When Darwin started on the voyage of the Beagle in 1831, he had no reason to doubt
the
immutability of species. The speculations of his grandfather Erasmus counted for nothing with him
because they were not supported by evidence, and those of Lamarck on the causes of evolution
had the additional issue of bringing the subject into disrepute by their fanciful nature. Furthermore,
in Lyell's Principles of Geology, to which Darwin owed so much for the general background of
uniformitarianism in place of catastrophism, the possibility of evolution was firmly rejected.
Three sets of observations started Darwin's revolt against the immutability of species.
The first was his studies of the fauna of the Galapagos Islands, where he found that
species of
finches differed slightly from island to island, while showing general resemblance not only to each
other but to the finches on the adjacent mainland of South America. If these species had been
separately created, why should there have been such a prodigal expenditure of 'creation' just there;
why should geographical propinquity have caused these ' creations' to resemble each other so
closely why in spite of the similarity in physical conditions between the islands of the Galapagos
Archipelago and the Cape Verde Islands are their faunas totally different, the former resembling that
of South America while the fauna of the latter resembles that of Africa ?
The second set of observations related to the fact that as he travelled over South
America, the
species occupying a particular niche in nature in some regions were replaced in neighbouring
regions by other species that were different, yet closely similar. Why are the rabbit-like animals on
the savannahs of La Plata built on the plan of the peculiar South American type of rodent, and not
on that of North America or the Old World?
The third set of observations was concerned with the fact that in the Pampas he found
fossil
remains of large mammals covered with armour like that of the armadillos now living on that
continent. Why were these extinct animals built on the same plan as those now living ?
On the view that species were immutable and had not changed since they were severally
created,
there was no rational answer to any of these questions, which would have to remain as
unfathomable mysteries. On the other hand, if species, like varieties, were subject to modification
during descent and to divergence into different lines of descent, all these questions could be
satisfactorily and simply answered. The finches of the Galapagos resemble each other and those of
South America because they are descended from a common ancestor; they differ from one another
because they are each adapted to modes of life restricted to their own particular island, one for
instance feeding on seeds on the ground and another on insects in trees. The volcanic nature and
physical conditions of the Galapagos Islands resemble those of the Cape Verde Islands, and yet
the Galapagos birds all differ from the birds of the Cape Verde Islands. Therefore, it is not the
physical conditions of the islands that determine their differences. These arose because the Cape
Verde Island birds share a common ancestor with the birds of Africa, whereas the Galapagos birds
share a common ancestor with those of South America. The hares of South America are built on
the South American rodent plan because all South American rodents are descended from a
common ancestor. The fossil Glyptodon resembles the living armadillos because they, also, share
a common ancestor and this case is particularly important because if living species show affinity
with extinct species, there is no necessity to believe that extinct types of animals have left no living
descendants. They may have representatives alive today, and this means that the whole wealth of
the paleontological record of fossils is available as material for the study of the problem of evolution.
In possession of a working hypothesis that species have undergone evolution and successive
origination by descent with modification from ancestral species shared in common with other
species, Darwin next proceeded to search the whole field of botanical and zoological knowledge for
evidence bearing on his hypothesis; for he realised that no general principle that explained the
evolution of animals was acceptable unless it also applied to plants. The result was one of the
most remarkable attacks on a problem ever made by the inductive method of searching for facts,
whatever their import might be.
In the first place, in cultivated plants and domestic animals such as the dahlia,
the potato, the
pigeon, and the rabbit, a large number of varieties have in each case been produced from a single
original stock. Descent with modification and divergence into several lines is therefore certainly
possible within the species.
Comparative anatomy reveals the existence of plans of structure in large groups of
organisms.
Plants may have vegetative leaves, and in some cases these are modified into parts of flowers.
Vertebrate animals have forelimbs that may be used for walking, running, swimming, or flying, but
in which the various parts of the skeleton correspond, bone for bone, from the upper arm to the last
joints of the fingers, whether the animal is a frog, a lizard, a turtle, a bird, a rabbit, a seal, a
bat, or
a man. This is what is meant by saying that such structures are homologous, and these
correspondences are inexplicable unless the animals are descended from a common ancestor.
Fundamental resemblance is therefore evidence of genetic affinity.
The study of comparative behaviour proves that related forms show gradations in their
instincts,
such as shamming death in insects and nest-building in birds. At the same time, related species
inhabiting different parts of the earth under very different conditions retain similar instincts, such
as
the habit of thrushes in England and in South America of lining nests with mud, or that of wrens in
England and North America of the males building 'cock-nests'. Why should this be, unless the
different species of thrushes and wrens are descended from common ancestors in each case ?
Embryology reveals remarkable similarity of structure between young embryos of animals
which in
the adult stage are as different as fish, lizard, fowl, and man .This similarity even extends to such
details as the manner in which the blood- vessels run from the heart to the dorsal aorta, a plan
which is of obvious significance in the case of the fish that breathes by means of gills, but not so
obvious in that of lizard, chick, or man where gill- pouches are formed in the embryo but soon
become transformed into different structures, and breathing is carried out by other means. This
similarity between embryos is explained by the affinity and descent from a common ancestor of the
groups to which they belong.
Embryology also provides evidence of vestiges of structures, which once performed
important
functions in the ancestors but now either perform different functions, or none at all .Examples of
such organs are the teeth of whalebone whales, the limbs of snakes, the wings of ostriches and
penguins, or the flowers of the feather-hyacinth. Since Darwin's time countless other examples
have been discovered, the most striking of which are the pineal gland which is a vestigial eye, and
vestiges of the egg- tooth found in marsupials although it is 75 million years since their ancestors
had to use an egg-tooth to crack the shell and hatch out of their eggs. Here again, descent from
common ancestral forms explains all these cases.
Knowledge of the fossil record in Darwin's time was so imperfect that nothing was
then available in
the way of series illustrating the course of evolution. Nevertheless, he noticed that in Tertiary strata,
the lower the horizon the fewer fossils there were belonging to species alive today. Paleontology
therefore showed that new species had appeared, and old species become extinct, not all at the
same time but in succession and gradually. Why should this be so unless new species have come
into existence from time to time by descent with modification from other species ?
Plants and animals are classified according to their resemblance and they are placed
in one or
other of a not very large number of groups such as ferns, conifers, molluscs, or mammals. But
within each of these groups, there is subdivision into other smaller groups, mammals being so
subdivided into rodents, carnivores, ungulates, and primates for example. Within these again there
is further subdivision, and the important point to notice is that Classification always places species
in groups that are contained within other larger groups. This is such a commonplace that its
significance is often overlooked. Why do organisms have to be classified like this ? Why are they
not strewn in single file up the ladder of the plant and animal kingdoms, or fortuitously like pebbles
on a beach, or arbitrarily like the stars imaginary constellations ? The reason is that the
arrangement of groups within groups is a natural classification reflecting the course of evolution.
It
is the result of descent from common ancestors and indication of affinity; the differences between
the groups are due modification and divergence during such descent.
Darwin also investigated the problem of inter-specific sterility and saw that it was
by no means
absolute, because numerous examples can be given of different species that produce hybrids, and
in so cases these hybrids are themselves fertile. From the point of view of breeding, therefore, such
species behave like varieties. Why, then, can species not have originated as varieties, by descent
modification from other species ?
The main steps in Darwin's proof of the fact of evolution were established by 1842
when he
committed them to paper in the form of a Sketch which he expanded into an Essay in 1844 though
neither was then published. Soon after this another naturalist, Alfred Russel Wallace, was led to
explore similar lines of research. From some simple observations on the distribution of organisms,
both geographically over the world and geologically in the fossil record, Wallace drew some equally
simple conclusions that are of great importance in the history of thought which led to the realisation
of evolution. They show that independently of Darwin and in complete ignorance of his work,
Wallace had hit upon the same solution of the problem of the mutability of species.
Wallace's observations were based on the facts, first, that large systematic groups
such as
Classes and Orders are usually distributed over the whole of the earth, whereas groups of low
systematic value such as families, genera, and species frequently have a very small localised
distribution. Secondly, 'when a group is confined to one district, and is rich in species, it is almost
invariably the case that the most closely allied species are found in the same locality or in closely
adjoining localities, and that therefore the natural sequence of the species by affinity is also
geographical'. Thirdly, in the fossil record, large groups extend through several geological
formations, and ' no group or species has come into existence twice'.
The conclusion which Wallace drew from these observations was that: ' Every species
has come
into existence coincident both in space and time with a pre-existing closely allied species.'
Thought out about 1845, written at Sarawak in 1855, and published in the same year, Wallace's
theory already allowed him to say that 'the natural series of affinities will also represent the order
in
which the several species came into existence, each one having had for its immediate antitype a
closely allied species existing at the time of its origin. It is evidently possible that two or three
distinct species may have had a common antitype, and that each of these may again have become
the antitypes from which other closely allied species were created.'
With the help of this principle, in which it is only necessary to substitute 'ancestor'
for 'antitype' for
the formulation of evolution to be complete, Wallace showed that it was possible to give a simple
explanation of natural classification, of the geographical distribution of plants and animals including
those of the Galapagos Islands, of the succession of forms in the fossil record, and of rudimentary
organs which would be inexplicable ' if each species had been created independently, and without
any necessary relations with pre-existing species'.
So much of the credit for the establishment of the fact of evolution has, rightly,
been accorded to
Darwin that it is only just that Wallace's contribution to this problem should be recognised and
honoured.
The evidence on which Darwin and Wallace based their demonstration that evolution
was a fact, is
not only valid to this day, but has been confirmed in all the branches of science concerned as well
as in many new fields. There was in their day not even an inkling of the possibilities of research
opened up to comparative physiology and biochemistry, or of serology as a quantitative indicator of
the amount of divergence that has taken place between related forms. Why should the chemical
substance involved in the mechanism of muscular contraction in most invertebrates be arginine,
whereas it is creatine in vertebrates and echinoderms, which on independent evidence are regarded
as related ? Why should serum immunised against man give precipitations of 64% when mixed
with blood of a gorilla, but 42% with that of an orang-utan, 29% with that of a baboon, and only 10%
with that of an ox ? Why should syphilis attack the chimpanzee more seriously than the orang-
utan, and the latter more seriously than the baboon? Why should the human ABO blood-group
system also be found in the apes ? The answer to all these question is that the organisms
concerned have undergone evolution from common ancestors, as a result of which members of the
various lines of descent share not only structural, mental, and genetical characters, but also
physiological and biochemical mechanisms, and immunological reactions.
Although Darwin already knew in 1837 that evolution was an inescapable conclusion
to be drawn
from the evidence, he did not allow himself to proceed any further with his discovery until he had
found an explanation of the fact of adaptation. In a general way, all plants and animals are adapted
to their environment, for otherwise they could not live. A man drowns in the sea; a fish dies out of
water. But there are some structures which show a particularly intimate relationship between the
organism and its conditions of life. Mistletoe is a parasite that requires a tree of certain species
to
live on, a particular insect to pollinate its flowers, and a thrush to eat its berries and deposit its
seeds on branches of the same species of tree. A woodpecker has two of its toes turned
backwards with which it grips the bark of a tree; it has stiff tail-feathers with which it props itself
against the tree; it has a very strong beak with which it bores holes in the tree trunk; and it has
an
abnormally long tongue with which it takes the grubs at the bottom of the holes. Other plants than
mistletoes and other birds than woodpeckers do not have all these adaptations, and therefore, if
evolution has occurred, it is necessary to give an objective explanation of how these adaptations
arose.
Darwin knew that all members of a species are not identical but show variation in
size, strength,
health, fertility, longevity, instincts, habits, mental attributes, and countless other characters.
He
soon perceived that such variation could be, and in fact was, turned to good account by man in the
course of artificial selection which he has practised in the production of cultivated plants and
domestic animals since the Neolithic Age. The key was selection which is the practice of breeding
only from those parents that possess the desired qualities. But how could selection operate on wild
plants and animals in nature since the beginning of life on earth without man or a conscious being
to direct it ? The solution of this puzzle occurred to Darwin accidentally when he read Malthus's
Essay on Population and realised that under the conditions of competition in which plants and
animals live, any variations would be preserved which increased the organisms' ability to leave
fertile offspring, while those variations which decreased it would be eliminated. In a state of nature,
selection works automatically, which is why Darwin called it Natural Selection. Furthermore, it is
not only individuals that natural selection eliminates but their potential offspring which, because
of
the fact of heredity, would have resembled them.
Darwin was then able to formulate a complete theory providing a rational explanation
of the causes
as well as of the fact of evolution in plants and animals. It is formally based on four propositions
which he already knew to be true, and three deductions which are now also known to be true. They
may be enumerated as follows.
1. Organisms produce a
far greater number of reproductive cells than ever give rise to mature
individuals.
2. The numbers of individuals
in species remain more or less constant.
3. Therefore there must
be a high rate of mortality.
4. The individuals in a
species are not all identical, but show variation in all characters.
5. Therefore some variants
will succeed better and others less well in the competition for
survival, and the parents of the next generation will be naturally selected from among those
members of the species that show variation in the direction of more effective adaptation to the
conditions of their environment.
6. Hereditary resemblance
between parent and offspring is a fact.
7. Therefore subsequent
generations will maintain and improve on the degree of adaptation
realised by their parents by gradual change.
This is the formal theory of evolution by natural selection, first announced jointly
on 1 July 1858 by
Darwin and Alfred Russel Wallace who had, again independently, come to the identical conclusion.
It represents a step in knowledge comparable to Newton's discovery of the law of gravitation.
The 'book of life'
The earliest chapters of earth history are written in the oldest rocks. These millions-of-years-
old
rocks reveal that the earth and its inhabitants have changed greatly during this vast span of time.
Luckily, the rocks and the fossils they contain have recorded these changes for us.
Earth's rocky "history book" reveals many interesting events. It indicates
that the world's climate
and geography have not always been as they are today. Indeed, they appear to have differed
greatly from one major period of time to the next. There is also evidence to suggest that these
physical changes in the environment had a profound effect on the plants and animals of the day.
Little by little and bit by bit, earth historians have pieced the puzzle together. Although the picture
is still far from complete, we do have a fairly accurate idea of the conditions on Earth throughout
most of its long history. Let us now turn back the pages of geologic time to the very beginning and
follow some of the more important changes that have taken place.
The oldest units of earth history appear at the bottom of the "geological calendar".
These very
ancient rocks represent Precambrian time—that part of earth history from the beginning of
geologic
history until the formation of the earliest fossil-bearing rocks of the Cambrian Period.
Earth's story begins, then, with the events recorded in rocks formed during the Archeozoic
Era.
These rocks—some of which are at least three thousand million years old—consist of
rocks that
were originally igneous or sedimentary, but that have since been greatly altered by heat and
pressure. In places these metamorphic rocks have been invaded by great plugs of granite. These
intrusive rocks provide evidence of underground movements of molten rock. Unfortunately, the
igneous activity, metamorphic changes, and structural deformation have greatly altered the original
Archeozoic rocks. Consequently, little can be learned about their original characteristics or any
evidence of life that might have been present. However, some of the rocks do contain
concentrations of carbon. These may have been formed from the remains of some as yet unknown
type of Archeozoic plant or animal.
The story of Late Precambrian time is revealed in the Proterozoic rocks. Composed
of igneous,
sedimentary, and metamorphic formations, these rocks tell of a time of volcanic activity, glaciation,
and considerable deposition of marine sediments. There is also evidence of an episode of mountain-
building.
The first direct evidence of prehistoric life is found in rocks of Proterozoic age.
These fossils consist
largely of the carbon impressions of soft- bodied animals. Huge masses of organic limestone
formed by sea- dwelling, lime-secreting algae are also known. In some parts of the world, the limy
remains of these plant-like organisms form thick beds of limestone that are now found many
thousands of feet above sea level.
The word "Paleozoic" literally means "ancient-life". This is an
appropriate name for this portion of
geologic history, for the life of this time was in an early stage of development. Fortunately, the
Paleozoic rocks have been subjected to less erosion and deformation than the Precambrian rocks
and there are many sedimentary strata which contain well-preserved fossils. Consequently, much
more is known about the Paleozoic record than is known about Precambrian time.
More than 600 million years have passed since the Paleozoic Era began and this great
span of
time has been separated into seven periods of unequal duration. How do we know when one period
stopped and another began? We cannot always tell for certain, but most periods appear to have
been separated from each other by relatively short, naturally occurring periods of broad continental
uplift. As the continents were raised the sea drained from the land. Each uplift was usually followed
by a period of submergence when the ocean again rolled over parts of the land masses. Sediments
were deposited with each advance of the sea and these were later converted into sedimentary rock.
There was much life in these prehistoric seas, and many of these organisms have been preserved
as fossils. Some of them can be used to determine the age of the rocks that contain them.
The Cambrian
Paleozoic time started
with the Cambrian Period that began about 600 million years ago. This
was a milestone in geologic history, for from Cambrian time onwards, we have a fairly good
record of the development of life on earth. Cambrian rocks were first studied and described in
Wales, and the name of the period is derived from the Latin word Cambria, which means Wales.
Although there is no way
of knowing for certain, the Cambrian Period apparently lasted for
about 100 million years. During this time some 30 per cent of North America was covered by
an ancient sea that washed over the land. Sediments deposited in the Cambrian seas were
later transformed into limestones, shales, sandstones, and other sedimentary rocks.
Cambrian fossils suggest
that the life of this time was dominated by a great variety of
invertebrates, or animals without backbones. The trilobites, relatives of the living horseshoe
crab, and the shellfish known as brachiopods were especially abundant. Many other
invertebrates and primitive plants inhabited Cambrian seas, but there is no evidence of any
creatures with backbones. Nor is there anything to suggest that life had yet invaded the land.
The Ordovician
The Ordovician Period was
also first studied in Wales and derives its name from an old Celtic
tribe, the Ordovices. Warm, shallow seas covered as much as 70 per cent of North America
during the 75 million years of Ordovician time and their waters contained many species of
plants and animals. Their fossil remains tell us that trilobites and brachiopods were still very
abundant. But they were joined by many unusual species of corals, clams, snails, and
cephalopods. The latter were extinct relatives of our modern squid and octopus. Some of these
ancient cephalopods had straight, cone- shaped shells as much as fifteen feet long.
Perhaps the most important
event of Ordovician time was the appearance of the first animals
with backbones—small armoured fishes called ostracoderms. These jawless fish are known
from tiny fragments of bony plates and scales found in the Rocky Mountain region of the United
States.
The nature of the Ordovician
fossils and sedimentary rocks suggests that the climate of this
period was uniformly temperate. Geologic evidence also indicates that there were no well-
defined climatic zones as we know them today.
The Silurian
The Silurian Period, like
the Ordovician, was named after an ancient Celtic tribe (the Silures)
and the rocks were also first studied in Wales. The central part of the United States was
flooded by a fairly widespread sea during much of Silurian time, but near the end of the period,
the water began to drain off the land. In places, landlocked bodies of water remained on the
continent and slowly evaporated. As the water evaporated, thick concentrations of salt gypsum
were deposited on the ocean floor in what is now Ohio, New York, Michigan, Pennsylvania, and
Ontario in Canada.
The warm Silurian sea was
teeming with brachiopods, corals, clams, and snails. Trilobites
were still abundant but had reached their peak and were beginning to dwindle in numbers. The
eurypterids, extinct scorpion-like creatures, are especially characteristic of Silurian time, and
may have been the forerunners of the air-breathing animals. There were still no backboned
creatures on the land but there were many fishes in the sea.
At some point during the
Silurian Period life established its first foothold on the land. The way
for terrestrial life was paved by rather simple, rootless plants whose remains have been found in
England and Australia.
Silurian rocks and fossils
suggest that the climate of this period must have been rather warm
and mild. The salt and gypsum deposits of Late Silurian time hint of an episode of desert-like
conditions for part of the country.
The Devonian
Named from exposures of
rocks first studied in Devonshire, the Devonian Period was a time of
great change. During the early part of the period, much of the North American continent was
exposed. However, there was a widespread invasion of the sea in Middle Devonian time.
Devonian life was characterized
by the spreading and development of land plants. Ferns and
seeding plants were numerous and their remains are commonly found as fossils.
Brachiopods were the dominant
Devonian invertebrates, but trilobites, corals, snails, and clams
were also well represented.
Fishes were many and various
and their many fossils have caused some geologists to call the
Devonian the "Age of Fishes". Especially notable were the great arthrodires. Some of these
sharklike animals were as much as thirty feet long.
An important event of Devonian
time was the appearance of the first four-footed vertebrate
animal. This early amphibian lived in water and on the land, much as our toads and frogs of
today.
As far as is known, Devonian
climates were mild and temperate throughout most of the period.
The nature of some Devonian fossils also suggests that some parts of the world were warm and
humid.
The Mississippian
The Mississippian Period
is named from exposures of rock first studied in the Upper
Mississippi River Valley of the United States. Much of this part of the United States was
covered by an ancient Mississippian sea and the land was relatively near sea level and had
little relief. Thick vegetation grew in these warm, moist swamplands and deposits of coal were
formed from their remains.
Life was thriving on land
and in the sea during Mississippian time and ferns, rushes, and other
water-loving plants grew in great profusion in the swamps. Insects were also present in great
numbers and amphibians were rapidly increasing. Brachiopods and cephalopods were
numerous in the sea, as were the crinoids, or "sea lilies".
The Pennsylvanian
Rocks of the Pennsylvanian
Period were first studied in the state of Pennsylvania. This part of
the United States was a region of low elevation during Pennsylvanian time and numerous
swamps dotted the landscape. Vegetation grew profusely in these moist lowlands and their
decaying remains were later transformed into valuable deposits of coal.
Marine life was plentiful
in the warm Pennsylvanian seas. Spiny brachiopods, sea lilies, corals,
snails, and clams were particularly numerous. The damp, jungle-like "coal forests" were
swarming with such great hordes of insects that the Pennsylvanian is sometimes called the
"Age of Insects".
The vertebrates were also
thriving and the amphibians were especially well adapted to the
swampy lowlands and were present in great numbers. A highlight of Pennsylvanian time is the
appearance of the first reptile. Unlike amphibians, which must undergo a water larval stage,
reptiles can spend their entire life on the land. The development of the early reptiles paved the
way for the widespread reptilian evolution that occurred during Permian time.
Pennsylvanian climates
were warm and moist. These conditions were ideal for the spread of
the great "coal forests" and the many plants and animals that lived in them.
The Permian
The Permian Period was
the closing chapter of the Paleozoic Era. Many changes took place
near the end of this period and animals that had been abundant for millions of years
disappeared from the face of the earth. It is not surprising, then, that the Permian has been
called "a time of great dying".
This period lasted for
about 50 million years. During this time the seas were rather restricted.
Rock-forming sediments were not widely deposited and exposures of Permian rocks are not
common in eastern North America. However, there are extensive Permian formations in
southeastern New Mexico, western Texas, Nebraska, Kansas, and the western United States.
Many of the rock formations of such scenic areas as Carlsbad Caverns National Park and
White Sands National Monument in New Mexico, Arizona's Grand Canyon, and the Garden of
the Gods in Colorado are Permian in age.
Drastic changes in climate
and geography took place near the end of Permian time. These
changes had a profound effect on the plants and animals of that time and hastened the
extinction of many species. Trilobites, which had been so numerous during much of Paleozoic
time, disappeared from the earth, never to return again. And the brachiopods— especially
the
spiny and more unusual species—were drastically reduced in numbers and variety.
The places vacated by these
vanishing creatures were quickly occupied by other species.
Cephalopods, clams, snails, and reef- building corals underwent remarkable growth and new
and unusual species were introduced.
Life also advanced on the
land. Reptiles and amphibians continued to evolve and have left some
interesting fossils. "Finback" reptiles such as Edaphosaurus and Dimetrodon sported large
fin-
like "sails" on their backs. Their remains are characteristic of a number of Permian formations.
The water-loving, swamp-dwelling
plants so abundant during the Pennsylvanian period were
greatly reduced during the Permian. Their place was taken by conifers (cone- bearing plants)
and other more modern species.
This "time of great
dying" was also a period marked by extreme climatic changes. Times of
desert-like dryness and glacial cold alternated with almost tropical, warm, and humid climates.
The rock record suggests that there may have been swamplike conditions in Asia and
Australia, and deserts in the southwestern United States. Meanwhile, sheets of glacial ice
blanketed parts of Australia, South Africa, and South America. Small wonder that certain
Permian species were unable to adapt to such drastic changes in the environment.
The geographic changes
of the Permian period were almost as dramatic as those of the
climate and life. Near the end of the period the final movements of the great Appalachian
Revolution gave rise to the Appalachian Mountains. This great range stretches from Nova
Scotia southward into Alabama. The rocks which were folded upward in this great orogenesis
were formed from sediment deposited in a branch of the sea that once occupied what is now
the Appalachian region.
The Mezozoic
The so-called "middle"
era of the geologic calendar was a turning point in the history of life.
Known literally as the time of "middle- life", the Mesozoic Era marked the transition from
the
relatively simple organisms of Paleozoic time to the more modern species of the Cenozoic Era.
Mesozoic seas were filled
with countless species of plants and animals and land- dwelling
organisms were equally abundant. But the true "stars" of this act in the drama of life were
the
reptiles. It is not surprising that this era is called the "Age of Reptiles", for dinosaurs
ruled the
land and equally strange reptiles filled the sea and air.
The Triassic
The "Age of Reptiles"
began with the Triassic Period about 230 million years ago. Named from
the Greek word trios (meaning three) it is called this because of the three- fold division
displayed by the Triassic rocks in central Germany.
The Triassic deposits of
the western United States have produced some spectacular scenery.
The Grand Canyon, Painted Desert, and Petrified Forest in Arizona, as well as Utah's Zion
Canyon all contain colourful formations of Triassic age.
The life of Triassic time
showed considerable advance over the plants and animals of the
Paleozoic. New species appeared in the sea and on land. In addition, some of the existing
forms underwent considerable expansion. The predominant land plants were the conifers (cone-
bearing trees), ferns, and the palmlike plants called cycads. Fossil conifers of Triassic age
occur among the great stone trees at Petrified Forest National Park in Arizona.
Marine invertebrates filled
the sea, and corals, clams, oysters, snails, and cephalopods were
especially common.
Sea-dwelling vertebrates
included many species of sharks and the bony fish were also well
represented. Living in the sea were strange sea-going reptiles such as the ichthyosaur, a
streamlined creature that resembled a swordfish. The equally peculiar plesiosaurs were also
present and some of these grew to be forty feet in length.
Reptiles also dominated
life on the land. The bones of phytosaurs, a group of reptiles that
superficially resemble crocodiles, are especially characteristic of certain Triassic formations.
The first dinosaurs also appeared during the Triassic period. They were relatively small,
however, when compared to the gigantic species which dominated life of the Jurassic and
Cretaceous Periods.
The great abundance of
fossil reptiles and amphibians suggests warm mild climates for much
of the earth during Triassic time. However, thick deposits of gypsum and salt indicate that
desert- like conditions must have been present in certain parts of the world during this period.
The Jurassic
Named from exposures in
the Jura Mountains located between Switzerland and France, the
Jurassic Period is well known for the large numbers of unusual reptiles that have been found in
its sedimentary formations.
Although there were abundant
and varied species of Jurassic plants, cycads were especially
abundant. Tree ferns were present, as were ginkgos, conifers, scouring rushes, and ferns.
Many invertebrates filled
the seas, and clams, snails, oysters, and cephalopods were
especially common. Marine vertebrates were well represented by sharks, fishes, and turtles.
Meanwhile, ichthyosaurs and plesiosaurs continued to thrive as they had during the Triassic.
You will recall that it was two such reptiles that Mary Anning discovered in England during the
early nineteenth century, embedded in the lias deposited during the Jurassic period.
But it was again the reptiles—especially
the dinosaurs—that dominated Jurassic life. Some,
like Brontosaurus, were four- footed plant-eaters that grew to be eighty feet long and weighed
tens of tons. These creatures provided food for the ferocious meat-eaters like Allosaurus. This
great beast of prey was about thirty-five feet long and his powerful jaws were well equipped with
sharp teeth. The peculiar plate- backed dinosaur, Stegosaurus, was another distinctive
Jurassic reptile.
The earliest known pterosaurs,
or flying reptiles, also appeared during the Jurassic time. These
remarkable winged "dragons" had batlike wings supported by arms, and long thin "fingers".
Rhamphorhyncus, with a wingspread of about two feet, is a typical Jurassic species.
Two very significant biologic
events took place during the Jurassic Period. The first was the
appearance of the first bird. Known from a feather, two skeletons, and the fragments of a third,
this important fossil was collected from a limestone quarry in southern Germany. Named
Archaeopteryx (which literally means "ancient wing"), this primitive bird still retained certain
reptile-like features. For example, its jaws contained teeth and there were claws on its wings.
This "early bird" did, nevertheless, have feathers. These clearly identify Archaeopteryx as
a bird.
The appearance of the mammals
was the second great event of the Jurassic period. Known
only from fragmental fossil remains, these early mammals appear to have been about the size
of a large rat. The structure of their teeth indicates that some of these early creatures were
plant-eaters while others ate meat.
The Cretaceous
England's famous white
chalk cliffs are typical of rocks of the Cretaceous Period, and they
contain many fossils. This is certainly to be expected, for Cretaceous rocks are among the
most fossiliferous in the world. Nor is it surprising that the name Cretaceous is derived from the
Latin word creta meaning chalk. This is certainly a most appropriate name, for Cretaceous
rocks typically consist of rather limy or chalky deposits. Typical Cretaceous strata can be
seen in the White Cliffs of Dover along the English Channel where these rocks were first
studied and described.
During the Cretaceous Period
the oceans covered the Atlantic and Gulf Coastal plains of the
United States. In addition, a lengthy arm of the sea extended inland from the Gulf of Mexico to
the Arctic Ocean, representing the last great submergence of the North American continent.
Plant life of the Early
Cretaceous period was characterized by ferns, conifers, and cycads. But
during Middle Cretaceous time the first angiosperms, or flowering plants, appeared. When the
period came to a close, Cretaceous plant life closely resembled that of today.
Cretaceous seas were warm
and relatively shallow. The fossil record reveals that they
contained hosts of invertebrates and many of these have been preserved as fossils. Snails,
clams, oysters, and other shellfish were especially numerous, as were the spiny- skinned sea
urchins. Particularly notable were the ammonites. These coiled cephalopods typically resemble
a coiled ram's horn. However, they also assumed other shapes. But despite their great
numbers, they were destined for extinction at the end of the Cretaceous Period.
Vertebrate life was represented
by a host of fish, amphibians, birds, and primitive mammals.
But as in the Triassic and Jurassic Periods, it was the reptiles who held sway over land, sea,
and air. There were duck-billed dinosaurs like Anatosaurus, horned forms such as Triceratops,
and tanklike, armoured species like Ankylosaurus. In addition to these plant-eaters, there were
monstrous carnivorous (meat-eating) dinosaurs such as Tyrannosaurus rex. Standing some
twenty feet tall, Tyrannosaurus walked on his hind legs, was forty to fifty feet long and had long
dagger-like teeth.
Marine reptiles were common
in Cretaceous seas and the still- numerous ichthyosaurs and
plesiosaurs were joined by the mosasaurs. Some of these "sea-going lizards" were as much
as fifty feet long. They were characterized by a flattened tail, sharp teeth, and four limbs that
were modified into paddle-like flippers. The sea also contained giant turtles. Some species
were as much as twelve feet long.
The flying reptiles continued
to make remarkable strides during the Cretaceous period. Perhaps
the best known pterosaur of this time was Pteranodon. Although its short, two- foot body
weighed only ten or twelve pounds, this peculiar beast had a wingspread of as much as twenty-
five feet!
Despite the great success
of the reptilian hordes of the Mesozoic Era, the dinosaurs—along
with the flying reptiles and most marine reptiles—became extinct at the end of the Cretaceous
Period. The cause of their extinction still remains one of science's greatest mysteries.
Cretaceous climates appear
to have been mild and temperate. However, it must have been
much colder in Australia, for there is evidence of glaciation there in Early Cretaceous time.
The Laramide Revolution,
a great orogenesis that produced the Rocky Mountain system,
punctuated the end of the Cretaceous Period and along with it the Mesozoic Era. Much folding
and faulting accompanied this great mountain-building movement and there is evidence of
considerable volcanic activity.
The Cenozoic
We are now living during
the Cenozoic Era. The word "Cenozoic" literally means "recent- life"
and, as the name suggests, the plants and animals of this era are characterized by the
presence of large numbers of modern species. Although many types of present-day
invertebrates appeared during the Cenozoic Era, the major biologic event was the phenomenal
expansion of the mammals. These warm- blooded creatures were so numerous and diverse
that the Cenozoic has been called the "Age of Mammals".
Cenozoic time began with
the Tertiary Period. The Tertiary derives its name from an old
outdated and abandoned classiciation of geologic time.
Plants of the Tertiary
Period closely resembled forms that are now living and the forests had a
decidedly modern appearance. The expansion of hardwood trees, flowering plants, and grasses
was particularly notable and probably furthered the expansion of the mammals.
Shellfish—especially
clams, oysters, and snails—were abundant in the sea. However, the
ammonites that had been so numerous during the Mesozoic Era were now extinct.
Birds were common during
Tertiary time and resembled many of our modern species. Their
fossils are not commonly found, however, for the fragile nature of their bodies normally prevents
fossilization. A few species of birds attained very great size and some birds lost the ability—or
need—to fly.
The relatively sudden extinction
of the dinosaur at the end of the Mesozoic Era triggered the
almost explosive development of the mammals. Horses appeared early in the period and were
about the size of a small dog. Certain Tertiary mammals were every bit as gigantic and bizarre
as the reptilian hordes of the Mesozoic Era. Consider, for example, the uintathere, a great
rhinoceros-like beast that weighed many tons and stood as much as seven feet tall at the
shoulder! Or, a titanothere such as Brontotherium whose elephant- like body and horned skull
gave it a most grotesque appearance. Also present were giant pigs and unusual camels and
deer.
Tertiary climates appear
to have been warm and somewhat humid in North America. But
temperatures dropped near the end of the period—a warning of the great sheets of ice that
were
soon to cover much of North America.
There was considerable
crustal unrest in the western United States near the end of Tertiary
time. These uplifts continued until the close of the period and were ended with the
Paleogeographic map of Tertiary time. The margins of the continent were flooded in places;
otherwise North America looked much as it does today.
The Cascadian Disturbance
elevated the Cascade Mountains of Oregon and Washington and
the Coast Ranges of California. Much volcanic activity accompanied the mountain- building in
the Pacific north-west as revealed by the extensive Columbia River lava flows. In addition, such
famous mountains as Mount Shasta and Lassen Peak in California, Mount Hood in Oregon,
and Washington's Mount Rainier were all associated with Tertiary volcanic activities.
The Quaternary
Period, like the Tertiary, got its name from a rock classification that is no
longer in use. The most recent chapter in earth history, this period has been divided into two
smaller units of time called the Pleistocene and Holocene (or Recent) Epochs.
Pleistocene time
was characterized by great continental glaciers that blanketed much of
Canada and the northern United States. Other massive ice sheets rode over parts of Northern
Europe and Siberia. At one time during this great Ice Age, approximately one- third of the
earth's land surface was buried beneath glacial ice.
The colder temperatures
of the Pleistocene Epoch had a profound effect on the life of that time.
Some forms which had been abundant during the Tertiary Period could not adjust to the frigid
glacial climate and, failing to adapt, they became extinct. But more hardy creatures such as
the mastodon, woolly mammoth, musk ox, and woolly rhinoceros adapted to the chilly climate
and ranged far and wide.
Among other well-known
Pleistocene animals are the sabre- toothed cat, giant dire wolf, huge
ground sloths, and the thick-shelled glyptodonts. The latter were large armadillo- like mammals
which were almost as large as a Volkswagen.
However, the big "news"
in Pleistocene time was the appearance of man. Although manlike
creatures or near humans developed much earlier, man as we know him today probably
appeared some 600,000 years ago. Considering the great age of the earth and how long life
has been present on it, man is clearly a relatively new addition to the geologic scene. The most
recent part of the Quaternary Period is the Holocene, or Recent Epoch. This, the latest
chapter
in Earth's history, began about 11,000 years ago and continues to this very instant. |
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