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.
Abich (Sur la structure et la geologic du Daghestan, 1862), the successive folds of Jurassic limestones and slates, all nearly parallel to the Caucasus, which form lofty, narrow plateaus.
The geologic record is, as perhaps is to be expected, exceedingly poor, except as regards the calcareous Siphonales, which are well represented at various horizons, from the Silurian to the Tertiary; even the Diatomaceae, which are found in great quantities in the Tertiary deposits, do not occur at all earlier than the chalk.
