But how can these tree-dwellers bring up their little ones in water? Some of them come down and lay their eggs in the ponds, and even sleep down in the mud in winter. Others lay their eggs in little puddles of water in the hollows of the trees, and there the young ones live their tadpole life; while in one curious tree-frog of Mexico, called the _Nototrema_, the mother has a pouch in her back, and the father places the eggs in it for the little tadpoles to live in a moist home till they leap out as perfect frogs. Nor is this the only case in which fathers and mothers take care of their young. In one species of frogs living near Paris, the father[63] winds the long string of gluey eggs round his thighs, and buries himself in the ground till the young tadpoles are ready to come out, and then he leaps into the water. And in one of the tongueless toads, the Surinam toad,[64] the mother’s soft skin swells up, forming ridges and hollows, and when her eggs are laid the father clasps them in his feet, and, leaping on her back, puts an egg into each hollow. Then the mother goes into the water, and remains there while each tadpole completes its changes in its own hole, jumping out at last a finished toad. Yet, in spite of curious habits such as these, the frogs and their companions on the whole lead a very monotonous life. They are, it is true, more intelligent than fish, and have learned to know more of the world, but in the long ages that have passed since their ancestors roamed in the coal-forest marshes, other and higher animals have taken possession of the land, and left room only for a few scattered groups of _amphibia_. Still, however, they remain hovering between two lives, and filling such spots as neither the fishes nor the land animals can occupy; and when we hear them croaking in the quiet night, or see them leaping on the marshy ground, they remind us that we have still living in our day, a link between the fish whose world is a world of waters, and the air-breathing animals which have become masters of the land. [Illustration: THE REPTILES IN THEIR PALMY DAYS.] CHAPTER V. THE COLD-BLOODED AIR-BREATHERS OF THE GLOBE IN TIMES BOTH PAST AND PRESENT. And now the transformation is complete, for when we pass on to the next division of backboned animals, the “Reptiles,” we hear nothing more of gills, nor air taken from the water, nor fins, nor fishes’ tails. From this time onward all the animals we shall study live with their heads in the air, even if their bodies may be in the water; they swim with their legs or, as in the case of the snakes, with their wriggling bodies, and they lay their eggs on the land where their young begin life at once as air-breathers.