Both of them have a solid vertebral column in their frames, and therefore both must be classed amongst the vertebrates. But within the limits of this group they differ very considerably from each other. The frog has four well-developed legs, its body terminates in a tail, and it breathes by means of lungs, like a bird, a dog, or a human being. The fish has fins, it swims in the water by means of these fins and its long rudder-like tail, and it breathes the air contained in the water by means of gills. When we arrange the vertebrates in a series, with man at their head, it is perfectly clear that the frog stands higher than the fish in regard to its whole structure. It is lower than the lizard, the bird, or the mammal, but at the same time it is a little nearer to these three than the fish is. That was recognised long ago by Linné, who assigned them a corresponding rank. The fishes are the lowest group of the vertebrates; the frogs belong to the group immediately above them. Now let us see how one of these frogs is developed to-day. The frogs are oviparous (egg-laying) animals. The mother frog lays her eggs in the water, and in the ordinary course of nature a new little frog develops from each of these eggs. But the object that develops from them is altogether different from the adult frog. This object is the familiar tadpole. At first it has no legs, but it has a long oar-like tail, with which it can make its way briskly in the water. It breathes in the water by means of gills just like a fish. It is only when the tadpole grows four legs, loses its tail, closes up the gills at its throat, and begins to breathe by the mouth and lungs instead, that it becomes a real frog. There can be no doubt whatever that the tadpole is very much more like the fish in all the most important particulars than the frog. Between the frog-egg and the frog itself we have a stage of development in each individual case of which we might almost say that the young frog has first to turn into a fish before it can become a frog. How are we to explain this? At first people supposed something like the following: All beings in nature are admirably adapted to their environment and their life-conditions. Whatever be the explanation of it, it is a simple fact. Now, the frog lays its eggs in the water. The young ones develop from these eggs, and find themselves in the water. The most practical adaptation for them is to swim about by means of a tail and breathe by means of gills like the fish. They do not reach land until later, and they creep on to it and have an equipment of the opposite character, with legs and lungs. But this explanation throws no light on the question why the frog lays its eggs in the water. However, there might be some utility or other, some need for protection, for instance, in that. Let us take a few other cases.