 Chapter
14 - Recapitulation and Conclusion
*
Recapitulation of the difficulties on the theory of Natural
Selection *
Recapitulation of the general and special circumstances
in its favour *
Causes of the general belief in the immutability of species
*
How far the theory of natural selection may be extended
*
Effects of its adoption on the study of Natural history
*
Concluding remarks
As this whole volume is one long argument, it may be convenient
to the reader to have the leading facts and inferences briefly
recapitulated.
That many and grave objections may be advanced against
the theory of descent with modification through natural
selection, I do not deny. I have endeavoured to give to
them their full force. Nothing at first can appear more
difficult to believe than that the more complex organs and
instincts should have been perfected not by means superior
to, though analogous with, human reason, but by the accumulation
of innumerable slight variations, each good for the individual
possessor. Nevertheless, this difficulty, though appearing
to our imagination insuperably great, cannot be considered
real if we admit the following propositions, namely, --
that gradations in the perfection of any organ or instinct,
which we may consider, either do now exist or could have
existed, each good of its kind, -- that all organs and instincts
are, in ever so slight a degree, variable, -- and, lastly,
that there is a struggle for existence leading to the preservation
of each profitable deviation of structure or instinct. The
truth of these propositions cannot, I think, be disputed.
It is, no doubt, extremely difficult even to conjecture
by what gradations many structures have been perfected,
more especially amongst broken and failing groups of organic
beings; but we see so many strange gradations in nature,
as is proclaimed by the canon, `Natura non facit saltum,'
that we ought to be extremely cautious in saying that any
organ or instinct, or any whole being, could not have arrived
at its present state by many graduated steps. There are,
it must be admitted, cases of special difficulty on the
theory of natural selection; and one of the most curious
of these is the existence of two or three defined castes
of workers or sterile females in the same community of ants
but I have attempted to show how this difficulty can be
mastered. With respect to the almost universal sterility
of species when first crossed, which forms so remarkable
a contrast with the almost universal fertility of varieties
when crossed, I must refer the reader to the recapitulation
of the facts given at the end of the eighth chapter, which
seem to me conclusively to show that this sterility is no
more a special endowment than is the incapacity of two trees
to be grafted together, but that it is incidental on constitutional
differences in the reproductive systems of the intercrossed
species. We see the truth of this conclusion in the vast
difference in the result, when the same two species are
crossed reciprocally; that is, when one species is first
used as the father and then as the mother.
The fertility of varieties when intercrossed and of their
mongrel offspring cannot be considered as universal; nor
is their very general fertility surprising when we remember
that it is not likely that either their constitutions or
their reproductive systems should have been profoundly modified.
Moreover, most of the varieties which have been experimentised
on have been produced under domestication; and as domestication
apparently tends to eliminate sterility, we ought not to
expect it also to produce sterility.
The sterility of hybrids is a very different case from
that of first crosses, for their reproductive organs are
more or less functionally impotent; whereas in first crosses
the organs on both sides are in a perfect condition. As
we continually see that organisms of all kinds are rendered
in some degree sterile from their constitutions having been
disturbed by slightly different and new conditions of life,
we need not feel surprise at hybrids being in some degree
sterile, for their constitutions can hardly fail to have
been disturbed from being compounded of two distinct organisations.
This parallelism is supported by another parallel, but directly
opposite, class of facts; namely, that the vigour and fertility
of all organic beings are increased by slight changes in
their conditions of life, and that the offspring of slightly
modified forms or varieties acquire from being crossed increased
vigour and fertility. So that, on the one hand, considerable
changes in the conditions of life and crosses between greatly
modified forms, lessen fertility; and on the other hand,
lesser changes in the conditions of life and crosses between
less modified forms, increase fertility.
Turning to geographical distribution, the difficulties
encountered on the theory of descent with modification are
grave enough. All the individuals of the same species, and
all the species of the same genus, or even higher group,
must have descended from common parents; and therefore,
in however distant and isolated parts of the world they
are now found, they must in the course of successive generations
have passed from some one part to the others. We are often
wholly unable even to conjecture how this could have been
effected. Yet, as we have reason to believe that some species
have retained the same specific form for very long periods,
enormously long as measured by years, too much stress ought
not to be laid on the occasional wide diffusion of the same
species; for during very long periods of time there will
always be a good chance for wide migration by many means.
A broken or interrupted range may often be accounted for
by the extinction of the species in the intermediate regions.
It cannot be denied that we are as yet very ignorant of
the full extent of the various climatal and geographical
changes which have affected the earth during modern periods;
and such changes will obviously have greatly facilitated
migration. As an example, I have attempted to show how potent
has been the influence of the Glacial period on the distribution
both of the same and of representative species throughout
the world. We are as yet profoundly ignorant of the many
occasional means of transport. With respect to distinct
species of the same genus inhabiting very distant and isolated
regions, as the process of modification has necessarily
been slow, all the means of migration will have been possible
during a very long period; and consequently the difficulty
of the wide diffusion of species of the same genus is in
some degree lessened.
As on the theory of natural selection an interminable
number of intermediate forms must have existed, linking
together all the species in each group by gradations as
fine as our present varieties, it may be asked, Why do we
not see these linking forms all around us? Why are not all
organic beings blended together in an inextricable chaos?
With respect to existing forms, we should remember that
we have no right to expect (excepting in rare cases) to
discover directly connecting links between them,
but only between each and some extinct and supplanted form.
Even on a wide area, which has during a long period remained
continuous, and of which the climate and other conditions
of life change insensibly in going from a district occupied
by one species into another district occupied by a closely
allied species, we have no just right to expect often to
find intermediate varieties in the intermediate zone. For
we have reason to believe that only a few species are undergoing
change at any one period; and all changes are slowly effected.
I have also shown that the intermediate varieties which
will at first probably exist in the intermediate zones,
will be liable to be supplanted by the allied forms on either
hand; and the latter, from existing in greater numbers,
will generally be modified and improved at a quicker rate
than the intermediate varieties, which exist in lesser numbers;
so that the intermediate varieties will, in the long run,
be supplanted and exterminated.
On this doctrine of the extermination of an infinitude
of connecting links, between the living and extinct inhabitants
of the world, and at each successive period between the
extinct and still older species, why is not every geological
formation charged with such links? Why does not every collection
of fossil remains afford plain evidence of the gradation
and mutation of the forms of life? We meet with no such
evidence, and this is the most obvious and forcible of the
many objections which may be urged against my theory. Why,
again, do whole groups of allied species appear, though
certainly they often falsely appear, to have come in suddenly
on the several geological stages? Why do we not find great
piles of strata beneath the Silurian system, stored with
the remains of the progenitors of the Silurian groups of
fossils? For certainly on my theory such strata must somewhere
have been deposited at these ancient and utterly unknown
epochs in the world's history.
I can answer these questions and grave objections only
on the supposition that the geological record is far more
imperfect than most geologists believe. It cannot be objected
that there has not been time sufficient for any amount of
organic change; for the lapse of time has been so great
as to be utterly inappreciable by the human intellect. The
number of specimens in all our museums is absolutely as
nothing compared with the countless generations of countless
species which certainly have existed. We should not be able
to recognise a species as the parent of any one or more
species if we were to examine them ever so closely, unless
we likewise possessed many of the intermediate links between
their past or parent and present states; and these many
links we could hardly ever expect to discover, owing to
the imperfection of the geological record. Numerous existing
doubtful forms could be named which are probably varieties;
but who will pretend that in future ages so many fossil
links will be discovered, that naturalists will be able
to decide, on the common view, whether or not these doubtful
forms are varieties? As long as most of the links between
any two species are unknown, if any one link or intermediate
variety be discovered, it will simply be classed as another
and distinct species. Only a small portion of the world
has been geologically explored. Only organic beings of certain
classes can be preserved in a fossil condition, at least
in any great number. Widely ranging species vary most, and
varieties are often at first local, -- both causes rendering
the discovery of intermediate links less likely. Local varieties
will not spread into other and distant regions until they
are considerably modified and improved; and when they do
spread, if discovered in a geological formation, they will
appear as if suddenly created there, and will be simply
classed as new species. Most formations have been intermittent
in their accumulation; and their duration, I am inclined
to believe, has been shorter than the average duration of
specific forms. Successive formations are separated from
each other by enormous blank intervals of time; for fossiliferous
formations, thick enough to resist future degradation, can
be accumulated only where much sediment is deposited on
the subsiding bed of the sea. During the alternate periods
of elevation and of stationary level the record will be
blank. During these latter periods there will probably be
more variability in the forms of life; during periods of
subsidence, more extinction.
With respect to the absence of fossiliferous formations
beneath the lowest Silurian strata, I can only recur to
the hypothesis given in the ninth chapter. That the geological
record is imperfect all will admit; but that it is imperfect
to the degree which I require, few will be inclined to admit.
If we look to long enough intervals of time, geology plainly
declares that all species have changed; and they have changed
in the manner which my theory requires, for they have changed
slowly and in a graduated manner. We clearly see this in
the fossil remains from consecutive formations invariably
being much more closely related to each other, than are
the fossils from formations distant from each other in time.
Such is the sum of the several chief objections and difficulties
which may justly be urged against my theory; and I have
now briefly recapitulated the answers and explanations which
can be given to them. I have felt these difficulties far
too heavily during many years to doubt their weight. But
it deserves especial notice that the more important objections
relate to questions on which we are confessedly ignorant;
nor do we know how ignorant we are. We do not know all the
possible transitional gradations between the simplest and
the most perfect organs; it cannot be pretended that we
know all the varied means of Distribution during the long
lapse of years, or that we know how imperfect the Geological
Record is. Grave as these several difficulties are, in my
judgement they do not overthrow the theory of descent with
modification.
Now let us turn to the other side of the argument. Under
domestication we see much variability. This seems to be
mainly due to the reproductive system being eminently susceptible
to changes in the conditions of life so that this system,
when not rendered impotent, fails to reproduce offspring
exactly like the parent-form. Variability is governed by
many complex laws, -- by correlation of growth, by use and
disuse, and by the direct action of the physical conditions
of life. There is much difficulty in ascertaining how much
modification our domestic productions have undergone; but
we may safely infer that the amount has been large, and
that modifications can be inherited for long periods. As
long as the conditions of life remain the same, we have
reason to believe that a modification, which has already
been inherited for many generations, may continue to be
inherited for an almost infinite number of generations.
On the other hand we have evidence that variability, when
it has once come into play, does not wholly cease; for new
varieties are still occasionally produced by our most anciently
domesticated productions.
Man does not actually produce variability; he only unintentionally
exposes organic beings to new conditions of life, and then
nature acts on the organisation, and causes variability.
But man can and does select the variations given to him
by nature, and thus accumulate them in any desired manner.
He thus adapts animals and plants for his own benefit or
pleasure. He may do this methodically, or he may do it unconsciously
by preserving the individuals most useful to him at the
time, without any thought of altering the breed. It is certain
that he can largely influence the character of a breed by
selecting, in each successive generation, individual differences
so slight as to be quite inappreciable by an uneducated
eye. This process of selection has been the great agency
in the production of the most distinct and useful domestic
breeds. That many of the breeds produced by man have to
a large extent the character of natural species, is shown
by the inextricable doubts whether very many of them are
varieties or aboriginal species.
There is no obvious reason why the principles which have
acted so efficiently under domestication should not have
acted under nature. In the preservation of favoured individuals
and races, during the constantly-recurrent Struggle for
Existence, we see the most powerful and ever-acting means
of selection. The struggle for existence inevitably follows
from the high geometrical ratio of increase which is common
to all organic beings. This high rate of increase is proved
by calculation, by the effects of a succession of peculiar
seasons, and by the results of naturalisation, as explained
in the third chapter. More individuals are born than can
possibly survive. A grain in the balance will determine
which individual shall live and which shall die, -- which
variety or species shall increase in number, and which shall
decrease, or finally become extinct. As the individuals
of the same species come in all respects into the closest
competition with each other, the struggle will generally
be most severe between them; it will be almost equally severe
between the varieties of the same species, and next in severity
between the species of the same genus. But the struggle
will often be very severe between beings most remote in
the scale of nature. The slightest advantage in one being,
at any age or during any season, over those with which it
comes into competition, or better adaptation in however
slight a degree to the surrounding physical conditions,
will turn the balance.
With animals having separated sexes there will in most
cases be a struggle between the males for possession of
the females. The most vigorous individuals, or those which
have most successfully struggled with their conditions of
life, will generally leave most progeny. But success will
often depend on having special weapons or means of defence,
or on the charms of the males; and the slightest advantage
will lead to victory.
As geology plainly proclaims that each land has undergone
great physical changes, we might have expected that organic
beings would have varied under nature, in the same way as
they generally have varied under the changed conditions
of domestication. And if there be any variability under
nature, it would be an unaccountable fact if natural selection
had not come into play. It has often been asserted, but
the assertion is quite incapable of proof, that the amount
of variation under nature is a strictly limited quantity.
Man, though acting on external characters alone and often
capriciously, can produce within a short period a great
result by adding up mere individual differences in his domestic
productions; and every one admits that there are at least
individual differences in species under nature. But, besides
such differences, all naturalists have admitted the existence
of varieties, which they think sufficiently distinct to
be worthy of record in systematic works. No one can draw
any clear distinction between individual differences and
slight varieties; or between more plainly marked varieties
and subspecies, and species. Let it be observed how naturalists
differ in the rank which they assign to the many representative
forms in Europe and North America.
If then we have under nature variability and a powerful
agent always ready to act and select, why should we doubt
that variations in any way useful to beings, under their
excessively complex relations of life, would be preserved,
accumulated, and inherited? Why, if man can by patience
select variations most useful to himself, should nature
fail in selecting variations useful, under changing conditions
of life, to her living products? What limit can be put to
this power, acting during long ages and rigidly scrutinising
the whole constitution, structure, and habits of each creature,
— favouring the good and rejecting the bad? I can
see no limit to this power, in slowly and beautifully adapting
each form to the most complex relations of life. The theory
of natural selection, even if we looked no further than
this, seems to me to be in itself probable. I have already
recapitulated, as fairly as I could, the opposed difficulties
and objections: now let us turn to the special facts and
arguments in favour of the theory.
On the view that species are only strongly marked and
permanent varieties, and that each species first existed
as a variety, we can see why it is that no line of demarcation
can be drawn between species, commonly supposed to have
been produced by special acts of creation, and varieties
which are acknowledged to have been produced by secondary
laws. On this same view we can understand how it is that
in each region where many species of a genus have been produced,
and where they now flourish, these same species should present
many varieties; for where the manufactory of species has
been active, we might expect, as a general rule, to find
it still in action; and this is the case if varieties be
incipient species. Moreover, the species of the large genera,
which afford the greater number of varieties or incipient
species, retain to a certain degree the character of varieties;
for they differ from each other by a less amount of difference
than do the species of smaller genera. The closely allied
species also of the larger genera apparently have restricted
ranges, and they are clustered in little groups round other
species -- in which respects they resemble varieties. These
are strange relations on the view of each species having
been independently created, but are intelligible if all
species first existed as varieties.
As each species tends by its geometrical ratio of reproduction
to increase inordinately in number; and as the modified
descendants of each species will be enabled to increase
by so much the more as they become more diversified in habits
and structure, so as to be enabled to seize on many and
widely different places in the economy of nature, there
will be a constant tendency in natural selection to preserve
the most divergent offspring of any one species. Hence during
a long-continued course of modification, the slight differences,
characteristic of varieties of the same species, tend to
be augmented into the greater differences characteristic
of species of the same genus. New and improved varieties
will inevitably supplant and exterminate the older, less
improved and intermediate varieties; and thus species are
rendered to a large extent defined and distinct objects.
Dominant species belonging to the larger groups tend to
give birth to new and dominant forms; so that each large
group tends to become still larger, and at the same time
more divergent in character. But as all groups cannot thus
succeed in increasing in size, for the world would not hold
them, the more dominant groups beat the less dominant. This
tendency in the large groups to go on increasing in size
and diverging in character, together with the almost inevitable
contingency of much extinction, explains the arrangement
of all the forms of life, in groups subordinate to groups,
all within a few great classes, which we now see everywhere
around us, and which has prevailed throughout all time.
This grand fact of the grouping of all organic beings seems
to me utterly inexplicable on the theory of creation.
As natural selection acts solely by accumulating slight,
successive, favourable variations, it can produce no great
or sudden modification; it can act only by very short and
slow steps. Hence the canon of `Natura non facit saltum,'
which every fresh addition to our knowledge tends to make
more strictly correct, is on this theory simply intelligible.
We can plainly see why nature is prodigal in variety, though
niggard in innovation. But why this should be a law of nature
if each species has been independently created, no man can
explain.
Many other facts are, as it seems to me, explicable on
this theory. How strange it is that a bird, under the form
of woodpecker, should have been created to prey on insects
on the ground; that upland geese, which never or rarely
swim, should have been created with webbed feet; that a
thrush should have been created to dive and feed on sub-aquatic
insects; and that a petrel should have been created with
habits and structure fitting it for the life of an auk or
grebe! and so on in endless other cases. But on the view
of each species constantly trying to increase in number,
with natural selection always ready to adapt the slowly
varying descendants of each to any unoccupied or ill-occupied
place in nature, these facts cease to be strange, or perhaps
might even have been anticipated.
As natural selection acts by competition, it adapts the
inhabitants of each country only in relation to the degree
of perfection of their associates; so that we need feel
no surprise at the inhabitants of any one country, although
on the ordinary view supposed to have been specially created
and adapted for that country, being beaten and supplanted
by the naturalised productions from another land. Nor ought
we to marvel if all the contrivances in nature be not, as
far as we can judge, absolutely perfect; and if some of
them be abhorrent to our ideas of fitness. We need not marvel
at the sting of the bee causing the bee's own death; at
drones being produced in such vast numbers for one single
act, and being then slaughtered by their sterile sisters;
at the astonishing waste of pollen by our fir-trees; at
the instinctive hatred of the queen bee for her own fertile
daughters; at ichneumonidae feeding within the live bodies
of caterpillars; and at other such cases. The wonder indeed
is, on the theory of natural selection, that more cases
of the want of absolute perfection have not been observed.
The complex and little known laws governing variation
are the same, as far as we can see, with the laws which
have governed the production of so-called specific forms.
In both cases physical conditions seem to have produced
but little direct effect; yet when varieties enter any zone,
they occasionally assume some of the characters of the species
proper to that zone. In both varieties and species, use
and disuse seem to have produced some effect; for it is
difficult to resist this conclusion when we look, for instance,
at the logger-headed duck, which has wings incapable of
flight, in nearly the same condition as in the domestic
duck; or when we look at the burrowing tucutucu, which is
occasionally blind, and then at certain moles, which are
habitually blind and have their eyes covered with skin;
or when we look at the blind animals inhabiting the dark
caves of America and Europe. In both varieties and species
correction of growth seems to have played a most important
part, so that when one part has been modified other parts
are necessarily modified. In both varieties and species
reversions to long-lost characters occur. How inexplicable
on the theory of creation is the occasional appearance of
stripes on the shoulder and legs of the several species
of the horse-genus and in their hybrids! How simply is this
fact explained if we believe that these species have descended
from a striped progenitor, in the same manner as the several
domestic breeds of pigeon have descended from the blue and
barred rock-pigeon!
On the ordinary view of each species having been independently
created, why should the specific characters, or those by
which the species of the same genus differ from each other,
be more variable than the generic characters in which they
all agree? Why, for instance, should the colour of a flower
be more likely to vary in any one species of a genus, if
the other species, supposed to have been created independently,
have differently coloured flowers, than if all the species
of the genus have the same coloured flowers? If species
are only well-marked varieties, of which the characters
have become in a high degree permanent, we can understand
this fact; for they have already varied since they branched
off from a common progenitor in certain characters, by which
they have come to be specifically distinct from each other;
and therefore these same characters would be more likely
still to be variable than the generic characters which have
been inherited without change for an enormous period. It
is inexplicable on the theory of creation why a part developed
in a very unusual manner in any one species of a genus,
and therefore, as we may naturally infer, of great importance
to the species, should be eminently liable to variation;
but, on my view, this part has undergone, since the several
species branched off from a common progenitor, an unusual
amount of variability and modification, and therefore we
might expect this part generally to be still variable. But
a part may be developed in the most unusual manner, like
the wing of a bat, and yet not be more variable than any
other structure, if the part be common to many subordinate
forms, that is, if it has been inherited for a very long
period; for in this case it will have been rendered constant
by long-continued natural selection.
Glancing at instincts, marvellous as some are, they offer
no greater difficulty than does corporeal structure on the
theory of the natural selection of successive, slight, but
profitable modifications. We can thus understand why nature
moves by graduated steps in endowing different animals of
the same class with their several instincts. I have attempted
to show how much light the principle of gradation throws
on the admirable architectural powers of the hive-bee. Habit
no doubt sometimes comes into play in modifying instincts;
but it certainly is not indispensable, as we see, in the
case of neuter insects, which leave no progeny to inherit
the effects of long-continued habit. On the view of all
the species of the same genus having descended from a common
parent, and having inherited much in common, we can understand
how it is that allied species, when placed under considerably
different conditions of life, yet should follow nearly the
same instincts; why the thrush of South America, for instance,
lines her nest with mud like our British species. On the
view of instincts having been slowly acquired through natural
selection we need not marvel at some instincts being apparently
not perfect and liable to mistakes, and at many instincts
causing other animals to suffer.
If species be only well-marked and permanent varieties,
we can at once see why their crossed offspring should follow
the same complex laws in their degrees and kinds of resemblance
to their parents, -- in being absorbed into each other by
successive crosses, and in other such points, -- as do the
crossed offspring of acknowledged varieties. On the other
hand, these would be strange facts if species have been
independently created, and varieties have been produced
by secondary laws.
If we admit that the geological record is imperfect in
an extreme degree, then such facts as the record gives,
support the theory of descent with modification. New species
have come on the stage slowly and at successive intervals;
and the amount of change, after equal intervals of time,
is widely different in different groups. The extinction
of species and of whole groups of species, which has played
so conspicuous a part in the history of the organic world,
almost inevitably follows on the principle of natural selection;
for old forms will be supplanted by new and improved forms.
Neither single species nor groups of species reappear when
the chain of ordinary generation has once been broken. The
gradual diffusion of dominant forms, with the slow modification
of their descendants, causes the forms of life, after long
intervals of time, to appear as if they had changed simultaneously
throughout the world. The fact of the fossil remains of
each formation being in some degree intermediate in character
between the fossils in the formations above and below, is
simply explained by their intermediate position in the chain
of descent. The grand fact that all extinct organic beings
belong to the same system with recent beings, falling either
into the same or into intermediate groups, follows from
the living and the extinct being the offspring of common
parents. As the groups which have descended from an ancient
progenitor have generally diverged in character, the progenitor
with its early descendants will often be intermediate in
character in comparison with its later descendants; and
thus we can see why the more ancient a fossil is, the oftener
it stands in some degree intermediate between existing and
allied groups. Recent forms are generally looked at as being,
in some vague sense, higher than ancient and extinct forms;
and they are in so far higher as the later and more improved
forms have conquered the older and less improved organic
beings in the struggle for life. Lastly, the law of the
n='448'> long endurance of allied forms on the same continent,
— of marsupials in Australia, of edentata in America,
and other such cases, -- is intelligible, for within a confined
country, the recent and the extinct will naturally be allied
by descent.
Looking to geographical distribution, if we admit that
there has been during the long course of ages much migration
from one part of the world to another, owing to former climatal
and geographical changes and to the many occasional and
unknown means of dispersal, then we can understand, on the
theory of descent with modification, most of the great leading
facts in Distribution. We can see why there should be so
striking a parallelism in the distribution of organic beings
throughout space, and in their geological succession throughout
time; for in both cases the beings have been connected by
the bond of ordinary generation, and the means of modification
have been the same. We see the full meaning of the wonderful
fact, which must have struck every traveller, namely, that
on the same continent, under the most diverse conditions,
under heat and cold, on mountain and lowland, on deserts
and marshes, most of the inhabitants within each great class
are plainly related; for they will generally be descendants
of the same progenitors and early colonists. On this same
principle of former migration, combined in most cases with
modification, we can understand, by the aid of the Glacial
period, the identity of some few plants, and the close alliance
of many others, on the most distant mountains, under the
most different climates; and likewise the close alliance
of some of the inhabitants of the sea in the northern and
southern temperate zones, though separated by the whole
intertropical ocean. Although two areas may present the
same physical conditions of life, we need feel no surprise
at their inhabitants being widely different, if they have
been for a long period completely separated from each other;
for as the relation of organism to organism is the most
important of all relations, and as the two areas will have
received colonists from some third source or from each other,
at various periods and in different proportions, the course
of modification in the two areas will inevitably be different.
On this view of migration, with subsequent modification,
we can see why oceanic islands should be inhabited by few
species, but of these, that many should be peculiar. We
can clearly see why those animals which cannot cross wide
spaces of ocean, as frogs and terrestrial mammals, should
not inhabit oceanic islands; and why, on the other hand,
new and peculiar species of bats, which can traverse the
ocean, should so often be found on islands far distant from
any continent. Such facts as the presence of peculiar species
of bats, and the absence of all other mammals, on oceanic
islands, are utterly inexplicable on the theory of independent
acts of creation.
The existence of closely allied or representative species
in any two areas, implies, on the theory of descent with
modification, that the same parents formerly inhabited both
areas; and we almost invariably find that wherever many
closely allied species inhabit two areas, some identical
species common to both still exist. Wherever many closely
allied yet distinct species occur, many doubtful forms and
varieties of the same species likewise occur. It is a rule
of high generality that the inhabitants of each area are
related to the inhabitants of the nearest source whence
immigrants might have been derived. We see this in nearly
all the plants and animals of the Galapagos archipelago,
of Juan Fernandez, and of the other American islands being
related in the most striking manner to the plants and animals
of the neighbouring American mainland; and those of the
Cape de Verde archipelago and other African islands to the
African mainland. It must be admitted that these facts receive
no explanation on the theory of creation.
The fact, as we have seen, that all past and present organic
beings constitute one grand natural system, with group subordinate
to group, and with extinct groups often falling in between
recent groups, is intelligible on the theory of natural
selection with its contingencies of extinction and divergence
of character. On these same principles we see how it is,
that the mutual affinities of the species and genera within
each class are so complex and circuitous. We see why certain
characters are far more serviceable than others for classification;
-- why adaptive characters, though of paramount importance
to the being, are of hardly any importance in classification;
why characters derived from rudimentary parts, though of
no service to the being, are often of high classificatory
value; and why embryological characters are the most valuable
of all. The real affinities of all organic beings are due
to inheritance or community of descent. The natural system
is a genealogical arrangement, in which we have to discover
the lines of descent by the most permanent characters, however
slight their vital importance may be.
The framework of bones being the same in the hand of a
man, wing of a bat, fin of the porpoise, and leg of the
horse, -- the same number of vertebrae forming the neck
of the giraffe and of the elephant, -- and innumerable other
such facts, at once explain themselves on the theory of
descent with slow and slight successive modifications. The
similarity of pattern in the wing and leg of a bat, though
used for such different purposes, -- in the jaws and legs
of a crab, -- in the petals, stamens, and pistils of a flower,
is likewise intelligible on the view of the gradual modification
of parts or organs, which were alike in the early progenitor
of each class. On the principle of successive variations
not always supervening at an early age, and being inherited
at a corresponding not early period of life, we can clearly
see why the embryos of mammals, birds, reptiles, and fishes
should be so closely alike, and should be so unlike the
adult forms. We may cease marvelling at the embryo of an
air-breathing mammal or bird having branchial slits and
arteries running in loops, like those in a fish which has
to breathe the air dissolved in water, by the aid of well-developed
branchiae.
Disuse, aided sometimes by natural selection, will often
tend to reduce an organ, when it has become useless by changed
habits or under changed conditions of life; and we can clearly
understand on this view the meaning of rudimentary organs.
But disuse and selection will generally act on each creature,
when it has come to maturity and has to play its full part
in the struggle for existence, and will thus have little
power of acting on an organ during early life; hence the
organ will not be much reduced or rendered rudimentary at
this early age. The calf, for instance, has inherited teeth,
which never cut through the gums of the upper jaw, from
an early progenitor having well-developed teeth; and we
may believe, that the teeth in the mature animal were reduced,
during successive generations, by disuse or by the tongue
and palate having been fitted by natural selection to browse
without their aid; whereas in the calf, the teeth have been
left untouched by selection or disuse, and on the principle
of inheritance at corresponding ages have been inherited
from a remote period to the present day. On the view of
each organic being and each separate organ having been specially
created, how utterly inexplicable it is that parts, like
the teeth in the embryonic calf or like the shrivelled wings
under the soldered wing-covers of some beetles, should thus
so frequently bear the plain stamp of inutility! Nature
may be said to have taken pains to reveal, by rudimentary
organs and by homologous structures, her scheme of modification,
which it seems that we wilfully will not understand.
I have now recapitulated the chief facts and considerations
which have thoroughly convinced me that species have changed,
and are still slowly changing by the preservation and accumulation
of successive slight favourable variations. Why, it may
be asked, have all the most eminent living naturalists and
geologists rejected this view of the mutability of species?
It cannot be asserted that organic beings in a state of
nature are subject to no variation; it cannot be proved
that the amount of variation in the course of long ages
is a limited quantity; no clear distinction has been, or
can be, drawn between species and well-marked varieties.
It cannot be maintained that species when intercrossed are
invariably sterile, and varieties invariably fertile; or
that sterility is a special endowment and sign of creation.
The belief that species were immutable productions was almost
unavoidable as long as the history of the world was thought
to be of short duration; and now that we have acquired some
idea of the lapse of time, we are too apt to assume, without
proof, that the geological record is so perfect that it
would have afforded us plain evidence of the mutation of
species, if they had undergone mutation.
But the chief cause of our natural unwillingness to admit
that one species has given birth to other and distinct species,
is that we are always slow in admitting any great change
of which we do not see the intermediate steps. The difficulty
is the same as that felt by so many geologists, when Lyell
first insisted that long lines of inland cliffs had been
formed, and great valleys excavated, by the slow action
of the coast-waves. The mind cannot possibly grasp the full
meaning of the term of a hundred million years; it cannot
add up and perceive the full effects of many slight variations,
accumulated during an almost infinite number of generations.
Although I am fully convinced of the truth of the views
given in this volume under the form of an abstract, I by
no means expect to convince experienced naturalists whose
minds are stocked with a multitude of facts all viewed,
during a long course of years, from a point of view directly
opposite to mine. It is so easy to hide our ignorance under
such expressions as the `plan of creation,' `unity of design,'
&c., and to think that we give an explanation when we
only restate a fact. Any one whose disposition leads him
to attach more weight to unexplained difficulties than to
the explanation of a certain number of facts will certainly
reject my theory. A few naturalists, endowed with much flexibility
of mind, and who have already begun to doubt on the immutability
of species, may be influenced by this volume; but I look
with confidence to the future, to young and rising naturalists,
who will be able to view both sides of the question with
impartiality. Whoever is led to believe that species are
mutable will do good service by conscientiously expressing
his conviction; for only thus can the load of prejudice
by which this subject is overwhelmed be removed.
Several eminent naturalists have of late published their
belief that a multitude of reputed species in each genus
are not real species; but that other species are real, that
is, have been independently created. This seems to me a
strange conclusion to arrive at. They admit that a multitude
of forms, which till lately they themselves thought were
special creations, and which are still thus looked at by
the majority of naturalists, and which consequently have
every external characteristic feature of true species, --
they admit that these have been produced by variation, but
they refuse to extend the same view to other and very slightly
different forms. Nevertheless they do not pretend that they
can define, or even conjecture, which are the created forms
of life, and which are those produced by secondary laws.
They admit variation as a vera causa in one case,
they arbitrarily reject it in another, without assigning
any distinction in the two cases. The day will come when
this will be given as a curious illustration of the blindness
of preconceived opinion. These authors seem no more startled
at a miraculous act of creation than at an ordinary birth.
But do they really believe that at innumerable periods in
the earth's history certain elemental atoms have been commanded
suddenly to flash into living tissues? Do they believe that
at each supposed act of creation one individual or many
were produced? Were all the infinitely numerous kinds of
animals and plants created as eggs or seed, or as full grown?
and in the case of mammals, were they created bearing the
false marks of nourishment from the mother's womb? Although
naturalists very properly demand a full explanation of every
difficulty from those who believe in the mutability of species,
on their own side they ignore the whole subject of the first
appearance of species in what they consider reverent silence.
It may be asked how far I extend the doctrine of the modification
of species. The question is difficult to answer, because
the more distinct the forms are which we may consider, by
so much the arguments fall away in force. But some arguments
of the greatest weight extend very far. All the members
of whole classes can be connected together by chains of
affinities, and all can be classified on the same principle,
in groups subordinate to groups. Fossil remains sometimes
tend to fill up very wide intervals between existing orders.
Organs in a rudimentary condition plainly show that an early
progenitor had the organ in a fully developed state; and
this in some instances necessarily implies an enormous amount
of modification in the descendants. Throughout whole classes
various structures are formed on the same pattern, and at
an embryonic age the species closely resemble each other.
Therefore I cannot doubt that the theory of descent with
modification embraces all the members of the same class.
I believe that animals have descended from at most only
four or five progenitors, and plants from an equal or lesser
number.
Analogy would lead me one step further, namely, to the
belief that all animals and plants have descended from some
one prototype. But analogy may be a deceitful guide. Nevertheless
all living things have much in common, in their chemical
composition, their germinal vesicles, their cellular structure,
and their laws of growth and reproduction. We see this even
in so trifling a circumstance as that the same poison often
similarly affects plants and animals; or that the poison
secreted by the gall-fly produces monstrous growths on the
wild rose or oak-tree. Therefore I should infer from analogy
that probably all the organic beings which have ever lived
on this earth have descended from some one primordial form,
into which life was first breathed.
When the views entertained in this volume on the origin
of species, or when analogous views are generally admitted,
we can dimly foresee that there will be a considerable revolution
in natural history. Systematists will be able to pursue
their labours as at present; but they will not be incessantly
haunted by the shadowy doubt whether this or that form be
in essence a species. This I feel sure, and I speak after
experience, will be no slight relief. The endless disputes
whether or not some fifty species of British brambles are
true species will cease. Systematists will have only to
decide (not that this will be easy) whether any form be
sufficiently constant and distinct from other forms, to
be capable of definition; and if definable, whether the
differences be sufficiently important to deserve a specific
name. This latter point will become a far more essential
consideration than it is at present; for differences, however
slight, between any two forms, if not blended by intermediate
gradations, are looked at by most naturalists as sufficient
to raise both forms to the rank of species. Hereafter we
shall be compelled to acknowledge that the only distinction
between species and well-marked varieties is, that the latter
are known, or believed, to be connected at the present day
by intermediate gradations, whereas species were formerly
thus connected. Hence, without quite rejecting the consideration
of the present existence of intermediate gradations between
any two forms, we shall be led to weigh more carefully and
to value higher the actual amount of difference between
them. It is quite possible that forms now generally acknowledged
to be merely varieties may hereafter be thought worthy of
specific names, as with the primrose and cowslip; and in
this case scientific and common language will come into
accordance. In short, we shall have to treat species in
the same manner as those naturalists treat genera, who admit
that genera are merely artificial combinations made for
convenience. This may not be a cheering prospect; but we
shall at least be freed from the vain search for the undiscovered
and undiscoverable essence of the term species.
The other and more general departments of natural history
will rise greatly in interest. The terms used by naturalists
of affinity, relationship, community of type, paternity,
morphology, adaptive characters, rudimentary and aborted
organs, &c., will cease to be metaphorical, and will
have a plain signification. When we no longer look at an
organic being as a savage looks at a ship, as at something
wholly beyond his comprehension; when we regard every production
of nature as one which has had a history; when we contemplate
every complex structure and instinct as the summing up of
many contrivances, each useful to the possessor, nearly
in the same way as when we look at any great mechanical
invention as the summing up of the labour, the experience,
the reason, and even the blunders of numerous workmen; when
we thus view each organic being, how far more interesting,
I speak from experience, will the study of natural history
become!
A grand and almost untrodden field of inquiry will be
opened, on the causes and laws of variation, on correlation
of growth, on the effects of use and disuse, on the direct
action of external conditions, and so forth. The study of
domestic productions will rise immensely in value. A new
variety raised by man will be a far more important and interesting
subject for study than one more species added to the infinitude
of already recorded species. Our classifications will come
to be, as far as they can be so made, genealogies; and will
then truly give what may be called the plan of creation.
The rules for classifying will no doubt become simpler when
we have a definite object in view. We possess no pedigrees
or armorial bearings; and we have to discover and trace
the many diverging lines of descent in our natural genealogies,
by characters of any kind which have long been inherited.
Rudimentary organs will speak infallibly with respect to
the nature of long-lost structures. Species and groups of
species, which are called aberrant, and which may fancifully
be called living fossils, will aid us in forming a picture
of the ancient forms of life. Embryology will reveal to
us the structure, in some degree obscured, of the prototypes
of each great class.
When we can feel assured that all the individuals of the
same species, and all the closely allied species of most
genera, have within a not very remote period descended from
one parent, and have migrated from some one birthplace;
and when we better know the many means of migration, then,
by the light which geology now throws, and will continue
to throw, on former changes of climate and of the level
of the land, we shall surely be enabled to trace in an admirable
manner the former migrations of the inhabitants of the whole
world. Even at present, by comparing the differences of
the inhabitants of the sea on the opposite sides of a continent,
and the nature of the various inhabitants of that continent
in relation to their apparent means of immigration, some
light can be thrown on ancient geography.
The noble science of Geology loses glory from the extreme
imperfection of the record. The crust of the earth with
its embedded remains must not be looked at as a well-filled
museum, but as a poor collection made at hazard and at rare
intervals. The accumulation of each great fossiliferous
formation will be recognised as having depended on an unusual
concurrence of circumstances, and the blank intervals between
the successive stages as having been of vast duration. But
we shall be able to gauge with some security the duration
of these intervals by a comparison of the preceding and
succeeding organic forms. We must be cautious in attempting
to correlate as strictly contemporaneous two formations,
which include few identical species, by the general succession
of their forms of life. As species are produced and exterminated
by slowly acting and still existing causes, and not by miraculous
acts of creation and by catastrophes; and as the most important
of all causes of organic change is one which is almost independent
of altered and perhaps suddenly altered physical conditions,
namely, the mutual relation of organism to organism, --
the improvement of one being entailing the improvement or
the extermination of others; it follows, that the amount
of organic change in the fossils of consecutive formations
probably serves as a fair measure of the lapse of actual
time. A number of species, however, keeping in a body might
remain for a long period unchanged, whilst within this same
period, several of these species, by migrating into new
countries and coming into competition with foreign associates,
might become modified; so that we must not overrate the
accuracy of organic change as a measure of time. During
early periods of the earth's history, when the forms of
life were probably fewer and simpler, the rate of change
was probably slower; and at the first dawn of life, when
very few forms of the simplest structure existed, the rate
of change may have been slow in an extreme degree. The whole
history of the world, as at present known, although of a
length quite incomprehensible by us, will hereafter be recognised
as a mere fragment of time, compared with the ages which
have elapsed since the first creature, the progenitor of
innumerable extinct and living descendants, was created.
In the distant future I see open fields for far more important
researches. Psychology will be based on a new foundation,
that of the necessary acquirement of each mental power and
capacity by gradation. Light will be thrown on the origin
of man and his history.
Authors of the highest eminence seem to be fully satisfied
with the view that each species has been independently created.
To my mind it accords better with what we know of the laws
impressed on matter by the Creator, that the production
and extinction of the past and present inhabitants of the
world should have been due to secondary causes, like those
determining the birth and death of the individual. When
I view all beings not as special creations, but as the lineal
descendants of some few beings which lived long before the
first bed of the Silurian system was deposited, they seem
to me to become ennobled. Judging from the past, we may
safely infer that not one living species will transmit its
unaltered likeness to a distant futurity. And of the species
now living very few will transmit progeny of any kind to
a far distant futurity; for the manner in which all organic
beings are grouped, shows that the greater number of species
of each genus, and all the species of many genera, have
left no descendants, but have become utterly extinct. We
can so far take a prophetic glance into futurity as to foretel
that it will be the common and widely-spread species, belonging
to the larger and dominant groups, which will ultimately
prevail and procreate new and dominant species. As all the
living forms of life are the lineal descendants of those
which lived long before the Silurian epoch, we may feel
certain that the ordinary succession by generation has never
once been broken, and that no cataclysm has desolated the
whole world. Hence we may look with some confidence to a
secure future of equally inappreciable length. And as natural
selection works solely by and for the good of each being,
all corporeal and mental endowments will tend to progress
towards perfection.
It is interesting to contemplate an entangled bank, clothed
with many plants of many kinds, with birds singing on the
bushes, with various insects flitting about, and with worms
crawling through the damp earth, and to reflect that these
elaborately constructed forms, so different from each other,
and dependent on each other in so complex a manner, have
all been produced by laws acting around us. These laws,
taken in the largest sense, being Growth with Reproduction;
inheritance which is almost implied by reproduction; Variability
from the indirect and direct action of the external conditions
of life, and from use and disuse; a Ratio of Increase so
high as to lead to a Struggle for Life, and as a consequence
to Natural Selection, entailing Divergence of Character
and the Extinction of less-improved forms. Thus, from the
war of nature, from famine and death, the most exalted object
which we are capable of conceiving, namely, the production
of the higher animals, directly follows. There is grandeur
in this view of life, with its several powers, having been
originally breathed into a few forms or into one; and that,
whilst this planet has gone cycling on according to the
fixed law of gravity, from so simple a beginning endless
forms most beautiful and most wonderful have been, and are
being, evolved.
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