Ian & Stuart's Australian Mac: Not for Sale/Another.not.for.sale (Australia).iso/hold me in your arms/ES Cyberama/Wallace-On The Tendency Of Vari

Text File | 1994-07-30 | 27KB | 437 lines

                                      1858
          ON THE TENDENCY OF VARIETIES TO DEPART INDEFINITELY FROM THE
                                 ORIGINAL TYPE
                            by Alfred Russel Wallace
                              written at Ternate,
                                 February, 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 of 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;
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! 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 and 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. 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 difference 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 reason 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 much 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 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. The
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.

  The variety would now have replaced the species, of which it would
be a more perfectly developed and more highly organized form. It would
be in all respects better adapted to secure its safety, and to prolong
its individual existence and that of the race. Such a variety could
not return to the original form; for that form is an inferior one, and
could never compete with it for existence. Granted, therefore, a
"tendency" to reproduce the original type of the species, still the
variety must ever remain preponderant in numbers, and under adverse
physical conditions again alone survive. But this new, improved, and
populous race might itself, in course of time, give rise to new
varieties, exhibiting several diverging modifications of form, any
of which, tending to increase the facilities for preserving existence,
must by the same general law, in their turn become predominant.
Here, then, we have progression and continued divergence deduced
from the general laws which regulate the existence of animals in a
state of nature, and from the undisputed fact that varieties do
frequently occur. It is not, however, contended that this result would
be invariable; a change of physical conditions in the district might
at times materially modify it, rendering the race which had been the
most capable of supporting existence under the former conditions now
the least so, and even causing the extinction of the newer and, for
a time, superior race, while the old or parent species and its first
inferior varieties continued to flourish. Variations in unimportant
parts might also occur, having no perceptible effect on the
life-preserving powers; and the varieties so furnished might run a
course parallel with the parent species, either giving rise to further
variations or returning to the former type. All we argue for is,
that certain varieties have a tendency to maintain their existence
longer than the original species, and this tendency must make itself
felt; for though the doctrine of chances or averages can never be
trusted to on a limited scale, yet, if applied to high numbers, the
results come nearer to what theory demands, and, as we approach to
an infinity of examples, become strictly accurate. Now the scale on
which nature works is so vast- the numbers of individuals and
periods of time with which she deals approach so near to infinity,
that any cause, however slight, and however liable to be veiled and
counteracted by accidental circumstances, must in the end produce
its full legitimate results.




       The Partial Reversion of Domesticated Varieties explained.

  Let us now turn to domesticated animals, and inquire how varieties
produced among them are affected by the principles here enunciated.
The essential difference in the condition of wild and domestic animals
is this,- that among the former, their well-being and very existence
depend upon the full exercise and healthy condition of all their
senses and physical powers, whereas, among the latter, these are
only partially exercised, and in some cases are absolutely unused. A
wild animal has to search, and often to labour, for every mouthful
of food- to exercise sight, hearing, and smell in seeking it, and in
avoiding dangers, in procuring shelter from the inclemency of the
seasons, and in providing for the subsistence and safety of its
offspring. There is no muscle of its body that is not called into
daily and hourly activity; there is no sense or faculty that is not
strengthened by continual exercise. The domestic animal, on the
other hand, has food provided for it, is sheltered, and often
confined, to guard it against the vicissitudes of the seasons, is
carefully secured from the attacks of its natural enemies, and
seldom even rears its young without human assistance. Half of its
senses and faculties are quite useless; and the other half are but
occasionally called into feeble exercise, while even its muscular
system is only irregularly called into action.
  Now when a variety of such an animal occurs, having increased
power or capacity in any organ or sense, such increase is totally
useless, is never called into action, and may even exist without the
animal ever becoming aware of it. In the wild animal, on the contrary,
all its faculties and powers being brought into full action for the
necessities of existence, any increase becomes immediately
available, is strengthened by exercise, and must even slightly
modify the food, the habits, and the whole economy of the race. It
creates as it were a new animal, one of superior powers, and which
will necessarily increase in numbers and outlive those inferior to it.
  Again, in the domesticated animal all variations have an equal
chance of continuance; and those which would decidedly render a wild
animal unable to compete with its fellows and continue its existence
are no disadvantage whatever in a state of domesticity. Our quickly
fattening pigs, short-legged sheep, pouter pigeons, and poodle dogs
could never have come into existence in a state of nature, because the
very first step towards such inferior forms would have led to the
rapid extinction of the race; still less could they now exist in
competition with their wild allies. The great speed but slight
endurance of the race horse, the unwielding strength of the
ploughman's team, would both be useless in a state of nature. If
turned wild on the pampas, such animals would probably soon become
extinct, or under favorable circumstances might each lose those
extreme qualities which would never be called into action, and in a
few generations would revert to a common type, which must be that in
which the various powers and faculties are so proportioned to each
other as to be best adapted to procure food and secure safety,- that
in which by the full exercise of every part of his organization the
animal can alone continue to live. Domestic varieties, when turned
wild, must return to something near the type of the original wild
stock, or become altogether extinct.*

  *That is, they will vary, and the variations which tend to adapt
them to the wild state, and therefore approximate them to wild
animals, will be preserved. Those individuals which do not vary
sufficiently will perish.


  Lamarck's Hypothesis very different from that now advanced.

  We see, then, that no inferences as to varieties in a state of
nature can be deduced from the observation of those occurring among
domestic animals. The two are so much opposed to each other in every
circumstance of their existence, that what applies to the one is
almost sure not to apply to the other. Domestic animals are
abnormal, irregular, artificial; they are subject to varieties which
never occur and never can occur in a state of nature; their very
existence depends altogether on human care: so far are many of them
removed from that just proportion of faculties, that true balance of
organization, by means of which alone an animal left to its own
resources can preserve its existence and continue its race.
  The hypothesis of Lamarck- that progressive changes in species
have been produced by the attempts of animals to increase the
development of their own organs, and thus modify their structure and
habits- has been repeatedly and easily refuted by all writers on the
subject of varieties and species, and it seems to have been considered
that when this was done the whole question has been finally settled;
but the view here developed renders such an hypothesis quite
unnecessary, by showing that similar results must be produced by the
action of principles constantly at work in nature. The powerful
retractile talons of the falcon- and the cat-tribes have not been
produced or increased by the volition of those animals; but among
the different varieties which occurred in the earlier and less
highly organized forms of these groups, those always survived
longest which had the greatest facilities for seizing their prey.
Neither did the giraffe acquire its long neck by desiring to reach the
foliage of the more lofty shrubs, and constantly stretching its neck
for the purpose, but because any varieties which occurred among its
antitypes with a longer neck than usual at once secured a fresh
range of pasture over the same ground as their shorter-necked
companions, and on the first scarcity of food were thereby enabled
to outlive them. Even the peculiar colours of many animals, especially
insects, so closely resembling the soil or the leaves or the trunks on
which they habitually reside, are explained on the same principle; for
though in the course of ages varieties of many tints may have
occurred, yet those races having colours best adapted to concealment
from their enemies would inevitably survive the longest. We have
also here an acting cause to account for that balance so often
observed in nature,- a deficiency in one set of organs always being
compensated by an increased development of some others- powerful wings
accompanying weak feet, or great velocity making up for the absence of
defensive weapons; for it has been shown that all varieties in which
an unbalanced deficiency occurred could not long continue their
existence. The action of this principle is exactly like that of the
centrifugal governor of the steam engine, which checks and corrects
any irregularities almost before they become evident; and in like
manner no unbalanced deficiency in the animal kingdom can ever reach
any conspicuous magnitude, because it would make itself felt at the
very first step, by rendering existence difficult and extinction
almost sure to follow. An origin such as is here advocated will also
agree with the peculiar character of the modifications of form and
structure which obtain in organized beings- the many lines of
divergence from a central type, the increasing efficiency and power of
a particular organ through a succession of allied species, and the
remarkable persistence of unimportant parts such as colour, texture of
plumage and hair, form of horns or crests, through a series of species
differing considerably in more essential characters. It also furnishes
us with a reason for that "more specialized structure" which Professor
Owen states to be a characteristic of recent compared with extinct
forms, and which would evidently be the result of the progressive
modification of any organ applied to a special purpose in the animal
economy.


                              Conclusion.

  We believe we have now shown that there is a tendency in nature to
the continued progression of certain classes of varieties further
and further from the original type- a progression to which there
appears no reason to assign any definite limits- and that the same
principle which produces this result in a state of nature will also
explain why domestic varieties have a tendency to revert to the
original type. This progression, by minute steps, in various
directions, but always checked and balanced by the necessary
conditions, subject to which alone existence can be preserved, may, it
is believed, be followed out so as to agree with all the phenomena
presented by organized beings, their extinction and succession in past
ages, and all the extraordinary modifications of form, instinct, and
habits which they exhibit.
                               Written at Ternate, February, 1858


                             -THE END-