The climate in late geologic time was very different from that which prevails to-day.
This principle has been demonstrated recently among Tertiary rhinoceroses and titanotheres, in which remains of four or five genetic series in the same geologic deposits have been discovered.
This is not due to environmental conditions solely, because senescent branches of normal progressive groups are found in all geologic horizons, beginning, for gastropods, in the Lower Cambrian.
These were successive stages in the geologic process which has created, and is still actively modifying, the state.
For its significance as a former (geologic) connexion between the Sea of Azov and the Caspian Sea, see Caspian Sea.
