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 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.
§ 2
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, 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.
