| Comparison with 1861 |
|
group, the later and more highly perfected sub-groups, from branching out and seizing on many new places in the polity of Nature, will constantly tend to supplant and destroy the earlier and less improved sub-groups. Small and broken groups and sub-groups will finally
..| ..... 1860 1861 1866 1869 1872 | | tend to 1859 |
| now have living and 1861 1866 |
| may now have living and 1859 1860 |
| have left 1869 1872 |
|
|
On
the
degree
to
which
Organisation
tends
to
advance.
—
|
| Natural selection
acts, as we have seen, exclusively
by the preservation and accumulation of variations, which are
beneficial under the organic and inorganic conditions of life
to which each creature is
at each successive period exposed.
The ultimate result will be
that each creature will
tend
to become more and more improved in relation to its
conditions
of life.
This improvement will, I think,
inevitably lead
to the gradual advancement of the organisation of the greater number of living beings throughout the world. But here we enter on a very intricate subject, for naturalists have not defined to each other's
satisfaction what is meant by an advance in organisation. Amongst the vertebrata the degree of intellect and an approach in structure to man clearly come into play. It might be thought that the amount of change which the various parts and organs undergo
in their development from the embryo to maturity would suffice as a standard of comparison; but there are cases, as with certain parasitic crustaceans, in which several parts of the structure become less perfect, so that the mature animal cannot be called higher than its larva. Von Baer's
standard seems the most widely applicable and the best, namely, the amount of differentiation of the different
parts (in
the adult state,
as I should be inclined to add)
and
their specialisation for different functions; or, as Milne Edwards would express it, the completeness of the division of physiological labour. But we shall see how obscure a
subject this
is if we look, for instance, to fish,
amongst which some naturalists rank those as highest which, like the sharks, approach nearest to reptiles;
whilst other naturalists rank the common bony or teleostean fishes as the highest, inasmuch as they are most strictly fish-like, and differ most from the other vertebrate classes. Still
more plainly we see
the obscurity of the subject by turning to plants, with
which the standard of intellect is of course quite excluded; and here some botanists rank those plants as highest which have every organ, as sepals, petals, stamens, and pistils, fully developed in each flower; whereas other botanists, probably with more truth, look at the plants which have their several organs much modified and somewhat
reduced in number as being of
the highest
rank.
|
|
| If we look at the
differentiation and specialisation of the several organs of
each being when adult (and this will include the advancement of the brain for intellectual purposes)
as the best standard of highness of organisation,
natural selection clearly leads towards highness;
for all physiologists admit that the specialisation of organs, inasmuch as they perform in this state
their functions better, is an advantage to each being; and hence the accumulation of variations tending towards specialisation is within the scope of natural selection. On the other hand, we can see, bearing in mind that all organic beings are striving to increase at a high ratio and to seize on every ill-occupied
place in the economy of nature, that it is quite possible for natural selection gradually to fit an organic
being to a situation in which several organs would be superfluous and
useless: in such cases there might
be retrogression in the scale of organisation. Whether organisation on the whole has actually advanced from the remotest geological periods to the present day will be more conveniently discussed in our chapter on Geological Succession. |
|
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 towards perfection in all organic beings, seems to have felt this difficulty so strongly, that he was led to suppose that new and simple forms were
continually being produced by spontaneous generation. I need hardly say that Science in her present state does not countenance the belief that living creatures are now ever produced from inorganic matter. ↑| 1 blocks not present in 1859 1860 1861 1866; present in 1869 1872 | | Science
has not as yet proved the truth of this
belief, whatever the future may reveal.
|
|
|
| Nearly the same remarks are applicable if we look to the great existing differences in the
grades of organisation which occur within almost every
great group; for instance, to
the co-existence
of mammals and fish in the vertebrata,—
to the co-existence of man and the ornithorhynchus
amongst mammalia— or
of the shark and amphioxus,
which latter fish in the extreme simplicity of its structure closely
approaches the invertebrate classes. But mammals and fish hardly come into competition with each other; the advancement of certain mammals or of the whole class
to the highest grade of organisation
would not lead to their taking the place of, and thus exterminating,
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 live
under some
disadvantages
compared with fishes.
In this latter class, members of the shark family would not, it is probable, tend to supplant the amphioxus;
the struggle for existence in the case of the amphioxus
apparently will lie with members of the invertebrate classes.
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 organisation, on the whole, may have advanced and be advancing
throughout the world, yet the scale will still
present all
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 organised forms seem
to have been preserved to the present day
from inhabiting peculiar
or isolated
stations, where they have been subjected to less severe competition, and where they have existed in
scanty numbers,
which, as already explained,
retards
the chance of favourable
variations arising. |
|
| Finally, I believe that lowly
organised forms now exist in numbers
throughout the world, and in nearly every class,
from various causes. In some cases favourable variations
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 may have
been what we must call retrogression of organisation. But the main cause lies in the circumstance
that under very simple conditions of life a high organisation 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 thus
injured. |
|
A difficulty, diametrically opposite to this which we have just been considering, has been advanced, namely,
looking
to the dawn
of life, when all organic beings, as we may imagine,
presented the simplest structure, how
could
the first steps in advancement or in the differentiation and specialisation
of parts have arisen? I can make no sufficient answer; and can only say that as we have no facts to guide us, all speculation on the subject would be baseless and 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
through their preservation either
the whole mass of individuals might become
modified, or two distinct forms might arise. But
I must recur to what was stated
towards the close of the Introduction, where I say that
no one ought to feel surprise at much remaining as yet unexplained on the origin of species, if due allowance be made
for our profound ignorance on the mutual relations of the inhabitants of the world during
the many past epochs in its history.
↑| Subtitle not present 1859 1860 1861 1872 |
|
Various
Objections
considered.
1866 1869 |
|
| I will here notice a few miscellaneous objections which have been advanced against my views, as some of the previous discussions may perhaps thus be made clearer. It has been argued that
as none of the animals and plants of Egypt, of which we know anything, have changed during the last 3000 years, so probably none have been modified in other
parts
of the world. The many animals which have remained unchanged since the commencement of the glacial period would have been an incomparably stronger case, for these have been exposed to great changes of climate and have migrated over great distances; whereas, in Egypt, during the last 3000 years, the conditions of life, as far as we know, have remained absolutely uniform. The fact of little or no modification having been effected since the glacial period would be of some avail against those who believe in the existence of
an innate and necessary law of development, but is powerless against the doctrine of natural selection,
which only implies
that variations occasionally occurring in single species are under favourable conditions preserved.
As Mr. Fawcett has well asked, what would be thought of a man who argued that
because he could show that Mont Blanc and the other Alpine peaks had exactly the same height 3000 years ago as at present, consequently that these mountains had never been slowly upraised, and that the height of other mountains in other parts of the world had not recently been increased by slow degrees? |
|
| It has been objected, if natural selection be so powerful,
why
has not this or that organ been recently
modified and improved? Why has not the proboscis of the hive-bee been lengthened so as to reach the nectar in
the flower of the
red-clover? Why has not the ostrich acquired the power of flight? But granting that these organs have happened to vary
in the right direction,
granting that there has been time sufficient for the slow work of natural selection, checked as it
will be by intercrossing and the tendency to reversion, who will pretend that he knows the natural history
of any one organic being sufficiently well to say whether
any particular change would be
to
its advantage? Can we feel sure that a long proboscis would not be a disadvantage to the hive-bee in sucking the innumerable small flowers which it frequents? Can we feel sure that a long proboscis would not, by correlation
of growth,
almost necessarily give increased size to other parts of the mouth, perhaps interfering with the delicate cell-constructing work? In the case of the ostrich
a moment's
reflection will show that
an enormous supply of food would be necessary in
this bird of the desert,
to supply force to
move its huge body through the air. But such ill-considered objections are hardly worth notice. |
|
| The celebrated palæontologist, Professor
Bronn, in
his German translation of this work, has advanced various good objections to my views, and other remarks in its favour.
Of the objections, some seem to me unimportant, some few are owing to misapprehension, and some are incidentally noticed in various parts of this volume. On the erroneous supposition that all the species of a region are believed by me to be changing at the same time, he justly asks how it is that all the forms of life do not present a fluctuating and inextricably confused body? but it is sufficient for us if some few forms at any one time are variable, and few will dispute that this is the case. He asks, how
can it be
on the principle of natural selection
that
a variety should
live in abundance side by side with the parent species;
for the
variety during its formation is supposed to have
supplanted
the intermediate forms between itself and the parent species,
and yet it has not supplanted the parent species itself,
for both are supposed now to live side by side? If the variety and parent species
have become fitted to
slightly different habits of life, they might live together; though
in the case of animals which freely cross and move about, varieties seem to be almost always confined to distinct localities. But is it the case that varieties of plants and of the lower animals are often found in abundance side by side with the parent forms? Laying aside the
polymorphic species
in which the
innumerable variations that occur
seem neither advantageous nor disadvantageous to the species, and have not been fixed; laying aside also temporary variations, such as albinism, &c., my impression is that varieties and the supposed parent species
are generally found,
inhabiting either
distinct stations, high land or low land, dry or moist districts, or distinct regions. |
|
| Again, Professor Bronn truly remarks, that distinct species do not differ from each other in single characters alone, but in many; and he asks, how it comes that natural selection should always have simultaneously affected many parts of the organisation? Probably the whole amount of difference has not been simultaneously effected; and the unknown laws of correlation will certainly account for, but not strictly explain, much simultaneous modification. Anyhow, we see in our domestic varieties the very same fact: though our domestic
races may differ much in some one organ from the other races of the same species, yet the other
parts of the organisation will always be found in some degree different. Professor Bronn likewise asks with striking effect how, for instance in the mouse or hare genus, natural selection will
account for the several species (descended, I may remark, from a parent of unknown character) having longer or shorter tails, longer or shorter ears, and fur of different colours; how will
it account for one species of plant having pointed and another species obtuse
leaves? I can give no definite answer to such questions; but I might ask in return, were these differences, on the doctrine of independent creation, formed for no purpose? If of use, or if due to correlation of growth, they could assuredly be formed through the natural preservation of such useful or correlated variations. I believe in the doctrine of descent with modification, notwithstanding that this or that particular change of structure cannot be accounted for, because this doctrine groups together and explains, as we shall see in the latter
chapters, many general phenomena of nature. |
|
| A distinguished botanist, Mr. H. C. Watson, believes that I have overrated the importance of the principle of divergence of character (in which, however, he apparently believes), and that convergence of character, as it may be called, has likewise played a part. This is an intricate subject which need not be here discussed. I will only say
that if two species of two closely allied genera produced a number of new and divergent species, I can believe that these new forms might sometimes approach each other so closely that they would for convenience
sake be classed in the same new genus, and thus two genera would converge into one; but from the strength of the principle of inheritance, it seems hardly credible that the two groups of new species would not at least form two sections of the supposed new single genus.
|
|
| Mr. Watson has also objected that the continued action of natural selection
with
divergence of character
will
tend to make an indefinite number of specific forms. As far as mere inorganic conditions are concerned, it seems probable that a sufficient number of species would soon become adapted to all considerable diversities of heat, moisture, &c.; but I fully admit that the mutual relations of organic beings are more important; and as the number of species in any
country
goes on increasing, the organic conditions of life will
become more and more complex. Consequently there seems at first sight to be
no limit to the amount of profitable diversification of structure, and therefore no limit to the number of species which might be produced. We do not know that even the most prolific area is fully stocked with specific forms: at the Cape of Good Hope and in Australia, which support such an astonishing number of species, many European plants have become naturalised. But geology shows us, at least within the whole immense
tertiary period,
that
the number of species of shells, and,
probably,
of mammals, has not greatly or at all increased. 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 any
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 these
cases will
be rapid, whereas the production of new species will
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 become in any country
indefinitely increased, will, on the principle often explained, present within a given period few favourable
variations; consequently, the process of giving birth to new specific forms will
thus be retarded. When any species becomes very rare, close interbreeding will help in
exterminating
it; at least
authors have thought that this comes into play in accounting for the deterioration of Aurochs
in Lithuania, of Red Deer in Scotland, and of Bears
in Norway, &C.
As far as animals are concerned, some species are closely adapted to prey on some one other being; but if this other being had been rare, it would not have been any advantage to the animal to have been produced in close relation to its prey: therefore, it would not have been produced by natural selection.
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; and, consequently, they will tend to
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 S. E.
corner of Australia, where, apparently, there are many invaders from different quarters of the world,
the endemic Australian species have been greatly reduced in number. How much weight to attribute to these several considerations I do
not pretend to assign;
but conjointly they must limit in each country the tendency to an indefinite augmentation of specific forms. |
Summary
of
Chapter.
—
|
Chapter.
—
1861 | |
Chapter
.—
1859 1860 | |
Chapter.
1866 1869 1872 |
|
If, | If, 1861 1866 | | If 1859 1860 1869 1872 |
|
