According to the best established and prevailing views, the great sea-beds, the deep basins, are the original features of the "face of the earth." They are the first depressions that were made after the planet's surface was solidified and its structure began to shrink from cooling. These depressions have probably been increasing vertically through all the geological ages, and therein lies the cause of the constant increase of the land to the present time. But the constancy of this increase is a fact only as a whole; for there have been times when the sea rose over the shore and overflowed a large part of the land. The last great transgression of this kind occurred at the beginning of the Miocene Tertiary, when extensive tracts of the Old World were covered with water. Parts of Italy, Portugal, southern France, northern Switzerland, southern and northern Germany, the Vienna basin, and the Hungarian plain, and of the lowlands of eastern Europe and the Black Sea region away into the interior of Asia to Persia, with the plains of North Africa still bear the marine deposits of that period, with the remains of organisms, mostly of extinct species, that inhabited the adjoining seas. The waters retired to their beds during the Pliocene, and the present boundary-lines of the land regions, aside from a few erosions, were shaped during the same period. Similar processes took place repeatedly in the earlier periods of the earth's growth, and to them are ascribed most of the changes that have taken place in the species of animals and plants; for these encroachments of the sea forced the living world into a narrower space, and entailed a fierce struggle for existence, in which the less valiant species succumbed. The retreat of the sea again permitted a fruitful development of life and the origination of new species.
An important circumstance has been brought to light in the investigation of the fluctuations of the ocean. The continents have been overflowed several times. Suess, who has made the most thorough study of the subject, has recognized six principal periods of submersion, and as many of dryness. But no indubitably deep-sea deposits later than the Silurian are to be found on the present continents. The great sea-beds are primitive; and primitive likewise are the socles of the continents, standing as equally sturdy champions with the briny flood, sometimes in the alternations of the contest lying under it, overcome by the sea, at other times shaking it off and sending it back within its lines.
The cause of these processes is still awaiting explanation. Celsius and Linnæus, who observed the gain of land on the Baltic coast during the last century, expressed the opinion that the sea was retreating. This view was contested at the beginning of the present century by Leopold von Buch, who thought that Scandinavia was rising. Lyell and Darwin advanced the theory of the
