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 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 a
lliance 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 purpose,--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 i
ntercrossed 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