the theory of Professor Forbes. Our knowledge of the organic world
during any geological epoch is necessarily very imperfect. Looking at
the vast numbers of species and groups that have been discovered by
geologists, this may be doubted; but we should compare their numbers not
merely with those that now exist upon the earth, but with a far larger
amount. We have no reason for believing that the number of species on
the earth at any former period was much less than at present; at all
events the aquatic portion, with which geologists have most
acquaintance, was probably often as great or greater. Now we know that
there have been many complete changes of species; new sets of organisms
have many times been introduced in place of old ones which have become
extinct, so that the total amount which have existed on the earth from
the earliest geological period must have borne about the same proportion
to those now living, as the whole human race who have lived and died
upon the earth, to the population at the present time. Again, at each
epoch, the whole earth was no doubt, as now, more or less the theatre of
life, and as the successive generations of each species died, their
exuviae and preservable parts would be deposited over every portion of
the then existing seas and oceans, which we have reason for supposing to
have been more, rather than less, extensive than at present. In order
then to understand our possible knowledge of the early world and its
inhabitants, we must compare, not the area of the whole field of our
geological researches with the earth's surface, but the area of the
examined portion of each formation separately with the whole earth. For
example, during the Silurian period all the earth was Silurian, and
animals were living and dying, and depositing their remains more or less
over the whole area of the globe, and they were probably (the species at
least) nearly as varied in different latitudes and longitudes as at
present. What proportion do the Silurian districts bear to the whole
surface of the globe, land and sea (for far more extensive Silurian
districts probably exist beneath the ocean than above it), and what
portion of the known Silurian districts has been actually examined for
fossils? Would the area of rock actually laid open to the eye be the
thousandth or the ten-thousandth part of the earth's surface? Ask the
same question with regard to the Oolite or the Chalk, or even to
particular beds of these when they differ considerably in their
fossils, and you may then get some notion of how small a portion of the
whole we know.
But yet more important is the probability, nay almost the certainty,
that whole formations containing the records of vast geological periods
are entirely buried beneath the ocean, and for ever beyond our reach.
Most of the gaps in the geological series may thus be filled up, and
vast numbers of unknown and unimaginable animals, which might help to
elucidate the affinities of the numerous isolated groups which are a
perpetual puzzle to the zoologist, may there be buried, till future
revolutions may raise them in their turn above the waters, to afford
materials for the study of whatever race of intelligent beings may then
have succeeded us. These considerations must lead us to the conclusion,
that our knowledge of the whole series of the former inhabitants of the
earth is necessarily most imperfect and fragmentary,--as much so as our
knowledge of the present organic world would be, were we forced to make
our collections and observations only in spots equally limited in area
and in number with those actually laid open for the collection of
fossils. Now, the hypothesis of Professor Forbes is essentially one that
assumes to a great extent the completeness of our knowledge of the whole
series of organic beings which have existed on the earth. This
appears to be a fatal objection to it, independently of all other
considerations. It may be said that the same objections exist against
every theory on such a subject, but this is not necessarily the case.
The hypothesis put forward in this paper depends in no degree upon the
completeness of our knowledge of the former condition of the organic
world, but takes what facts we have as fragments of a vast whole, and
deduces from them something of the nature and proportions of that whole
which we can never know in detail. It is founded upon isolated groups of
facts, recognizes their isolation, and endeavours to deduce from them
the nature of the intervening portions.
_Rudimentary Organs._
Another important series of facts, quite in accordance with, and even
necessary deductions from, the law now developed, are those of
rudimentary organs. That these really do exist, and in most cases have
no special function in the animal oeconomy, is admitted by the first
authorities in comparative anatomy. The minute limbs hidden beneath the
skin in many of the snake-like lizards, the anal hooks of the boa
constrictor, the complete series of jointed finger-bones in the paddle
of the Manatus and whale, are a few of the most familiar instances. In
botany a similar class of facts has been long recognised. Abortive
stamens, rudimentary floral envelopes and undeveloped carpels, are of
the most frequent occurrence. To every thoughtful naturalist the
question must arise, What are these for? What have they to do with the
great laws of creation? Do they not teach us something of the system of
Nature? If each species has been created independently, and without any
necessary relations with pre-existing species, what do these rudiments,
these apparent imperfections mean? There must be a cause for them; they
must be the necessary results of some great natural law. Now, if, as it
has been endeavoured to be shown, the great law which has regulated the
peopling of the earth with animal and vegetable life is, that every
change shall be gradual; that no new creature shall be formed widely
differing from anything before existing; that in this, as in everything
else in Nature, there shall be gradation and harmony,--then these
rudimentary organs are necessary, and are an essential part of the
system of Nature. Ere the higher Vertebrata were formed, for instance,
many steps were required, and many organs had to undergo modifications
from the rudimental condition in which only they had as yet existed. We
still see remaining an antitypal sketch of a wing adapted for flight in
the scaly flapper of the penguin, and limbs first concealed beneath the
skin, and then weakly protruding from it, were the necessary gradations
before others should be formed fully adapted for locomotion.[C] Many
more of these modifications should we behold, and more complete series
of them, had we a view of all the forms which have ceased to live. The
great gaps that exist between fishes, reptiles, birds, and mammals would
then, no doubt, be softened down by intermediate groups, and the whole
organic world would be seen to be an unbroken and harmonious system.
+--------------------------------------------------------------+
| [C] The theory of Natural Selection has now taught us that |
| these are not the steps by which limbs have been formed; and |
| that most rudimentary organs have been produced by abortion, |
| owing to disuse, as explained by Mr. Darwin. |
+--------------------------------------------------------------+
_Conclusion._
It has now been shown, though most briefly and imperfectly, how the law
that "_Every species has come into existence coincident both in time and
space with a pre-existing closely allied species_," connects together
and renders intelligible a vast number of independent and hitherto
unexplained facts. The natural system of arrangement of organic beings,
their geographical distribution, their geological sequence, the
phaenomena of representative and substituted groups in all their
modifications, and the most singular peculiarities of anatomical
structure, are all explained and illustrated by it, in perfect
accordance with the vast mass of facts which the researches of modern
naturalists have brought together, and, it is believed, not materially
opposed to any of them. It also claims a superiority over previous
hypotheses, on the ground that it not merely explains, but necessitates
what exists. Granted the law, and many of the most important facts in
Nature could not have been otherwise, but are almost as necessary
deductions from it, as are the elliptic orbits of the planets from the
law of gravitation.
II.
ON THE TENDENCY OF VARIETIES TO DEPART INDEFINITELY FROM THE ORIGINAL
TYPE.[D]
+--------------------------------------------------------------+
| [D] Written at Ternate, February, 1858; and published in the |
| Journal of the Proceedings of the Linnaean Society for |
| August, 1858. |
+--------------------------------------------------------------+
_Instability of Varieties supposed to prove the permanent distinctness
of Species._
One of the strongest arguments which have been adduced to prove the
original and permanent distinctness of species is, that _varieties_
produced in a state of domesticity are more or less unstable, and often
have a tendency, if left to themselves, to return to the normal form of
the parent species; and this instability is considered to be a
distinctive peculiarity of all varieties, even of those occurring among
wild animals in a state of nature, and to constitute a provision for
preserving unchanged the originally created distinct species.
In the absence or scarcity of facts and observations as to _varieties_
occurring among wild animals, this argument has had great weight with
naturalists, and has led to a very general and somewhat prejudiced
belief in the stability of species. Equally general, however, is the
belief in what are called "permanent or true varieties,"--races of
animals which continually propagate their like, but which differ so
slightly (although constantly) from some other race, that the one is
considered to be a _variety_ of the other. Which is the _variety_ and
which the original _species_, there is generally no means of
determining, except in those rare cases in which the one race has been
known to produce an offspring unlike itself and resembling the other.
This, however, would seem quite incompatible with the "permanent
invariability of species," but the difficulty is overcome by assuming
that such varieties have strict limits, and can never again vary further
from the original type, although they may return to it, which, from the
analogy of the domesticated animals, is considered to be highly
probable, if not certainly proved.
It will be observed that this argument rests entirely on the assumption,
that _varieties_ occurring in a state of nature are in all respects
analogous to or even identical with those of domestic animals, and are
governed by the same laws as regards their permanence or further
variation. But it is the object of the present paper to show that this
assumption is altogether false, that there is a general principle in
nature which will cause many _varieties_ to survive the parent species,
and to give rise to successive variations departing further and further
from the original type; and which also produces, in domesticated
animals, the tendency of varieties to return to the parent form.
_The Struggle for Existence._
The life of wild animals is a struggle for existence. The full exertion
of all their faculties and all their energies is required to preserve
their own existence and provide for that of their infant offspring. The
possibility of procuring food during the least favourable seasons, and
of escaping the attacks of their most dangerous enemies, are the primary
conditions which determine the existence both of individuals and of
entire species. These conditions will also determine the population of a
species; and by a careful consideration of all the circumstances we may
be enabled to comprehend, and in some degree to explain, what at first
sight appears so inexplicable--the excessive abundance of some species,
while others closely allied to them are very rare.
_The Law of Population of Species._
The general proportion that must obtain between certain groups of
animals is readily seen. Large animals cannot be so abundant as small
ones; the carnivora must be less numerous than the herbivora; eagles and
lions can never be so plentiful as pigeons and antelopes; and the wild
asses of the Tartarian deserts cannot equal in numbers the horses of the
more luxuriant prairies and pampas of America. The greater or less
fecundity of an animal is often considered to be one of the chief causes
of its abundance or scarcity; but a consideration of the facts will show
us that it really has little or nothing to do with the matter. Even the
least prolific of animals would increase rapidly if unchecked, whereas
it is evident that the animal population of the globe must be
stationary, or perhaps, through the influence of man, decreasing.
Fluctuations there may be; but permanent increase, except in restricted
localities, is almost impossible. For example, our own observation must
convince us that birds do not go on increasing every year in a
geometrical ratio, as they would do, were there not some powerful check
to their natural increase. Very few birds produce less than two young
ones each year, while many have six, eight, or ten; four will certainly
be below the average; and if we suppose that each pair produce young
only four times in their life, that will also be below the average,
supposing them not to die either by violence or want of food. Yet at
this rate how tremendous would be the increase in a few years from a
single pair! A simple calculation will show that in fifteen years each
pair of birds would have increased to nearly ten millions![E] whereas we
have no reason to believe that the number of the birds of any country
increases at all in fifteen or in one hundred and fifty years. With such
powers of increase the population must have reached its limits, and
have become stationary, in a very few years after the origin of each
species. It is evident, therefore, that each year an immense number of
birds must perish--as many in fact as are born; and as on the lowest
calculation the progeny are each year twice as numerous as their
parents, it follows that, whatever be the average number of individuals
existing in any given country, _twice that number must perish
annually_,--a striking result, but one which seems at least highly
probable, and is perhaps under rather than over the truth. It would
therefore appear that, as far as the continuance of the species and the
keeping up the average number of individuals are concerned, large broods
are superfluous. On the average all above _one_ become food for hawks
and kites, wild cats or weasels, or perish of cold and hunger as winter
comes on. This is strikingly proved by the case of particular species;
for we find that their abundance in individuals bears no relation
whatever to their fertility in producing offspring.
+--------------------------------------------------------------+
| [E] This is under estimated. The number would really amount |
| to more than two thousand millions! |
+--------------------------------------------------------------+
Perhaps the most remarkable instance of an immense bird population is
that of the passenger pigeon of the United States, which lays only one,
or at most two eggs, and is said to rear generally but one young one.
Why is this bird so extraordinarily abundant, while others producing two
or three times as many young are much less plentiful? The explanation is
not difficult. The food most congenial to this species, and on which it
thrives best, is abundantly distributed over a very extensive region,
offering such differences of soil and climate, that in one part or
another of the area the supply never fails. The bird is capable of a
very rapid and long-continued flight, so that it can pass without
fatigue over the whole of the district it inhabits, and as soon as the
supply of food begins to fail in one place is able to discover a fresh
feeding-ground. This example strikingly shows us that the procuring a
constant supply of wholesome food is almost the sole condition requisite
for ensuring the rapid increase of a given species, since neither the
limited fecundity, nor the unrestrained attacks of birds of prey and of
man are here sufficient to check it. In no other birds are these
peculiar circumstances so strikingly combined. Either their food is more
liable to failure, or they have not sufficient power of wing to search
for it over an extensive area, or during some season of the year it
becomes very scarce, and less wholesome substitutes have to be found;
and thus, though more fertile in offspring, they can never increase
beyond the supply of food in the least favourable seasons.
Many birds can only exist by migrating, when their food becomes scarce,
to regions possessing a milder, or at least a different climate, though,
as these migrating birds are seldom excessively abundant, it is evident
that the countries they visit are still deficient in a constant and
abundant supply of wholesome food. Those whose organization does not
permit them to migrate when their food becomes periodically scarce, can
never attain a large population. This is probably the reasons why
woodpeckers are scarce with us, while in the tropics they are among the
most abundant of solitary birds. Thus the house sparrow is more
abundant than the redbreast, because its food is more constant and
plentiful,--seeds of grasses being preserved during the winter, and our
farm-yards and stubble-fields furnishing an almost inexhaustible supply.
Why, as a general rule, are aquatic, and especially sea birds, very
numerous in individuals? Not because they are more prolific than others,
generally the contrary; but because their food never fails, the
sea-shores and river-banks daily swarming with a fresh supply of small
mollusca and crustacea. Exactly the same laws will apply to mammals.
Wild cats are prolific and have few enemies; why then are they never as
abundant as rabbits? The only intelligible answer is, that their supply
of food is more precarious. It appears evident, therefore, that so long
as a country remains physically unchanged, the numbers of its animal
population cannot materially increase. If one species does so, some
others requiring the same kind of food must diminish in proportion. The
numbers that die annually must be immense; and as the individual
existence of each animal depends upon itself, those that die must be
the weakest--the very young, the aged, and the diseased--while those
that prolong their existence can only be the most perfect in health and
vigour--those who are best able to obtain food regularly, and avoid
their numerous enemies. It is, as we commenced by remarking, "a struggle
for existence," in which the weakest and least perfectly organized must
always succumb.
_The Abundance or Rarity of a Species dependent upon its more or less
perfect Adaptation to the Conditions of Existence._
It seems evident that what takes place among the individuals of a
species must also occur among the several allied species of a
group,--viz., that those which are best adapted to obtain a regular
supply of food, and to defend themselves against the attacks of their
enemies and the vicissitudes of the seasons, must necessarily obtain and
preserve a superiority in population; while those species which from
some defect of power or organization are the least capable of
counteracting the vicissitudes of food-supply, &c., must diminish in
numbers, and, in extreme cases, become altogether extinct. Between these
extremes the species will present various degrees of capacity for
ensuring the means of preserving life; and it is thus we account for the
abundance or rarity of species. Our ignorance will generally prevent us
from accurately tracing the effects to their causes; but could we become
perfectly acquainted with the organization and habits of the various
species of animals, and could we measure the capacity of each for
performing the different acts necessary to its safety and existence
under all the varying circumstances by which it is surrounded, we might
be able even to calculate the proportionate abundance of individuals
which is the necessary result.
If now we have succeeded in establishing these two points--1st, _that
the animal population of a country is generally stationary, being kept
down by a periodical deficiency of food, and other checks_; and, 2nd,
_that the comparative abundance or scarcity of the individuals of the
several species is entirely due to their organization and resulting
habits, which, rendering it more difficult to procure a regular supply
of food and to provide for their personal safety in some cases than in
others, can only be balanced by a difference in the population which
have to exist in a given area_--we shall be in a condition to proceed to
the consideration of _varieties_, to which the preceding remarks have a
direct and very important application.
_Useful Variations will tend to Increase; useless or hurtful Variations
to Diminish._
Most or perhaps all the variations from the typical form of a species
must have some definite effect, however slight, on the habits or
capacities of the individuals. Even a change of colour might, by
rendering them more or less distinguishable, affect their safety; a
greater or less development of hair might modify their habits. More
important changes, such as an increase in the power or dimensions of the
limbs or any of the external organs, would more or less affect their
mode of procuring food or the range of country which they could inhabit.
It is also evident that most changes would affect, either favourably or
adversely, the powers of prolonging existence. An antelope with shorter
or weaker legs must necessarily suffer more from the attacks of the
feline carnivora; the passenger pigeon with less powerful wings would
sooner or later be affected in its powers of procuring a regular supply
of food; and in both cases the result must necessarily be a diminution
of the population of the modified species. If, on the other hand, any
species should produce a variety having slightly increased powers of
preserving existence, that variety must inevitably in time acquire a
superiority in numbers. These results must follow as surely as old age,
intemperance, or scarcity of food produce an increased mortality. In
both cases there may be many individual exceptions; but on the average
the rule will invariably be found to hold good. All varieties will
therefore fall into two classes--those which under the same conditions
would never reach the population of the parent species, and those which
would in time obtain and keep a numerical superiority. Now, let some
alteration of physical conditions occur in the district--a long period
of drought, a destruction of vegetation by locusts, the irruption of
some new carnivorous animal seeking "pastures new"--any change in fact
tending to render existence more difficult to the species in question,
and tasking its utmost powers to avoid complete extermination; it is
evident that, of all the individuals composing the species, those
forming the least numerous and most feebly organized variety would
suffer first, and, were the pressure severe, must soon become extinct.
The same causes continuing in action, the parent species would next
suffer, would gradually diminish in numbers, and with a recurrence of
similar unfavourable conditions might also become extinct. Tho superior
variety would then alone remain, and on a return to favourable
circumstances would rapidly increase in numbers and occupy the place of
the extinct species and variety.
_Superior Varieties will ultimately Extirpate the original Species._