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.
- Deciduous trees.