though varying greatly in the different zones, has been upon the whole highly favorable to life in general. There is no reason to suppose that it was less so during the whole of Quaternary time, notwithstanding the occurrence in the northern hemisphere of several glacial periods. The Pliocene period that preceded this was perhaps more favorable to life than the early Quaternary, being more equable. The remainder of the Tertiary (Miocene and Eocene) was much warmer, so that vegetation now found at temperate latitudes grew above the arctic circle. Palms were common in the present temperate zone, and animal life was at its high tide. As we go back from the Tertiary through the Cretaceous to the Jurassic, we find evidence that the temperatures generally prevailing were too high to be favorable to the highest development of either plants or animals. We reach at last an almost exclusively coniferous and cycadean flora and a chiefly reptilian and amphibian fauna. When we reach the coal measures, the evidence all points to the existence of a very high temperature over the whole surface of the earth. There was very little difference between that of the equator and that of the poles. The same class of plants grew in Brazil and on Bear Island near Spitzbergen. It is altogether probable that the sun never shone upon the surface of the earth, but that from the almost seething waters there rose a vast atmosphere of vapor through which no ray of sunshine could ever penetrate.
We need not go farther back. It is obvious that none of the higher life of the present could have existed under these conditions. Aside from the necessity of vast periods of time to evolve these higher forms, the world was not ready for them until near the time when they appeared in late Mesozoic and Tertiary time. In a word, instead of there being a decline of life or a deterioration in the conditions necessary to the highest forms of organic existence, there has been a constant improvement in those conditions from the earliest geologic time to the present, and there is nothing to indicate that the optimum conditions have yet been reached. There is every probability that the habitability of the globe is still increasing and will continue
