(Study Material) Zoology Study Material For AIPMT and State PMT Examination (Principles of Evolution)
Study Material : Zoology Study Material For AIPMT and State PMT Examination (Principles of Evolution)
First Principle of Evolution
For the moment it will be enough to state two leading principles. The first is
that there is no such thing as a "law of evolution" in the sense in
which many people understand that phrase. It is now sufficiently well known
that, when science speaks of a law, it does not mean that there is some rule
that things MUST act in such and such a way. The law is a mere general
expression of the fact that they DO act in that way. But many imagine that there
is some principle within the living organism which impels it onward to a higher
level of organisation. That is entirely an error. There is no "law of
progress." If an animal is fitted to secure its livelihood and breed
posterity in certain surroundings, it may remain unchanged indefinitely if these
surroundings do not materially change. So the duckmole of Australia and the
tuatara of New Zealand have retained primitive features for millions of years;
so the aboriginal Australian and the Fuegian have remained stagnant, in their
isolation, for a hundred thousand years or more; so the Chinaman, in his
geographical isolation, has remained unchanged for two thousand years. There is
no more a "conservative instinct" in Chinese than there is a
"progressive instinct" in Europeans. The difference is one of history
and geography, as we shall see.
To make this important principle still clearer, let us imagine some primitive
philosopher observing the advance of the tide over a level beach. He must
discover two things: why the water comes onward at all, and why it advances
along those particular channels. We shall see later how men of science explain
or interpret the mechanism in a living thing which enables it to advance, when
it does advance. For the present it is enough to say that new-born animals and
plants are always tending to differ somewhat from their parents, and we now
know, by experiment, that when some exceptional influence is brought to bear on
the parent, the young may differ considerably from her. But, if the parents were
already in harmony with their environment, these variations on the part of the
young are of no consequence. Let the environment alter, however, and some of
these variations may chance to make the young better fitted than the parent was.
The young which happen to have the useful variation will have an advantage over
their brothers or sisters, and be more likely to survive and breed the next
generation. If the change in the environment (in the food or climate, for
instance) is prolonged and increased for hundreds of thousands of years, we
shall expect to find a corresponding change in the animals and plants.
We shall find such changes occurring throughout the story of the earth. At one
important point in the story we shall find so grave a revolution in the face of
nature that twenty-nine out of every thirty species of animals and plants on the
earth are annihilated. Less destructive and extreme changes have been taking
place during nearly the whole of the period we have to cover, entailing a more
gradual alteration of the structure of animals and plants; but we shall
repeatedly find them culminating in very great changes of climate, or of the
distribution of land and water, which have subjected the living population of
the earth to the most searching tests and promoted every variation toward a more
effective organization
Second Principle of Evolution
And the second guiding principle I wish to lay down in advance is that these
great changes in the face of the earth, which explain the progress of organisms,
may very largely be reduced to one simple agency--the battle of the land and the
sea. When you gaze at some line of cliffs that is being eaten away by the waves,
or reflect on the material carried out to sea by the flooded river, you
are--paradoxical as it may seem--beholding a material process that has had a
profound influence on the development of life. The Archaean continent that we
described was being reduced constantly by the wash of rain, the scouring of
rivers, and the fretting of the waves on the coast. It is generally thought that
these wearing agencies were more violent in early times, but that is disputed,
and we will not build on it. In any case, in the course of time millions of tons
of matter were scraped off the Archaean continent and laid on the floor of the
sea by its rivers. This meant a very serious alteration of pressure or weight on
the surface of the globe, and was bound to entail a reaction or restoration of
the balance.
The rise of the land and formation of mountains used to be ascribed mainly to
the cooling and shrinking of the globe of the earth. The skin (crust), it was
thought, would become too large for the globe as it shrank, and would wrinkle
outwards, or pucker up into mountain-chains. The position of our greater
mountain-chains sprawling across half the earth (the Pyrenees to the Himalaya,
and the Rocky Mountains to the Andes), seems to confirm this, but the question
of the interior of the earth is obscure and disputed, and geologists generally
conceive the rise of land and formation of mountains in a different way. They
are due probably to the alteration of pressure on the crust in combination with
the instability of the interior. The floors of the seas would sink still lower
under their colossal burdens, and this would cause some draining of the
land-surface. At the same time the heavy pressure below the seas and the
lessening of pressure over the land would provoke a reaction. Enormous masses of
rock would be forced toward and underneath the land-surface, bending, crumpling,
and upheaving it as if its crust were but a leather coat. As a result, masses of
land would slowly rise above the plain, to be shaped into hills and valleys by
the hand of later time, and fresh surfaces would be dragged out of the deep,
enlarging the fringes of the primitive continents, to be warped and crumpled in
their turn at the next era of pressure.
In point of geological fact, the story of the earth has been one prolonged
series of changes in the level of land and water, and in their respective
limits. These changes have usually been very gradual, but they have always
entailed changes (in climate, etc. ) of the greatest significance in the
evolution of life. What was the swampy soil of England in the Carboniferous
period is now sometimes thousands of feet beneath us; and what was the floor of
a deep ocean over much of Europe and Asia at another time is now to be found on
the slopes of lofty Alps, or 20,000 feet above the sea-level in Thibet. Our
story of terrestrial life will be, to a great extent, the story of how animals
and plants changed their structure in the long series of changes which this
endless battle of land and sea brought over the face of the earth. Europe, as a
continent, has had more "ups and downs" than America in the course of
geological time.
This meagre picture of the battle of land and sea, with interludes of great
volcanic activity and even of an ice age, represents nearly all we know of the
first half of the world's story from geology. It is especially disappointing in
regard to the living population. The very few fossils we find in the upper
Archaean rocks are so similar to those we shall discuss in the next chapter that
we may disregard them, and the seams of carbon-shales, iron-ore, and limestone,
suggest only, at the most, that life was already abundant. We must turn
elsewhere for some information on the origin and early development of life.
The question of the origin of life I will dismiss with a brief account of the
various speculations of recent students of science. Broadly speaking, their
views fall into three classes. Some think that the germs of life may have come
to the earth from some other body in the universe; some think that life was
evolved out of non-living matter in the early ages of the earth, under
exceptional conditions which we do not at present know, or can only dimly
conjecture; and some think that life is being evolved from non-life in nature
to-day, and always has been so evolving. The majority of scientific men merely
assume that the earliest living things were no exception to the general process
of evolution, but think that we have too little positive knowledge to speculate
profitably on the manner of their origin.
Views over Evolution
The first view, that the germs of life may have come to this planet on a
meteoric visitor from some other world, as a storm-driven bird may take its
parasites to some distant island, is not without adherents to-day. It was put
forward long ago by Lord Kelvin and others; it has been revived by the
distinguished Swede, Professor Svante Arrhenius. The scientific objection to it
is that the more intense (ultra-violet) rays of the sun would frill such germs
as they pass through space. But a broader objection, and one that may dispense
us from dwelling on it, is that we gain nothing by throwing our problems upon
another planet. We have no ground for supposing that the earth is less capable
of evolving life than other planets.
The second view is that, when the earth had passed through its white-hot stage,
great masses of very complex chemicals, produced by the great heat, were found
on its surface. There is one complex chemical substance in particular, called
cyanogen, which is either an important constituent of living matter, or closely
akin to it. Now we need intense heat to produce this substance in the
laboratory. May we not suppose that masses of it were produced during the
incandescence of the earth, and that, when the waters descended, they passed
through a series of changes which culminated in living plasm? Such is the "cyanogen
hypothesis" of the origin of life, advocated by able physiologists such as
Pfluger, Verworn, and others. It has the merit of suggesting a reason why life
may not be evolving from non-life in nature to-day, although it may have so
evolved in the Archaean period.
Other students suggest other combinations of carbon-compounds and water in the
early days. Some suggest that electric action was probably far more intense in
those ages; others think that quantities of radium may have been left at the
surface. But the most important of these speculations on the origin of life in
early times, and one that has the merit of not assuming any essentially
different conditions then than we find now, is contained in a recent
pronouncement of one of the greatest organic chemists in Europe, Professor
Armstrong. He says that such great progress has been made in his science--the
science of the chemical processes in living things--that "their cryptic
character seems to have disappeared almost suddenly." On the strength of
this new knowledge of living matter, he ventures to say that "a series of
lucky accidents" could account for the first formation of living things out
of non-living matter in Archaean times. Indeed, he goes further. He names
certain inorganic substances, and says that the blowing of these into pools by
the wind on the primitive planet would set afoot chemical combinations which
would issue in the production of living matter.
It is evident that the popular notion that scientific men have declared that
life cannot be evolved from non-life is very far astray. This blunder is usually
due to a misunderstanding of the dogmatic statement which one often reads in
scientific works that "every living thing comes from a living thing."
This principle has no reference to remote ages, when the conditions may have
been different. It means that to-day, within our experience, the living thing is
always born of a living parent. However, even this is questioned by some
scientific men of eminence, and we come to the third view.
Professor Nageli, a distinguished botanist, and Professor Haeckel, maintain that
our experience, as well as the range of our microscopes, is too limited to
justify the current axiom. They believe that life may be evolving constantly
from inorganic matter. Professor J. A. Thomson also warns us that our experience
is very limited, and, for all we know, protoplasm may be forming naturally in
our own time. Mr. Butler Burke has, under the action of radium, caused the birth
of certain minute specks which strangely imitate the behaviour of bacteria. Dr.
Bastian has maintained for years that he has produced living things from
non-living matter. In his latest experiments, described in the book quoted,
purely inorganic matter is used, and it is previously subjected, in hermetically
sealed tubes, to a heat greater than what has been found necessary to kill any
germs whatever.
Evidently the problem of the origin of life is not hopeless, but our knowledge
of the nature of living matter is still so imperfect that we may leave detailed
speculation on its origin to a future generation. Organic chemistry is making
such strides that the day may not be far distant when living matter will be made
by the chemist, and the secret of its origin revealed. For the present we must
be content to choose the more plausible of the best-informed speculations on the
subject.
But while the origin of life is obscure, the early stages of its evolution come
fairly within the range of our knowledge. To the inexpert it must seem strange
that, whereas we must rely on pure speculation in attempting to trace the origin
of life, we can speak with more confidence of those early developments of plants
and animals which are equally buried in the mists of the Archaean period. Have
we not said that nothing remains of the procession of organisms during half the
earth's story but a shapeless seam of carbon or limestone? A simple illustration
will serve to justify the procedure we are about to adopt. Suppose that the
whole of our literary and pictorial references to earlier stages in the
development of the bicycle, the locomotive, or the loom, were destroyed. We
should still be able to retrace the phases of their evolution, because we should
discover specimens belonging to those early phases lingering in our museums, in
backward regions, and elsewhere. They might yet be useful in certain
environments into which the higher machines have not penetrated. In the same
way, if all the remains of prehistoric man and early civilisation were lost, we
could still fairly retrace the steps of the human race, by gathering the lower
tribes and races, and arranging them in the order of their advancement. They are
so many surviving illustrations of the stages through which mankind as a whole
has passed.
Age-long Procession of Life
Just in the same way we may marshal the countless species of animals and plants
to-day in such order that they will, in a general way, exhibit to us the
age-long procession of life. From the very start of living evolution certain
forms dropped out of the onward march, and have remained, to our great
instruction, what their ancestors were millions of years ago. People create a
difficulty for themselves by imagining that, if evolution is true, all animals
must evolve. A glance at our own fellows will show the error of this. Of one
family of human beings, as a French writer has said, one only becomes a
Napoleon; the others remain Lucien, Jerome, or Joseph. Of one family of animals
or trees, some advance in one or other direction; some remain at the original
level. There is no "law of progress." The accidents of the world and
hereditary endowment impel some onward, and do not impel others. Hence at nearly
every great stage in the upward procession through the ages some regiment of
plants or animals has dropped out, and it represents to-day the stage of life at
which it ceased to progress. In other words, when we survey the line of the
hundreds of thousands of species which we find in nature to-day, we can trace,
amid their countless variations and branches, the line of organic evolution in
the past; just as we could, from actual instances, study the evolution of a
British house, from the prehistoric remains in Devonshire to a mansion in Park
Lane or a provincial castle.
Another method of retracing the lost early chapters in the development of life
is furnished by embryology. The value of this method is not recognised by all
embryologists, but there are now few authorities who question the substantial
correctness of it, and we shall, as we proceed, see some remarkable applications
of it. In brief, it is generally admitted that an animal or plant is apt to
reproduce, during its embryonic development, some of the stages of its ancestry
in past time. This does not mean that a higher animal, whose ancestors were at
one time worms, at another time fishes, and at a later time reptiles, will
successively take the form of a little worm, a little fish, and a little
reptile. The embryonic life itself has been subject to evolution, and this
reproduction of ancestral forms has been proportionately disturbed. Still, we
shall find that animals will tend, in their embryonic development, to reproduce
various structural features which can only be understood as reminiscences of
ancestral organs. In the lower animals the reproduction is much less disturbed
than in the higher, but even in the case of man this law is most strikingly
verified. We shall find it useful sometimes at least in confirming our
conclusions as to the ancestry of a particular group.
We have, therefore, two important clues to the missing chapters in the story of
evolution. Just as the scheme of the evolution of worlds is written broadly
across the face of the heavens to-day, so the scheme of the evolution of life is
written on the face of living nature; and it is written again, in blurred and
broken characters, in the embryonic development of each individual. With these
aids we set out to restore the lost beginning of the epic of organic evolution.
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