THE INVASION OF THE DRY LAND BY LIFE

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 of 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 the moisture 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 Palæozoic seas into{v1-24} 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 Palæozoic rocks, the fish appear, first of all the backboned 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 crumpling 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 to-day 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{v1-25} 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 æons of the Proterozoic Age and the early Palæozoic 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 watercourse as they spread.

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 lung-books 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,{v1-26} 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.

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 Palæozoic 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,[10] no grasses nor trees that shed their leaves in winter;[11] the first “flora” consisted{v1-27} 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 stems 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{v1-28} of these first swamp forests constitute the main coal-measures of the world to-day.

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.[12] 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 Palæozoic 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 Palæozoic 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.