place of the less improved hill breed; and thus the two breeds, which
originally existed in greater numbers, will come into close contact with
each other, without the interposition of the supplanted, intermediate
hill-variety.
To sum up, I believe that species come to be tolerably well-defined
objects, and do not at any one period present an inextricable chaos of
varying and intermediate links: firstly, because new varieties are very
slowly formed, for variation is a very slow process, and natural selection
can do nothing until favourable variations chance to occur, and until a
place in the natural polity of the country can be better filled by some
modification of some one or more of its inhabitants. And such new places
will depend on slow changes of climate, or on the occasional immigration of
new inhabitants, and, probably, in a still more important degree, on some
of the old inhabitants becoming slowly modified, with the new forms thus
produced and the old ones acting and reacting on each other. So that, in
any one region and at any one time, we ought only to see a few species
presenting slight modifications of structure in some degree permanent; and
this assuredly we do see.
Secondly, areas now continuous must often have existed within the recent
period in isolated portions, in which many forms, more especially amongst
the classes which unite for each birth and wander much, may have separately
been rendered sufficiently distinct to rank as representative species. In
this case, intermediate varieties between the several representative
species and their common parent, must formerly have existed in each broken
portion of the land, but these links will have been supplanted and
exterminated during the process of natural selection, so that they will no
longer exist in a living state.
Thirdly, when two or more varieties have been formed in different portions
of a strictly continuous area, intermediate varieties will, it is probable,
at first have been formed in the intermediate zones, but they will
generally have had a short duration. For these intermediate varieties
will, from reasons already assigned (namely from what we know of the actual
distribution of closely allied or representative species, and likewise of
acknowledged varieties), exist in the intermediate zones in lesser numbers
than the varieties which they tend to connect. From this cause alone the
intermediate varieties will be liable to accidental extermination; and
during the process of further modification through natural selection, they
will almost certainly be beaten and supplanted by the forms which they
connect; for these from existing in greater numbers will, in the aggregate,
present more variation, and thus be further improved through natural
selection and gain further advantages.
Lastly, looking not to any one time, but to all time, if my theory be true,
numberless intermediate varieties, linking most closely all the species of
the same group together, must assuredly have existed; but the very process
of natural selection constantly tends, as has been so often remarked, to
exterminate the parent forms and the intermediate links. Consequently
evidence of their former existence could be found only amongst fossil
remains, which are preserved, as we shall in a future chapter attempt to
show, in an extremely imperfect and intermittent record.
On the origin and transitions of organic beings with peculiar habits and
structure. -- It has been asked by the opponents of such views as I hold,
how, for instance, a land carnivorous animal could have been converted into
one with aquatic habits; for how could the animal in its transitional state
have subsisted? It would be easy to show that within the same group
carnivorous animals exist having every intermediate grade between truly
aquatic and strictly terrestrial habits; and as each exists by a struggle
for life, it is clear that each is well adapted in its habits to its place
in nature. Look at the Mustela vison of North America, which has webbed
feet and which resembles an otter in its fur, short legs, and form of tail;
during summer this animal dives for and preys on fish, but during the long
winter it leaves the frozen waters, and preys like other polecats on mice
and land animals. If a different case had been taken, and it had been
asked how an insectivorous quadruped could possibly have been converted
into a flying bat, the question would have been far more difficult, and I
could have given no answer. Yet I think such difficulties have very little
weight.
Here, as on other occasions, I lie under a heavy disadvantage, for out of
the many striking cases which I have collected, I can give only one or two
instances of transitional habits and structures in closely allied species
of the same genus; and of diversified habits, either constant or
occasional, in the same species. And it seems to me that nothing less than
a long list of such cases is sufficient to lessen the difficulty in any
particular case like that of the bat.
Look at the family of squirrels; here we have the finest gradation from
animals with their tails only slightly flattened, and from others, as Sir
J. Richardson has remarked, with the posterior part of their bodies rather
wide and with the skin on their flanks rather full, to the so-called flying
squirrels; and flying squirrels have their limbs and even the base of the
tail united by a broad expanse of skin, which serves as a parachute and
allows them to glide through the air to an astonishing distance from tree
to tree. We cannot doubt that each structure is of use to each kind of
squirrel in its own country, by enabling it to escape birds or beasts of
prey, or to collect food more quickly, or, as there is reason to believe,
by lessening the danger from occasional falls. But it does not follow from
this fact that the structure of each squirrel is the best that it is
possible to conceive under all natural conditions. Let the climate and
vegetation change, let other competing rodents or new beasts of prey
immigrate, or old ones become modified, and all analogy would lead us to
believe that some at least of the squirrels would decrease in numbers or
become exterminated, unless they also became modified and improved in
structure in a corresponding manner. Therefore, I can see no difficulty,
more especially under changing conditions of life, in the continued
preservation of individuals with fuller and fuller flank-membranes, each
modification being useful, each being propagated, until by the accumulated
effects of this process of natural selection, a perfect so-called flying
squirrel was produced.
Now look at the Galeopithecus or flying lemur, which formerly was falsely
ranked amongst bats. It has an extremely wide flank-membrane, stretching
from the corners of the jaw to the tail, and including the limbs and the
elongated fingers: the flank membrane is, also, furnished with an extensor
muscle. Although no graduated links of structure, fitted for gliding
through the air, now connect the Galeopithecus with the other Lemuridae,
yet I can see no difficulty in supposing that such links formerly existed,
and that each had been formed by the same steps as in the case of the less
perfectly gliding squirrels; and that each grade of structure had been
useful to its possessor. Nor can I see any insuperable difficulty in
further believing it possible that the membrane-connected fingers and
fore-arm of the Galeopithecus might be greatly lengthened by natural
selection; and this, as far as the organs of flight are concerned, would
convert it into a bat. In bats which have the wing-membrane extended from
the top of the shoulder to the tail, including the hind-legs, we perhaps
see traces of an apparatus originally constructed for gliding through the
air rather than for flight.
If about a dozen genera of birds had become extinct or were unknown, who
would have ventured to have surmised that birds might have existed which
used their wings solely as flappers, like the logger-headed duck
(Micropterus of Eyton); as fins in the water and front legs on the land,
like the penguin; as sails, like the ostrich; and functionally for no
purpose, like the Apteryx. Yet the structure of each of these birds is
good for it, under the conditions of life to which it is exposed, for each
has to live by a struggle; but it is not necessarily the best possible
under all possible conditions. It must not be inferred from these remarks
that any of the grades of wing-structure here alluded to, which perhaps may
all have resulted from disuse, indicate the natural steps by which birds
have acquired their perfect power of flight; but they serve, at least, to
show what diversified means of transition are possible.
Seeing that a few members of such water-breathing classes as the Crustacea
and Mollusca are adapted to live on the land, and seeing that we have
flying birds and mammals, flying insects of the most diversified types, and
formerly had flying reptiles, it is conceivable that flying-fish, which now
glide far through the air, slightly rising and turning by the aid of their
fluttering fins, might have been modified into perfectly winged animals.
If 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?
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