record of life is such as it would be if the warmth were due to a blanket of carbonic acid and water-vapor, and the temperature fell in accordance with Tyndall's law.
It is also very suggestive that, while, in the earlier periods, the changes in plants and animals were world-wide, the Tertiary changes were more and more confined to high latitudes, as if the cold were setting down from the poles toward the equator. Such was the effect to be expected if the early warmth was due to the warm blanket of CO2 and aqueous vapor. If that was growing thinner, it would be long before any sensible effect would be produced; but, when it did appear, it would first manifest itself near the poles, where less solar heat was received, and where so much depended upon the heat being retained, and from the polar regions it would spread toward the equator. With these facts in view, there is no difficulty in seeing why the flora of temperate or even warmer regions should have had their origin in very high latitudes, since it was there that a temperature first appeared which was adapted to their needs.
I have purposely avoided speaking of how much CO2 the air can contain and support life. I doubt very much the possibility of saying what the limits were in those remote times when not merely every species, but every genus, was exceedingly unlike any now living. They may have been adapted to conditions fatal to any creature known to us. It is certain that as the air grew purer the early animals died, and were replaced by others more like those now living. Present animals, or even human beings, according to Professor Remsen, of Johns Hopkins University, can breathe an atmosphere containing five per cent of carbonic acid "without experiencing serious or even disagreeable effects." That is, the present amount of CO2 could be increased one hundred and fifty times, and more, without "even disagreeable effects." If this be true, the fact that the animals of those early times flourished, is no reason why we may not believe that the atmosphere contained many hundred times as much carbonic acid as it does now.
Accounting for the uniformity of biological conditions, including in that term heat, light, and actinic forces, solves only a part of the climatic problem. The cold which followed must also be accounted for, as well as the return of a mild climate to regions so long covered with ice. The former was a corollary of the causes already discussed. It was due to the combined effect of a perpendicular axis and a purer atmosphere, aided by those high latitude uplifts which occurred at or soon after the close of the Pliocene. The warm blanket being removed, the natural effects of an upright axis began to show themselves. It was the same as if the sun got no farther north than it does now on the 21st of March. Since the cold of the vernal equinox is in part the residuum of winter, it will be near the truth to say that, with the axis perpendicular, the temperature would be the same as now in April. The present flora would die out, and it would
