3 Natural Selection and Changes of Species

Now here it will be well to put plainly certain general facts about this new thing, life, that was creeping in the shallow waters and intertidal muds of the early Palaeozoic period, and which is perhaps confined to our planet alone in all the immensity of space.

Life differs from all things whatever that are without life in certain general aspects. There are the most wonderful differences among living things to-day, but all living things past and present agree in possessing a certain power of growth, all living things take nourishment, all living things move about as they feed and grow, though the movement be no more than the spread of roots through the soil, or of branches in the air. Moreover, living things reproduce; they give rise to other living things, either by growing and then dividing or by means of seeds or spores or eggs or other ways of producing young. Reproduction is a characteristic of life.

No living thing goes on living for ever. There seems to be a limit of growth for every kind of living thing. Among very small and simple living things, such as that microscopic blob of living matter the Amoeba, an individual may grow and then divide completely into two new individuals, which again may divide in their turn. Many other microscopic creatures live actively for a time, grow, and then become quiet and inactive, enclose themselves in an outer covering and break up wholly into a number of still smaller things, spores, which are released and scattered and again grow into the likeness of their parent. Among more complex creatures the reproduction is not usually such simple division, though division does occur even in the case of many creatures big enough to be visible to the unassisted eye. But the rule with almost all larger beings is that the individual grows up to a certain limit of size. Then, before it becomes unwieldy, its growth declines and stops. As it reaches its full size it matures, it begins to produce young, which are either born alive or hatched from eggs. But all of its body does not produce young. Only a special part does that. After the individual has lived and produced offspring for some time, it ages, and dies. It does so by a sort of necessity. There is a practical limit to its life as well as to its growth. These things are as true of plants as they are of animals. And they are not true of things that do not live. Non-living things, such as crystals, grow, but they have no set limits of growth or size, they do not move of their own accord and there is no, stir within them. Crystals once formed may last unchanged for millions of years. There is no reproduction for any non-living thing.

This growth and dying and reproduction of living things leads to some very wonderful consequences. The young which a living thing produces are either directly, or after some intermediate stages and changes (such as the changes of a caterpillar and butterfly), like the parent living thing. But they are never exactly like it or like each other. There is always a slight difference, which we speak of as individuality. A thousand butterflies this year may produce two or three thousand next year; these latter will look to us almost exactly like their predecessors, but each one will have just that slight difference. It is hard for us to see individuality in butterflies because we do not observe them very closely, but it is easy for us to see it in men. All the men and women in the world now are descended from the men and women of A.D. 1800, but not one of us now is exactly the same as one of that vanished generation. And what is true of men and butterflies is true of every sort of living thing, of plants as of animals. Every species changes all its individualities in each generation. That is true of all the minute creatures that swarmed and reproduced and died in the Archaeozoic and Proterozoic seas, as it is of men to-day.

Every species of living things is continually dying and being born again, as a, multitude of fresh individuals.

Consider, then, what must happen to a new-born generation of living things of any species. Some of the individuals will be stronger or sturdier or better suited to succeed in life in some way than the rest, many individuals will be weaker or less suited. In particular single cases any sort of luck or accident may occur, but on the whole the better equipped individuals will live and grow up and reproduce themselves and the weaker will as a rule go under. The latter will be less able to get food, to fight their enemies and pull through. So that in each generation there is as it were a picking over of a species, a picking out of most of the weak or unsuitable and a preference for the strong and suitable. This process is called Natural Selection or the Survival of the Fittest.[^Fn_§§.1]

It follows, therefore, from the fact that living things grow and breed and die, that every species, so long as the conditions under which it lives remain the same, becomes more and more perfectly fitted to those conditions in every generation.

But now suppose those conditions change, then the sort of individual that used to succeed may now fail to succeed and a sort of individual that could not get on at all under the old conditions may now find its opportunity. These species will change, therefore, generation by generation; the old sort of individual that used to prosper and dominate will fail and die out and the new sort of individual will become the rule,?until the general character of the species changes.

Suppose, for example, there is some little furry whitey brown animal living in a bitterly cold land which is usually under snow. Such individuals as have the thickest, whitest fur will be least hurt by the cold, less seen by their enemies, and less conspicuous as they seek their prey. The fur of this species will thicken and its whiteness increase with every generation, until there is no advantage in carrying any more fur.

Imagine now a change of climate that brings warmth into the land, sweeps away the snows, makes white creatures glaringly, visible during the greater part of the year and thick fur an encumbrance. Then every individual with a touch of brown in its colouring and a thinner fur will find itself at an advantage, and very white and heavy fur will be a handicap. There will be a weeding out of the white in favour of the brown in each generation. If this change of climate come about too quickly, it may of course exterminate the species altogether; but if it come about gradually, the species, although it may have a hard time, may yet be able to change itself and adapt itself generation by generation. This change and adaptation is called the Modification of Species.

Fig. 16 – Life in the Later Paleozoic Age

Perhaps this change of climate does not occur all over the lands inhabited by the species, maybe it occurs only on one side of some great arm of the sea or some great mountain range or such-like divide, and not on the other. A warm ocean current like the Gulf Stream may be deflected, and flow so as to warm one side of the barrier, leaving the other still cold. Then on the cold side this species will still be going on to its utmost possible furriness and whiteness and on the other side it will be modifying towards brownness and a thinner coat. At the same time there will probably be other changes going on; a difference in the paws perhaps, because one half of the species will be frequently scratching through snow for its food, while the other will be scampering over brown earth. Probably also the difference of climate will mean differences in the sort of food available, and that may produce differences in the teeth and the digestive organs. And there may be changes in the sweat and oil glands of the skin due to the changes in the fur, and these will affect the excretory, organs and all the internal chemistry of the body. And so through all the structure of the creature. A time will come when the two separated varieties of this formerly single species will become so unlike each other as to be recognizably different species. Such a splitting up of a species in the course of generations into two or more species is called the Differentiation of Species.

And it should be clear to the reader that given these elemental facts of life, given growth and death and reproduction with individual variation in a world that changes, life must change in this way, modification and differentiation must occur, old species must disappear, and new ones appear. We have chosen for our instance here a familiar sort of animal, but what is true of furry beasts in snow and ice is true of all life, and equally true of the soft jellies and simple beginnings, that flowed and crawled for hundreds of millions of years between the tidal levels and in the shallow, warm waters of the Proterozoic seas.

The early life of the early world when the blazing sun rose and set in only a quarter of the time it now takes, when the warm seas poured in great tides over the sandy and muddy shores of the rocky lands and the air was full of clouds and steam, must have been modified and varied and species must have developed at a great pace. Life was probably as swift and short as the days and years; the generations, which natural selection picked over, followed one another in rapid succession.

Natural selection is a slower process with man than with any other creature. It takes twenty years or more before an ordinary human being in western Europe grows up and reproduces. In the case of most animals the new generation is on trial in a year or less. With such simple and lowly beings, however, as first appeared in the primordial seas, growth and reproduction was probably a matter of a few brief hours or even of a few brief minutes. Modification and differentiation of species must accordingly have been extremely rapid, and life had already developed a great variety of widely contrasted forms before it began to leave traces in the rocks. The Record of the Rocks does not begin, therefore, with any group of closely related forms from which all subsequent and existing creatures are descended. It begins in the midst of the game, with nearly every main division of the animal kingdom already represented. Plants are already plants, and animals animals. The curtain rises on a drama in the sea that has already begun, and has been going on for some time. The brachiopods are discovered already in their shells, accepting and consuming much the same sort of food that oysters and mussels do now; the great water scorpions crawl among the seaweeds, the trilobites roll up into balls, and unroll and scuttle away. In that ancient mud and among those early weeds there was probably as rich and abundant and active a life of infusoria and the like as one finds in a drop of ditchwater to-day. In the ocean waters, too, down to the utmost downward limit to which light could filter, then as now, there was an abundance of minute and translucent, and in many cases phosphorescent, beings.

But though the ocean and intertidal waters already swarmed with life, the land above the high-tide line was still, so far as we can guess, a stony wilderness without a trace of life.

4 The Invasion of the Dry Land by Life

4.1 Life and Water

Wherever the shore line ran there was life, and that life went on in and by and with water as its home, its medium, and its fundamental necessity.

The first jelly-like beginnings of life must have perished whenever they got out of the water, as jelly-fish dry up and perish on our beaches to-day. Drying up was the fatal thing for life in those days, against which at first it had no protection. But in a world of rain-pools and shallow seas and tides, any variation that enabled a living thing to hold out and keep its moisture during hours of low tide or drought met with every encouragement in the circumstances of the time. There must have been a constant risk of stranding. And, on the other hand, life had to keep rather near the shore and beaches in the shallows because it had need of air (dissolved of course in the water) and light.

No creature can breathe, no creature can digest its food, without water. We talk of breathing air, but what all living things really do is to breathe oxygen dissolved in water. The air we ourselves breathe must first be dissolved in our lungs; and all our food must be liquefied before it can be assimilated. Water-living creatures which are always under water, wave the freely exposed gills by which they breathe in that water, and extract the air dissolved in it. But a creature that is to be exposed for any time out of the water must have its body and its breathing apparatus protected from drying up. Before the seaweeds could creep up out of the Early Palaeozoic seas into the intertidal line of the beach, they had to develop a tougher outer skin to hold their moisture.

Before the ancestor of the sea scorpion could survive being left by the tide it had to develop its casing and armour. The trilobites probably developed their tough covering and rolled up into balls, far less as a protection against each other and any other enemies they may have possessed, than as a precaution against drying. And when presently, as we ascend the Palaeozoic rocks, the fish appear, first of all the back-boned or vertebrated animals, it is evident that a number of them are already adapted by the protection of their gills with gill covers and by a sort of primitive lung swimming-bladder, to face the same risk of temporary stranding.

Now the weeds and plants that were adapting themselves to intertidal conditions were also bringing themselves into a region of brighter light, and light is very necessary and precious to all plants. Any development of structure that would stiffen them and hold them up to the light, so that instead of crumping and flopping when the waters receded, they would stand up outspread, was a great advantage. And so we find them developing fibre and support, and the beginning of woody fibre in them. The early plants reproduced by soft spores, or half-animal «gametes», that were released in water, were distributed by water and could only germinate under water. The early plants were tied, and most lowly plants today are tied, by the conditions of their life cycle, to water. But here again there was a great advantage to be got by the development of some protection of the spores from drought that would enable reproduction to occur without submergence. So soon as a species could do that, it could live and reproduce and spread above the, high-water mark, bathed in light and out of reach of the beating and distress of the waves. The main classificatory divisions of the larger plants mark stages in the release of plant life, from the necessity of submergence by the development of woody support and of a method of reproduction that is more and more defiant of drying up. The lower plants are still the prisoner attendants of water. The lower mosses must live in damp, and even the development of the spore of the ferns demands at certain stages extreme wetness. The highest plants have carried freedom from water so far that they can live and reproduce if only there is some moisture in the soil below them. They have solved their problem of living out of water altogether.

The essentials of that problem were worked out through the vast aeons of the Proterozoic Age and the early Palaeozoic Age by nature’s method of experiment and trial. Then slowly, but in great abundance, a variety of new plants began to swarm away from the sea and over the lower lands, still keeping to swamp and lagoon and water-course as they spread.

4.2 The Earliest Animals

And after the plants came the animal life.

There is no sort of land animal in the world, as there is no sort of land plant, whose structure is not primarily that of a water-inhabiting being which has been adapted through the modification and differentiation of species to life out of the water. This adaptation is attained in various ways. In the case of the land scorpion the gill-plates of the primitive sea scorpion are sunken into the body so as to make the lungbooks secure from rapid evaporation. The gills of crustaceans, such as the crabs which run about in the air, are protected by the gill-cover extensions of the back shell or carapace. The ancestors of the insects developed a system of air pouches and air tubes, the tracheal tubes, which carry the air all over the body before it is dissolved. In the case of the vertebrated land animals, the gills of the ancestral fish were first supplemented and then replaced by a bag-like growth from the throat, the primitive lung swimming-bladder. To this day there survive certain mudfish which enable us to understand very clearly the method by which the vertebrated land animals worked their way out of the water. These creatures (e.g. the African lung fish) are found in tropical regions in which there is a rainy full season and a dry season, during which the rivers become mere ditches of baked mud. During the rainy season these fish swim about and breathe by gills like any other fish. As the waters of the river evaporate, these fish bury themselves in the mud, their gills go out of action, and the creature keeps, itself alive until the waters return by swallowing air, which passes into its swimming-bladder. The Australian lung fish, when it is caught by the drying up of the river in stagnant pools, and the water has become deaerated and foul, rises to the surface and gulps air. A newt in a pond does exactly the same thing. These creatures still remain at the transition stage, the stage at which the ancestors of the higher vertebrated animals were released from their restriction to an under-water life.

Fig. 22 – Australian Lung Fish

The amphibia (frogs, newts, tritons, etc.) still show in their life history all the stages in the process of this liberation. They are still dependent on water for their reproduction; their eggs must be laid in sunlit water, and there they must develop. The young tadpole has branching external gills that wave in the water; then a gill cover grows back over them and forms a gill chamber.

Then as the creature’s legs appear and its tail is absorbed, it begins to use its lungs, and its gills dwindle and vanish. The adult frog can live all the rest of its days in the air, but it can be drowned if it is kept steadfastly below water. When we come to the reptile, however, we find an egg which is protected from evaporation by a tough egg case, and this egg produces young which breathe by lungs from the very moment of hatching. The reptile is on all fours with the seeding plant in its freedom from the necessity to pass any stage of its life cycle in water.

The later Palaeozoic Rocks of the northern hemisphere give us the materials for a series of pictures of this slow spreading of life over the land. Geographically, all round the northern half of the World it was an age of lagoons and shallow seas very favourable to this invasion. The new plants, now that they had acquired the power to live this new aerial life, developed with an extraordinary richness and variety.

There were as yet no true flowering plants[2] no grasses nor trees that shed their leaves in winter;[3] the first «flora» consisted of great tree ferns, gigantic equisetums, cycad ferns, and kindred vegetation. Many of these plants took the form of huge-stemmed trees, of which great multitudes of trunks survive fossilized to this day. Some of these trees were over a hundred feet high, of orders and classes now vanished from the world. They stood with their sterns in the water, in which no doubt there was a thick tangle of soft mosses and green slime and fungoid growths that left few plain vestiges behind them. The abundant remains of these first swamp forests constitute the main coal measures of the world to-day.

Fig. 23 – Some Reptiles of the Later Palaeozoic Age

Amidst this luxuriant primitive vegetation crawled and glided and flew the first insects. They were rigid-winged, four-winged creatures, often very big, some of them having wings measuring a foot in length. There were numerous dragon flies – one found in the Belgian coal-measures had a wing span of twenty-nine inches! There were also a great variety of flying cockroaches. Scorpions abounded, and a number of early spiders, which, however, had no spinnerets for web making. Land snails appeared. So, too, did the first known step of our own ancestry upon land, the amphibia. As we ascend the higher levels of the Later Palaeozoic record, we find the process of air adaptation has gone as far as the appearance of true reptiles amidst the abundant and various amphibia.

The land life of the Upper Palaeozoic Age was the life of a green swamp forest without flowers or birds or the noises of modern insects. There were no big land beasts at all; wallowing amphibia, and primitive reptiles were the very highest creatures that life had so far produced. Whatever land lay away from the water or high above the water was still altogether barren and lifeless. But steadfastly, generation by generation, life was creeping away from the shallow sea-water of its beginning.