The Outline of Science, Vol. 1 (of 4)

J >> J. Arthur Thomson >> The Outline of Science, Vol. 1 (of 4)

Pages:
1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26

On another tack, however, there probably evolved a series of simple
predatory creatures, not able to build up organic matter from air,
water, and salts, but devouring their neighbours. These units were not
closed in with cellulose, but remained naked, with their living matter
or protoplasm flowing out in changeful processes, such as we see in the
Amoebae in the ditch or in our own white blood corpuscles and other
amoeboid cells. These were the originators of the animal kingdom. Thus
from very simple Protists the first animals and the first plants may
have arisen. All were still very minute, and it is worth remembering
that had there been any scientific spectator after our kind upon the
earth during these long ages, he would have lamented the entire absence
of life, although the seas were teeming. The simplest forms of life and
the protoplasm which Huxley called the physical basis of life will be
dealt with in the chapter on Biology in a later section of this work.

FIRST GREAT STEPS IN EVOLUTION

THE FIRST PLANTS--THE FIRST ANIMALS--BEGINNINGS OF BODIES--EVOLUTION OF
SEX--BEGINNING OF NATURAL DEATH

Sec. 1

The Contrast between Plants and Animals

However it may have come about, there is no doubt at all that one of the
first great steps in Organic Evolution was the forking of the
genealogical tree into Plants and Animals--the most important parting of
the ways in the whole history of Nature.

Typical plants have chlorophyll; they are able to feed at a low chemical
level on air, water, and salts, using the energy of the sunlight in
their photosynthesis. They have their cells boxed in by cellulose walls,
so that their opportunities for motility are greatly restricted. They
manufacture much more nutritive material than they need, and live far
below their income. They have no ready way of getting rid of any
nitrogenous waste matter that they may form, and this probably helps to
keep them sluggish.

Animals, on the other hand, feed at a high chemical level, on the
carbohydrates (e.g. starch and sugar), fats, and proteins (e.g. gluten,
albumin, casein) which are manufactured by other animals, or to begin
with, by plants. Their cells have not cellulose walls, nor in most cases
much wall of any kind, and motility in the majority is unrestricted.
Animals live much more nearly up to their income. If we could make for
an animal and a plant of equal weight two fractions showing the ratio of
the upbuilding, constructive, chemical processes to the down-breaking,
disruptive, chemical processes that go on in their respective bodies,
the ratio for the plant would be much greater than the corresponding
ratio for the animal. In other words, animals take the munitions which
plants laboriously manufacture and explode them in locomotion and
work; and the entire system of animate nature depends upon the
photosynthesis that goes on in green plants.

[Illustration: _From the Smithsonian Report, 1917_

A PIECE OF A REEF-BUILDING CORAL, BUILT UP BY A LARGE COLONY OF SMALL
SEA-ANEMONE-LIKE POLYPS, EACH OF WHICH FORMS FROM THE SALTS OF THE SEA A
SKELETON OR SHELL OF LIME

The wonderful mass of corals, which are very beautiful, are the skeleton
remains of hundreds of these little creatures.]

[Illustration: _Photo: J. J. Ward, F.E.S._

THE INSET CIRCLE SHOWS A GROUP OF CHALK-FORMING ANIMALS, OR
FORAMINIFERA, EACH ABOUT THE SIZE OF A VERY SMALL PIN'S HEAD

They form a great part of the chalk cliffs of Dover and similar deposits
which have been raised from the floor of an ancient sea.

THE ENORMOUSLY ENLARGED ILLUSTRATION IS THAT OF A COMMON FORAMINIFER
(POLYSTOMELLA) SHOWING THE SHELL IN THE CENTRE AND THE OUTFLOWING
NETWORK OF LIVING MATTER, ALONG WHICH GRANULES ARE CONTINUALLY
TRAVELLING, AND BY WHICH FOOD PARTICLES ARE ENTANGLED AND DRAWN IN

_Reproduced by permission of the Natural History Museum_ (_after Max
Schultze_).]

As the result of much more explosive life, animals have to deal with
much in the way of nitrogenous waste products, the ashes of the living
fire, but these are usually got rid of very effectively, e.g. in the
kidney filters, and do not clog the system by being deposited as
crystals and the like, as happens in plants. Sluggish animals like
sea-squirts which have no kidneys are exceptions that prove the rule,
and it need hardly be said that the statements that have been made in
regard to the contrasts between plants and animals are general
statements. There is often a good deal of the plant about the animal, as
in sedentary sponges, zoophytes, corals, and sea-squirts, and there is
often a little of the animal about the plant, as we see in the movements
of all shoots and roots and leaves, and occasionally in the parts of the
flower. But the important fact is that on the early forking of the
genealogical tree, i.e. the divergence of plants and animals, there
depended and depends all the higher life of the animal kingdom, not to
speak of mankind. The continuance of civilisation, the upkeep of the
human and animal population of the globe, and even the supply of oxygen
to the air we breathe, depend on the silent laboratories of the green
leaves, which are able with the help of the sunlight to use carbonic
acid, water, and salts to build up the bread of life.

Sec. 2

The Beginnings of Land Plants

It is highly probable that for long ages the waters covered the earth,
and that all the primeval vegetation consisted of simple Flagellates in
the universal Open Sea. But contraction of the earth's crust brought
about elevations and depressions of the sea-floor, and in places the
solid substratum was brought near enough the surface to allow the
floating plants to begin to settle down without getting out of the
light. This is how Professor Church pictures the beginning of a fixed
vegetation--a very momentous step in evolution. It was perhaps among
this early vegetation that animals had their first successes. As the
floor of the sea in these shallow areas was raised higher and higher
there was a beginning of dry land. The sedentary plants already spoken
of were the ancestors of the shore seaweeds, and there is no doubt that
when we go down at the lowest tide and wade cautiously out among the
jungle of vegetation only exposed on such occasions we are getting a
glimpse of very ancient days. _This_ is the forest primeval.

The Protozoa

Animals below the level of zoophytes and sponges are called Protozoa.
The word obviously means "First Animals," but all that we can say is
that the very simplest of them may give us some hint of the simplicity
of the original first animals. For it is quite certain that the vast
majority of the Protozoa to-day are far too complicated to be thought of
as primitive. Though most of them are microscopic, each is an animal
complete in itself, with the same fundamental bodily attributes as are
manifested in ourselves. They differ from animals of higher degree in
not being built up of the unit areas or corpuscles called cells. They
have no cells, no tissues, no organs, in the ordinary acceptation of
these words, but many of them show a great complexity of internal
structure, far exceeding that of the ordinary cells that build up the
tissues of higher animals. They are complete living creatures which have
not gone in for body-making.

In the dim and distant past there was a time when the only animals were
of the nature of Protozoa, and it is safe to say that one of the great
steps in evolution was the establishment of three great types of
Protozoa: (_a_) Some were very active, the Infusorians, like the slipper
animalcule, the night-light (Noctiluca), which makes the seas
phosphorescent at night, and the deadly Trypanosome, which causes
Sleeping Sickness. (_b_) Others were very sluggish, the parasitic
Sporozoa, like the malaria organism which the mosquito introduces into
man's body. (_c_) Others were neither very active nor very passive, the
Rhizopods, with out-flowing processes of living matter. This amoeboid
line of evolution has been very successful; it is represented by the
Rhizopods, such as Amoebae and the chalk-forming Foraminifera and the
exquisitely beautiful flint-shelled Radiolarians of the open sea. They
have their counterparts in the amoeboid cells of most multicellular
animals, such as the phagocytes which migrate about in the body,
engulfing and digesting intruding bacteria, serving as sappers and
miners when something has to be broken down and built up again, and
performing other useful offices.

Sec. 3

The Making of a Body

The great naturalist Louis Agassiz once said that the biggest gulf in
Organic Nature was that between the unicellular and the multicellular
animals (Protozoa and Metazoa). But the gulf was bridged very long ago
when sponges, stinging animals, and simple worms were evolved, and
showed, for the first time, a "body." What would one not give to be able
to account for the making of a body, one of the great steps in
evolution! No one knows, but the problem is not altogether obscure.

When an ordinary Protozoon or one-celled animal divides into two or
more, which is its way of multiplying, the daughter-units thus formed
float apart and live independent lives. But there are a few Protozoa in
which the daughter-units are not quite separated off from one another,
but remain coherent. Thus Volvox, a beautiful green ball, found in some
canals and the like, is a colony of a thousand or even ten thousand
cells. It has almost formed a body! But in this "colony-making"
Protozoon, and in others like it, the component cells are all of one
kind, whereas in true multicellular animals there are different kinds
of cells, showing division of labour. There are some other Protozoa in
which the nucleus or kernel divides into many nuclei within the cell.
This is seen in the Giant Amoeba (Pelomyxa), sometimes found in
duck-ponds, or the beautiful Opalina, which always lives in the hind
part of the frog's food-canal. If a portion of the living matter of
these Protozoa should gather round each of the nuclei, then _that would
be the beginning of a body_. It would be still nearer the beginning of a
body if division of labour set in, and if there was a setting apart of
egg-cells and sperm-cells distinct from body-cells.

It was possibly in some such way that animals and plants with a body
were first evolved. Two points should be noticed, that body-making is
not essentially a matter of size, though it made large size possible.
For the body of a many-celled Wheel Animalcule or Rotifer is no bigger
than many a Protozoon. Yet the Rotifer--we are thinking of Hydatina--has
nine hundred odd cells, whereas the Protozoon has only one, except in
forms like Volvox. Secondly, it is a luminous fact that _every
many-celled animal from sponge to man that multiplies in the ordinary
way begins at the beginning again as a "single cell,"_ the fertilised
egg-cell. It is, of course, not an ordinary single cell that develops
into an earthworm or a butterfly, an eagle, or a man; it is a cell in
which a rich inheritance, the fruition of ages, is somehow condensed;
but it is interesting to bear in mind the elementary fact that every
many-celled creature, reproduced in the ordinary way and not by budding
or the like, starts as a fertilised egg-cell. The coherence of the
daughter-cells into which the fertilised egg-cell divides is a
reminiscence, as it were, of the primeval coherence of daughter-units
that made the first body possible.

The Beginning of Sexual Reproduction

A freshwater Hydra, growing on the duckweed usually multiplies by
budding. It forms daughter-buds, living images of itself; a check comes
to nutrition and these daughter-buds go free. A big sea-anemone may
divide in two or more parts, which become separate animals. This is
asexual reproduction, which means that the multiplication takes place by
dividing into two or many portions, and not by liberating egg-cells and
sperm-cells. Among animals as among plants, asexual reproduction is very
common. But it has great disadvantages, for it is apt to be
physiologically expensive, and it is beset with difficulties when the
body shows great division of labour, and is very intimately bound into
unity. Thus, no one can think of a bee or a bird multiplying by division
or by budding. Moreover, if the body of the parent has suffered from
injury or deterioration, the result of this is bound to be handed on to
the next generation if asexual reproduction is the only method.

[Illustration: _Photos: J. J. Ward, F.E.S._

A PLANT-LIKE ANIMAL, OR ZOOPHYTE, CALLED OBELIA

Consisting of a colony of small polyps, whose stinging tentacles are
well shown greatly enlarged in the lower photograph.]

[Illustration: _Reproduced by permission of "The Quart. Journ. Mic.
Sci."_

TRYPANOSOMA GAMBIENSE

(Very highly magnified.)

The microscopic animal Trypanosome, which causes Sleeping Sickness. The
study of these organisms has of late years acquired an immense
importance on account of the widespread and dangerous maladies to which
some of them give rise. It lives in the blood of man, who is infected by
the bite of a Tse-tse fly which carries the parasite from some other
host.]

[Illustration: VOLVOX

The Volvox is found in some canals and the like. It is one of the first
animals to suggest the beginning of a body. It is a colony of a thousand
or even ten thousand cells, but they are all cells of one kind. In
_multicellular_ animals the cells are of _different_ kinds with
different functions. Each of the ordinary cells (marked 5) has two
lashes or flagella. Daughter colonies inside the Parent colony are being
formed at 3, 4, and 2. The development of germ-cells is shown at 1.]

[Illustration: PROTEROSPONGIA

One of the simplest multicellular animals, illustrating the beginning of
a body. There is a setting apart of egg-cells and sperm-cells, distinct
from body-cells; the collared lashed cells on the margin are different
in kind from those farther in. Thus, as in indubitable multicellular
animals, division of labour has begun.]

Splitting into two or many parts was the old-fashioned way of
multiplying, but one of the great steps in evolution was the discovery
of a better method, namely, sexual reproduction. The gist of this is
simply that during the process of body-building (by the development of
the fertilised egg-cell) certain units, _the germ-cells_, do not share
in forming ordinary tissues or organs, but remain apart, continuing the
full inheritance which was condensed in the fertilised egg-cell. _These
cells kept by themselves are the originators of the future reproductive
cells of the mature animal_; they give rise to the egg-cells and the
sperm-cells.

The advantages of this method are great. (1) The new generation is
started less expensively, for it is easier to shed germ-cells into the
cradle of the water than to separate off half of the body. (2) It is
possible to start a great many new lives at once, and this may be of
vital importance when the struggle for existence is very keen, and when
parental care is impossible. (3) The germ-cells are little likely to be
prejudicially affected by disadvantageous dints impressed on the body of
the parent--little likely unless the dints have peculiarly penetrating
consequences, as in the case of poisons. (4) A further advantage is
implied in the formation of two kinds of germ-cells--the ovum or
egg-cell, with a considerable amount of building material and often with
a legacy of nutritive yolk; the spermatozoon or sperm-cell, adapted to
move in fluids and to find the ovum from a distance, thus securing
change-provoking cross-fertilisation.

Sec. 4

The Evolution of Sex

Another of the great steps in organic evolution was the differentiation
of two different physiological types, the male or sperm-producer and the
female or egg-producer. It seems to be a deep-seated difference in
constitution, which leads one egg to develop into a male, and another,
lying beside it in the nest, into a female. In the case of pigeons it
seems almost certain, from the work of Professor Oscar Riddle, that
there are two kinds of egg, a male-producing egg and a female-producing
egg, which differ in their yolk-forming and other physiological
characters.

In sea-urchins we often find two creatures superficially
indistinguishable, but the one is a female with large ovaries and the
other is a male with equally large testes. Here the physiological
difference does not affect the body as a whole, but the reproductive
organs or gonads only, though more intimate physiology would doubtless
discover differences in the blood or in the chemical routine
(metabolism). In a large number of cases, however, there are marked
superficial differences between the sexes, and everyone is familiar with
such contrasts as peacock and peahen, stag and hind. In such cases the
physiological difference between the sperm-producer and the
ovum-producer, for this is the essential difference, saturates through
the body and expresses itself in masculine and feminine structures and
modes of behaviour. The expression of the masculine and feminine
characters is in some cases under the control of hormones or chemical
messengers which are carried by the blood from the reproductive organs
throughout the body, and pull the trigger which brings about the
development of an antler or a wattle or a decorative plume or a capacity
for vocal and saltatory display. In some cases it is certain that the
female carries in a latent state the masculine features, but these are
kept from expressing themselves by other chemical messengers from the
ovary. Of these chemical messengers more must be said later on.

Recent research has shown that while the difference between male and
female is very deep-rooted, corresponding to a difference in gearing, it
is not always clear-cut. Thus a hen-pigeon may be very masculine, and a
cock-pigeon very feminine. The difference is in degree, not in kind.

Sec. 5

What is the meaning of the universal or almost universal inevitableness
of death? A Sequoia or "Big Tree" of California has been known to live
for over two thousand years, but eventually it died. A centenarian
tortoise has been known, and a sea-anemone sixty years of age; but
eventually they die. What is the meaning of this apparently inevitable
stoppage of bodily life?

The Beginning of Natural Death

There are three chief kinds of death, (_a_) The great majority of
animals come to a violent end, being devoured by others or killed by
sudden and extreme changes in their surroundings. (_b_) When an animal
enters a new habitat, or comes into new associations with other
organisms, it may be invaded by a microbe or by some larger parasite to
which it is unaccustomed and to which it can offer no resistance. With
many parasites a "live-and-let-live" compromise is arrived at, but new
parasites are apt to be fatal, as man knows to his cost when he is
bitten by a tse-tse fly which infects him with the microscopic animal (a
Trypanosome) that causes Sleeping Sickness. In many animals the
parasites are not troublesome as long as the host is vigorous, but if
the host is out of condition the parasites may get the upper hand, as in
the so-called "grouse disease," and become fatal. (_c_) But besides
violent death and microbic (or parasitic) death, there is natural death.
This is in great part to be regarded as the price paid for a body. A
body worth having implies complexity or division of labour, and this
implies certain internal furnishings of a more or less stable kind in
which the effects of wear and tear are apt to accumulate. It is not the
living matter itself that grows old so much as the framework in which it
works--the furnishings of the vital laboratory. There are various
processes of rejuvenescence, e.g. rest, repair, change, reorganisation,
which work against the inevitable processes of senescence, but sooner or
later the victory is with ageing. Another deep reason for natural death
is to be found in the physiological expensiveness of reproduction, for
many animals, from worms to eels, illustrate natural death as the
nemesis of starting new lives. Now it is a very striking fact that to a
large degree the simplest animals or Protozoa are exempt from natural
death. They are so relatively simple that they can continually
recuperate by rest and repair; they do not accumulate any bad debts.
Moreover, their modes of multiplying, by dividing into two or many
units, are very inexpensive physiologically. It seems that in some
measure this bodily immortality of the Protozoa is shared by some simple
many-celled animals like the freshwater Hydra and Planarian worms. Here
is an interesting chapter in evolution, the evolution of means of
evading or staving off natural death. Thus there is the well-known case
of the Paloloworm of the coral-reefs where the body breaks up in
liberating the germ-cells, but the head-end remains fixed in a crevice
of the coral, and buds out a new body at leisure.

Along with the evolution of the ways of avoiding death should be
considered also the gradual establishment of the length of life best
suited to the welfare of the species, and the punctuation of the
life-history to suit various conditions.

[Illustration: _Photo: J. J. Ward, F.E.S._

GREEN HYDRA

A little freshwater polyp, about half an inch long, with a crown of
tentacles round the mouth. It is seen giving off a bud, a clear
illustration of asexual reproduction. When a tentacle touches some small
organism the latter is paralysed and drawn into the mouth.]

[Illustration: _Photo: J. J. Ward, F.E.S._

EARTHWORM

Earthworms began the profitable habit of moving with one end of the body
always in front, and from worms to man the great majority of animals
have bilateral symmetry.]

[Illustration: DIAGRAM ILLUSTRATING THE BEGINNING OF INDIVIDUAL LIFE

1. An immature _sperm_-cell, with 4 chromosomes (nuclear bodies)
represented as rods.

2. A mature sperm-cell, with 2 chromosomes.

3. An immature _egg_-cell, with 4 chromosomes represented as curved
bodies.

4. A mature egg-cell, with 2 chromosomes.

5. The spermatozoon fertilises the ovum, introducing 2 chromosomes.

6. The fertilised ovum, with 4 chromosomes, 2 of paternal origin and 2
of maternal origin.

7. The chromosomes lie at the equator, and each is split longitudinally.
The centrosome introduced by the spermatozoon has divided into two
centrosomes, one at each pole of the nucleus. These play an important
part in the division or segmentation of the egg.

8. The fertilised egg has divided into two cells. Each cell has 2
paternal and 2 maternal chromosomes.]

[Illustration: _Reproduced from the Smithsonian Report, 1917._

GLASS MODEL OF A SEA-ANEMONE

A long tubular sea-anemone, with a fine crown of tentacles around the
mouth. The suggestion of a flower is very obvious. By means of stinging
lassoes on the tentacles minute animals on which it feeds are paralysed
and captured for food.]

[Illustration: THIS DRAWING SHOWS THE EVOLUTION OF THE BRAIN FROM FISH
TO MAN

The Cerebrum, the seat of intelligence, increases in proportion to the
other parts. In mammals it becomes more and more convoluted. The brain,
which lies in one plane in fishes, becomes gradually curved on itself.
In birds it is more curved than the drawing shows.]

Sec. 6

Great Acquisitions

In animals like sea-anemones and jellyfishes the general symmetry of the
body is radial; that is to say, there is no right or left, and the body
might be halved along many planes. It is a kind of symmetry well suited
for sedentary or for drifting life. But worms began the profitable habit
of moving with one end of the body always in front, and from worms to
man the great majority of animals have bilateral symmetry. They have a
right and a left side, and there is only one cut that halves the body.
This kind of symmetry is suited for a more strenuous life than radial
animals show; it is suited for pursuing food, for avoiding enemies, for
chasing mates. And _with the establishment of bilateral symmetry must be
associated the establishment of head-brains_, the beginning of which is
to be found in some simple worm-types.

Among the other great acquisitions gradually evolved we may notice: a
well-developed head with sense-organs, the establishment of large
internal surfaces such as the digestive and absorptive wall of the
food-canal, the origin of quickly contracting striped muscle and of
muscular appendages, the formation of blood as a distributing medium
throughout the body, from which all the parts take what they need and to
which they also contribute.

Another very important acquisition, almost confined (so far as is known)
to backboned animals, was the evolution of what are called glands of
internal secretion, such as the thyroid and the supra-renal. These
manufacture subtle chemical substances which are distributed by the
blood throughout the body, and have a manifold influence in regulating
and harmonising the vital processes. Some of these chemical messengers
are called hormones, which stimulate organs and tissues to greater
activity; others are called chalones, which put on a brake. Some
regulate growth and others rapidly alter the pressure and composition
of the blood. Some of them call into active development certain parts of
the body which have been, as it were, waiting for an appropriate
trigger-pulling. Thus, at the proper time, the milk-glands of a
mammalian mother are awakened from their dormancy. This very interesting
outcome of evolution will be dealt with in another portion of this work.

Pages:
1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26