We have already seen that these victories of the sea are just as stimulating, in a different way, to animals as the victories of the land. American geologists are tracing, in a very instructive way, the effect on that early population of the encroachment of the sea. In each arm of the sea is a distinctive fauna. Life is still very parochial; the great cosmopolitans, the fishes, have not yet arrived. As the land is revelled, the arms of the sea approach each other, and at last mingle their waters and their populations, with stimulating effect. Provincial characters are modified, and cosmopolitan characters increase in the great central sea of America. The vast shallow waters provide a greatly enlarged theatre for the life of the time, and it flourishes enormously. Then, at the end of the Ordovician, the land begins to rise once more. Whether it was due to a fresh shrinking of the crust, or to the simple process we have described, or both, we need not attempt to determine; but both in Europe and America there is a great emergence of land. The shore-tracts and the shallow water are narrowed, the struggle is intensified in them, and we pass into the Silurian age with a greatly reduced number but more advanced variety of animals. In the Silurian age the sea advances once more, and the shore-waters expand. There is another great "expansive evolution" of life. But the Silurian age closes with a fresh and very extensive emergence of the land, and this time it will have the most important consequences. For two new things have meantime appeared on the earth. The fish has evolved in the waters, and the plant, at least, has found a footing on the land. These geological changes which we have summarised and which have been too little noticed until recently in evolutionary studies, occupied 7,000,000 years, on the lowest estimate, and probably twice that period. The impatient critic of evolutionary hypotheses is apt to forget the length of these early periods. We shall see that in the last two or three million years of the earth's story most extraordinary progress has been made in plant and animal development, and can be very fairly traced. How much advance should we allow for these seven or fourteen million years of swarming life and changing environments? We cannot nearly cover the whole ground of paleontology for the period, and must be content to notice some of the more interesting advances, and then deal more fully with the evolution of the fish, the forerunner of the great land animals.