Our modern tailless Amphibia, frogs and toads, appear much later in the story of the earth, but they are not without interest here on account of the remarkable capacity which they show to adapt themselves to different surroundings. There are frogs, like the tree-frog of Martinique, and others in regions where water is scarce, which never pass through the tadpole stage; or, to be quite accurate, they lose the gills and tail in the egg, as higher land-animals do. On the other hand, there is a modern Amphibian, the axolotl of Mexico, which retains the gills throughout life, and never lives on land. Dr. Gadow has shown that the lake in which it lives is so rich in food that it has little inducement to leave it for the land. Transferred to a different environment, it may pass to the land, and lose its gills. These adaptations help us to understand the rich variety of Amphibian forms that appeared in the changing conditions of the Carboniferous world. When we think of the diet of the Amphibia we are reminded of the other prominent representatives of land life at the time. Snails, spiders, and myriapods crept over the ground or along the stalks of the trees, and a vast population of insects filled the air. We find a few stray wings in the Silurian, and a large number of wings and fragments in the Devonian, but it is in the Coal-forest that we find the first great expansion of insect life, with a considerable development of myriapods, spiders, and scorpions. Food was enormously abundant, and the insect at least had no rival in the air, for neither bird nor flying reptile had yet appeared. Hence we find the same generous growth as amongst the Amphibia. Large primitive "may-flies" had wings four or five inches long; great locust-like creatures had fat bodies sometimes twenty inches in length, and soared on wings of remarkable breadth, or crawled on their six long, sprawling legs. More than a thousand species of insects, and nearly a hundred species of spiders and fifty of myriapods, are found in the remains of the Coal-forests.