The alternate rise and fall of the waters covering the geosynclines during these periods caused a continual alteration in the types of marine fauna that swarmed in these shallow seas. Only those types managed to survive that could adapt themselves to the new condition of each period. This explains the difference of the faunas in successive rock formations, so that a trained eye can distinguish at a glance an Ordovician from a Cilurian fauna, and this from a Devon ian, and so forth.
But if the Interpulsation periods were periods of trial for the marine faunas, they were favourable to the advance of the land faunas. Indeed, "wherever some of the marine forms were cut off locally in basins left by the retreat of the sea, these basins, on drying, forced the survivors on to the land or to enter the rivers. Thus the fresh-water and the air-breathing land fauna arose under the compelling influence of changing environment.
From the survivors of the river types of earlier days the reptilian fauna developed. River lizards adventured upon the land, and from them developed the dinosaurus. During the era of Reptilian life, or the Mesozoic period, as it is called, the most bizarre and groetsques types, as well as the-giants of that period, developed. Their bulk brought about their destruction, for the food supply was not sufficient for their requirements,
Text read from the printed page and cleaned; old type leaves the occasional error.