dispersed throughout the whole system. We know, for instance, that
even a minute fragment of a leaf of a begonia will reproduce the whole
plant; and that if a fresh-water worm is chopped into small pieces,
each will reproduce the whole animal. Considering also the minuteness
of the gemmules and the permeability of all organic tissues, the
thorough dispersion of the gemmules is not surprising. That matter
may be readily transferred without the aid of vessels from part to
part of the body, we have a good instance in a case recorded by Sir J.
Paget of a lady, whose hair lost its color at each successive attack
of neuralgia and recovered it again in the course of a few days. With
plants, however, and probably with compound animals, such as corals,
the gemmules do not ordinarily spread from bud to bud, but are confined
to the parts developed from each separate bud; and of this fact no
explanation can be given.
TWO OBJECTIONS ANSWERED.
[Page 380.]
But we have here to encounter two objections which apply not only to
the regrowth of a part, or of a bisected individual, but to fissiparous
generation and budding. The first objection is that the part which is
reproduced is in the same stage of development as that of the being
which has been operated on or bisected; and in the case of buds,
that the new beings thus produced are in the same stage as that of
the budding parent. Thus a mature salamander, of which the tail has
been cut off, does not reproduce a larval tail; and a crab does not
reproduce a larval leg. In the case of budding it was shown in the
first part of this chapter that the new being thus produced does not
retrograde in development--that is, does not pass through those earlier
stages which the fertilized germ has to pass through. Nevertheless, the
organisms operated on or multiplying themselves by buds must, by our
hypothesis, include innumerable gemmules derived from every part or
unit of the earlier stages of development; and why do not such gemmules
reproduce the amputated part or the whole body at a corresponding early
stage of development?
The second objection, which has been insisted on by Delpino, is that
the tissues, for instance, of a mature salamander or crab, of which
a limb has been removed, are already differentiated and have passed
through their whole course of development; and how can such tissues in
accordance with our hypothesis attract and combine with the gemmules of
the part which is to be reproduced? In answer to these two objections
we must bear in mind the evidence which has been advanced, showing
that at least in a large number of cases the power of regrowth is a
localized faculty, acquired for the sake of repairing special injuries
to which each particular creature is liable; and, in the case of buds
or fissiparous generation, for the sake of quickly multiplying the
organism at a period of life when it can be supported in large numbers.
These considerations lead us to believe that in all such cases a stock
of nascent cells or of partially developed gemmules are retained for
this special purpose either locally or throughout the body, ready to
combine with the gemmules derived from the cells which come next in due
succession. If this be admitted, we have a sufficient answer to the
above two objections. Anyhow, pangenesis seems to throw a considerable
amount of light on the wonderful power of regrowth.
EFFECT OF MORBID ACTION.
[Page 392.]
We have as yet spoken only of the removal of parts, when not followed
by morbid action: but, when the operation is thus followed, it is
certain that the deficiency is sometimes inherited. In a former chapter
instances were given, as of a cow, the loss of whose horn was followed
by suppuration, and her calves were destitute of a horn on the same
side of their heads. But the evidence which admits of no doubt is
that given by Brown-Séquard with respect to Guinea-pigs, which, after
their sciatic nerves had been divided, gnawed off their own gangrenous
toes, and the toes of their offspring were deficient in at least
thirteen instances on the corresponding feet. The inheritance of the
lost part in several of these cases is all the more remarkable as only
one parent was affected; but we know that a congenital deficiency is
often transmitted from one parent alone--for instance, the offspring
of hornless cattle of either sex, when crossed with perfect animals,
are often hornless. How, then, in accordance with our hypothesis can
we account for mutilations being sometimes strongly inherited, if they
are followed by diseased action? The answer probably is that all the
gemmules of the mutilated or amputated part are gradually attracted
to the diseased surface during the reparative process, and are there
destroyed by the morbid action.
TRANSMISSION LIMITED.
[Page 396.]
The transmission of dormant gemmules during many successive generations
is hardly in itself more improbable, as previously remarked, than the
retention during many ages of rudimentary organs, or even only of
a tendency to the production of a rudiment; but there is no reason
to suppose that dormant gemmules can be transmitted and propagated
forever. Excessively minute and numerous as they are believed to be,
an infinite number, derived, during a long course of modification and
descent, from each unit of each progenitor, could not be supported or
nourished by the organism. But it does not seem improbable that certain
gemmules, under favorable conditions, should be retained and go on
multiplying for a much longer period than others. Finally, on the view
here given, we certainly gain some insight into the wonderful fact that
the child may depart from the type of both its parents, and resemble
its grandparents, or ancestors removed by many hundreds of generations.
* * * * *
[Page 398.]
The child, strictly speaking, does not grow into the man, but includes
germs which slowly and successively become developed and form the man.
In the child, as well as in the adult, each part generates the same
part. Inheritance must be looked at as merely a form of growth, like
the self-division of a lowly-organized unicellular organism. Reversion
depends on the transmission from the forefather to his descendants of
dormant gemmules, which occasionally become developed under certain
known or unknown conditions. Each animal and plant may be compared with
a bed of soil full of seeds, some of which soon germinate, some lie
dormant for a period, while others perish. When we hear it said that
a man carries in his constitution the seeds of an inherited disease,
there is much truth in the expression. No other attempt, as far as I
am aware, has been made, imperfect as this confessedly is, to connect
under one point of view these several grand classes of facts. An
organic being is a microcosm--a little universe, formed of a host of
self-propagating organisms, inconceivably minute and numerous as the
stars in heaven.
XV.
OBJECTIONS TO THE THEORY OF DESCENT WITH MODIFICATION CONSIDERED.
[Origin of
Species,
page 63.]
Several writers have misapprehended or objected to the term Natural
Selection. Some have even imagined that natural selection induces
variability, whereas it implies only the preservation of such
variations as arise and are beneficial to the being under its
conditions of life. No one objects to agriculturists speaking of the
potent effects of man’s selection; and in this case the individual
difference given by nature, which man for some object selects, must of
necessity first occur. Others have objected that the term selection
implies conscious choice in the animals which become modified; and it
has even been urged that, as plants have no volition, natural selection
is not applicable to them! In the literal sense of the word, no doubt,
natural selection is a false term; but who ever objected to chemists
speaking of the elective affinities of the various elements?--and yet
an acid can not strictly be said to elect the base with which it in
preference combines. It has been said that I speak of natural selection
as an active power or Deity; but who objects to an author speaking
of the attraction of gravity as ruling the movements of the planets?
Every one knows what is meant and is implied by such metaphorical
expressions; and they are almost necessary for brevity. So again it is
difficult to avoid personifying the word Nature; but I mean by Nature,
only the aggregate action and product of many natural laws, and by laws
the sequence of events as ascertained by us. With a little familiarity
such superficial objections will be forgotten.
MISREPRESENTATIONS CORRECTED.
[Origin of
Species,
page 421.]
As my conclusions have lately been much misrepresented, and it has
been stated that I attribute the modification of species exclusively
to natural selection, I may be permitted to remark that in the first
edition of this work, and subsequently, I placed in a most conspicuous
position--namely, at the close of the introduction--the following
words: “I am convinced that natural selection has been the main but not
the exclusive means of modification.” This has been of no avail. Great
is the power of steady misrepresentation; but the history of science
shows that fortunately this power does not long endure.
It can hardly be supposed that a false theory would explain, in so
satisfactory a manner as does the theory of natural selection, the
several large classes of facts above specified. It has recently been
objected that this is an unsafe method of arguing; but it is a method
used in judging of the common events of life, and has often been used
by the greatest natural philosophers. The undulatory theory of light
has thus been arrived at; and the belief in the revolution of the
earth on its own axis was until lately supported by hardly any direct
evidence. It is no valid objection that science as yet throws no
light on the far higher problem of the essence or origin of life. Who
can explain what is the essence of the attraction of gravity? No one
now objects to following out the results consequent on this unknown
element of attraction; notwithstanding that Leibnitz formerly accused
Newton of introducing “occult qualities and miracles into philosophy.”
I see no good reason why the views given in this volume should shock
the religious feelings of any one. It is satisfactory, as showing how
transient such impressions are, to remember that the greatest discovery
ever made by man, namely, the law of the attraction of gravity, was
also attacked by Leibnitz, “as subversive of natural, and inferentially
of revealed, religion.” A celebrated author and divine has written to
me that “he has gradually learned to see that it is just as noble a
conception of the Deity to believe that he created a few original forms
capable of self-development into other and needful forms, as to believe
that he required a fresh act of creation to supply the voids caused by
the action of his laws.”
LAPSE OF TIME AND EXTENT OF AREA.
[Origin of
Species,
page 82.]
The mere lapse of time by itself does nothing, either for or against
natural selection. I state this because it has been erroneously
asserted that the element of time has been assumed by me to play an
all-important part in modifying species, as if all the forms of life
were necessarily undergoing change through some innate law. Lapse of
time is only so far important, and its importance in this respect is
great, that it gives a better chance of beneficial variations arising,
and of their being selected, accumulated, and fixed. It likewise tends
to increase the direct action of the physical conditions of life, in
relation to the constitution of each organism.
If we turn to nature to test the truth of these remarks, and look
at any small isolated area, such as an oceanic island, although the
number of species inhabiting it is small, as we shall see in our
chapter on “Geographical Distribution,” yet of these species a very
large proportion are endemic--that is, have been produced there, and
nowhere else in the world. Hence an oceanic island at first sight
seems to have been highly favorable for the production of new species.
But we may thus deceive ourselves, for, to ascertain whether a small
isolated area, or a large open area like a continent, has been most
favorable for the production of new organic forms, we ought to make the
comparison within equal times; and this we are incapable of doing.
Although isolation is of great importance in the production of new
species, on the whole I am inclined to believe that largeness of area
is still more important, especially for the production of species which
shall prove capable of enduring for a long period, and of spreading
widely. Throughout a great and open area, not only will there be a
better chance of favorable variations, arising from the large number
of individuals of the same species there supported, but the conditions
of life are much more complex from the large number of already
existing species; and if some of these many species become modified
and improved, others will have to be improved in a corresponding
degree, or they will be exterminated. Each new form, also, as soon as
it has been much improved, will be able to spread over the open and
continuous area, and will thus come into competition with many other
forms. Moreover, great areas, though now continuous, will often, owing
to former oscillations of level, have existed in a broken condition; so
that the good effects of isolation will generally, to a certain extent,
have concurred. Finally, I conclude that, although small isolated
areas have been in some respects highly favorable for the production of
new species, yet that the course of modification will generally have
been more rapid on large areas; and what is more important, that the
new forms produced on large areas, which already have been victorious
over many competitors, will be those that will spread most widely, and
will give rise to the greatest number of new varieties and species.
They will thus play a more important part in the changing history of
the organic world.
WHY THE HIGHER FORMS HAVE NOT SUPPLANTED THE LOWER.
[Origin of
Species,
page 98.]
But it may be objected that if all organic beings thus tend to rise
in the scale, how is it that throughout the world a multitude of the
lowest forms still exist; and how is it that in each great class some
forms are far more highly developed than others? Why have not the
more highly developed forms everywhere supplanted and exterminated
the lower? Lamarck, who believed in an innate and inevitable tendency
toward perfection in all organic beings, seems to have felt this
difficulty so strongly that he was led to suppose that new and simple
forms are continually being produced by spontaneous generation. Science
has not as yet proved the truth of this belief, whatever the future
may reveal. On our theory the continued existence of lowly organisms
offers no difficulty; for natural selection, or the survival of the
fittest, does not necessarily include progressive development--it only
takes advantage of such variations as arise and are beneficial to each
creature under its complex relations of life. And it may be asked,
What advantage, as far as we can see, would it be to an infusorian
animalcule--to an intestinal worm--or even to an earth-worm, to be
highly organized? If it were no advantage, these forms would be left,
by natural selection, unimproved or but little improved, and might
remain for indefinite ages in their present lowly condition. And
geology tells us that some of the lowest forms, as the infusoria and
rhizopods, have remained for an enormous period in nearly their present
state. But to suppose that most of the many now existing low forms
have not in the least advanced since the first dawn of life would be
extremely rash; for every naturalist who has dissected some of the
beings now ranked as very low in the scale must have been struck with
their really wondrous and beautiful organization.
Nearly the same remarks are applicable if we look to the different
grades of organization within the same great group; for instance,
in the vertebrata, to the co-existence of mammals and fish--among
mammalia, to the co-existence of man and the ornithorhynchus--among
fishes, to the co-existence of the shark and the lancelet (Amphioxus),
which latter fish in the extreme simplicity of its structure approaches
the invertebrate classes. But mammals and fish hardly come into
competition with each other; the advancement of the whole class of
mammals, or of certain members in this class, to the highest grade
would not lead to their taking the place of fishes. Physiologists
believe that the brain must be bathed by warm blood to be highly
active, and this requires aërial respiration; so that warm-blooded
mammals when inhabiting the water lie under a disadvantage in having to
come continually to the surface to breathe. With fishes, members of the
shark family would not tend to supplant the lancelet; for the lancelet,
as I hear from Fritz Müller, has as sole companion and competitor on
the barren, sandy shore of South Brazil an anomalous annelid. The
three lowest orders of mammals, namely, marsupials, edentata, and
rodents, co-exist in South America in the same region with numerous
monkeys, and probably interfere little with each other. Although
organization, on the whole, may have advanced and be still advancing
throughout the world, yet the scale will always present many degrees
of perfection; for the high advancement of certain whole classes, or
of certain members of each class, does not at all necessarily lead to
the extinction of those groups with which they do not enter into close
competition. In some cases, as we shall hereafter see, lowly organized
forms appear to have been preserved to the present day, from inhabiting
confined or peculiar stations, where they have been subjected to less
severe competition, and where their scanty numbers have retarded the
chance of favorable variations arising.
Finally, I believe that many lowly organized forms now exist throughout
the world, from various causes. In some cases variations or individual
differences of a favorable nature may never have arisen for natural
selection to act on and accumulate. In no case, probably, has time
sufficed for the utmost possible amount of development. In some few
cases there has been what we must call retrogression of organization.
But the main cause lies in the fact that under very simple conditions
of life a high organization would be of no service--possibly would be
of actual disservice, as being of a more delicate nature, and more
liable to be put out of order and injured.
Looking to the first dawn of life, when all organic beings, as we may
believe, presented the simplest structure, how, it has been asked,
could the first steps in the advancement or differentiation of parts
have arisen?
* * * * *
[Page 100.]
As we have no facts to guide us, speculation on the subject is almost
useless. It is, however, an error to suppose that there would be no
struggle for existence, and, consequently, no natural selection, until
many forms had been produced: variations in a single species inhabiting
an isolated station might be beneficial, and thus the whole mass of
individuals might be modified, or two distinct forms might arise. But,
as I remarked toward the close of the Introduction, no one ought to
feel surprised at much remaining as yet unexplained on the origin of
species, if we make due allowance for our profound ignorance on the
mutual relations of the inhabitants of the world at the present time,
and still more so during past ages.
THE AMOUNT OF LIFE MUST HAVE A LIMIT.
[Origin of
Species,
page 101.]
What, then, checks an indefinite increase in the number of species?
The amount of life (I do not mean the number of specific forms)
supported on an area must have a limit, depending so largely as it
does on physical conditions; therefore, if an area be inhabited by
very many species, each or nearly each species will be represented by
few individuals; and such species will be liable to extermination from
accidental fluctuations in the nature of the seasons or in the number
of their enemies. The process of extermination in such cases would
be rapid, whereas the production of new species must always be slow.
Imagine the extreme case of as many species as individuals in England,
and the first severe winter or very dry summer would exterminate
thousands on thousands of species. Rare species, and each species
will become rare if the number of species in any country becomes
indefinitely increased, will, on the principle often explained,
present within a given period few favorable variations; consequently,
the process of giving birth to new specific forms would thus be
retarded. When any species becomes very rare, close interbreeding will
help to exterminate it; authors have thought that this comes into play
in accounting for the deterioration of the aurochs in Lithuania, of red
deer in Scotland, and of bears in Norway, etc. Lastly, and this I am
inclined to think is the most important element, a dominant species,
which has already beaten many competitors in its own home, will tend to
spread and supplant many others. Alph. de Candolle has shown that those
species which spread widely tend generally to spread _very_ widely;
consequently, they will tend to supplant and exterminate several
species in several areas, and thus check the inordinate increase of
specific forms throughout the world. Dr. Hooker has recently shown
that in the southeast corner of Australia, where, apparently, there
are many invaders from different quarters of the globe, the endemic
Australian species have been greatly reduced in number. How much weight
to attribute to these several considerations I will not pretend to
say; but conjointly they must limit in each country the tendency to an
indefinite augmentation of specific forms.
THE BROKEN BRANCHES OF THE TREE OF LIFE.
[Origin of
Species,
page 104.]
The affinities of all the beings of the same class have sometimes been
represented by a great tree. I believe this simile largely speaks the
truth. The green and budding twigs may represent existing species; and
those produced during former years may represent the long succession
of extinct species. At each period of growth all the growing twigs
have tried to branch out on all sides, and to overtop and kill the
surrounding twigs and branches, in the same manner as species and
groups of species have at all times overmastered other species in the
great battle for life. The limbs divided into great branches, and these
into lesser and lesser branches, were themselves once, when the tree
was young, budding twigs; and this connection of the former and present
buds by ramifying branches may well represent the classification of
all extinct and living species in groups subordinate to groups. Of the
many twigs which flourished when the tree was a mere bush, only two or
three, now grown into great branches, yet survive and bear the other
branches; so with the species which lived during long-past geological
periods, very few have left living and modified descendants. From
the first growth of the tree, many a limb and branch has decayed and
dropped off; and these fallen branches of various sizes may represent
those whole orders, families, and genera which have now no living
representatives, and which are known to us only in a fossil state. As
we here and there see a thin straggling branch springing from a fork
low down in a tree, and which by some chance has been favored and is
still alive on its summit, so we occasionally see an animal like the
ornithorhynchus or lepidosiren, which in some small degree connects by
its affinities two large branches of life, and which has apparently
been saved from fatal competition by having inhabited a protected
station. As buds give rise by growth to fresh buds, and these, if
vigorous, branch out and overtop on all sides many a feebler branch, so
by generation I believe it has been with the great Tree of Life, which
fills with its dead and broken branches the crust of the earth, and
covers the surface with its ever-branching and beautiful ramifications.
WHY WE DO NOT FIND TRANSITIONAL FORMS.
[Origin of
Species,
page 134.]
It may be urged that, when several closely-allied species inhabit
the same territory, we surely ought to find at the present time many
transitional forms.
* * * * *
[Page 137.]