Page:Popular Science Monthly Volume 29.djvu/378

364 rapidity of descent of the curve. The abscissas correspond to the equal decrements in equal times; the ordinates, to the fall in heat-retaining power. Up to the heavy line, counting from the right, the ordinates are proportional to the numbers in Professor Tyndall's table (page 354), which were obtained by measurements. The others are estimated by continuing the curve to the left. If we suppose the first abscissa on the right represents the Tertiary, the next three will represent the Mesozoic; the next twelve the Palæozoic, and an unknown number beyond will represent the Archæan.



All that is known about the changes of temperature in the geological climate is derived from observing the changes in the plants and animals. These changes were world-wide, and hence were due to a world-wide cause. None sufficiently broad has been imagined other than a change of temperature, or a change in the purity of the atmosphere. The peculiar life of the Palæozoic lasted several times as long as that of the Mezosoic; and that, in turn, lasted several times as long as that of the Tertiary. Or, calling the last 1, the ratio was 12:3:1. The millions of years of the Palæozoic brought a certain increase of purity and decrease of temperature, and were followed by an almost complete extermination of species, owing to their non-adaptation to the new conditions. The Mesozoic, although only one fourth as long, brought probably quite as great a change, and was followed by "one of the most complete exterminations on record." Through the Eocene and Miocene, although not more than one fifteenth as long as the preceding, the fall of temperature was actually greater than in them all combined, and this was far exceeded in the rest of the Tertiary, for, in the Miocene, cypresses and magnolias grew within 10° of the pole, and, at the end of the Pliocene, that dread winter began to set in which is known as the Glacial epoch. There was a correspondingly rapid change in the plants and animals. Whether it is a matter of accident, or whether they stand in relation of cause and effect, the