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.

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.

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 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 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.

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 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".

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 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 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 "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.

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.

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.

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.