Page:The origin of continents and oceans - Wegener, tr. Skerl - 1924.djvu/175

Rh but only about 30 km. thick, and have been covered with a “Panthalassa,” of an average thickness calculated by A. Penck as 2.64 km., which probably left exposed only small portions, or none at all, of the earth’s surface.

The correctness of this idea is in any case demonstrated on two grounds, namely, the evolution of life on the earth and the tectonic structure of the continental blocks.

“Certainly no one can seriously doubt but that the life of the fresh-water as well as of the dry land and of the air has come from the sea.” Before the Silurian we do not know of any air-breathing animals; the oldest remains of land plants are obtained from the Upper Silurian of Gothland. According to Gothan, those from the Lower Devonian are still, in the main, only moss-like plants without true foliage. “Traces of real, spreading leaves are rare in the Lower Devonian. Nearly all the plants were small, herb-like, and of slight rigidity.” On the other hand, the flora in the Upper Devonian is already similar to that of the Carboniferous, and evolves “through the appearance of large, well-developed, veined leaf-blades, by the accomplishment of the division of labour in the plant as a result of the development of the supporting and assimilating organs. … The character of the flora of the Lower Devonian, its lowly organization, its small size, etc., suggest the opinion, already expressed by Potonié, Lignier, Arber, among others, that the land flora was derived from the water. The advance observed in the Upper Devonian is to be understood as an adaptation to the new mode of life in the air.”

It appears, on the other hand, as if the sial crust