The study of fossil flora not only enables us to follow the evolution of plants from their remotest known ancestors to their present actual descendants, but it throws much light upon the past mysteries of the earth, and especially upon the climatic conditions which controlled its surface while the slow revolutions of organic life were going on. We know what numerous causes concur to form a climate; latitude and longitude, the direction of winds and of currents of water, the nature and relief of the soil, and the distance from the sea. All these causes have their respective known effects, and have acted in the past as they act now; yet we know that, if it were needful to determine the amount of influence due to each of these agencies in the different geological epochs, we could not do it; the difficulties are too great. But in one case, that of latitude, we can find out its ancient effects by analogy with what is passing under our eyes, and by abstracting all other influences. We know that the obliquity of the sun's rays increases with latitude, and that temperature diminishes in the same proportion; that the higher the latitude of a region the less heat has its climate. But we know also that vegetation marches with temperature, provided always that soil and moisture are favorable. The floras of the temperate and polar regions show clearly the decrease of temperature from the equator to the pole. There exists between a flora and the climate in which it lives a relation so close that, knowing the one, we can represent the other. Palms do not grow in Greenland nor fir-trees on the plains of equatorial Africa. Each climate has its flora, and each flora its climate.
Paleontology has established the permanence and universality of this law; but it has at the same time established a singular fact which remains inexplicable. It is this: the different climates of the earth have not always been what they are now, either as to temperature or distribution. We speak only of those epochs which have succeeded each other since the time of the most ancient known plants. If we transport ourselves in thought to a time toward the end of the Tertiary period, and then, leaving behind us the Quaternary epoch, follow the course of the ages, we find, as an increasing enlargement of the tropical zone, that which is equivalent to an increase of temperature for the whole earth. More extended in the Pliocene epoch than in our day, this zone was still greater in the Miocene epoch, and yet greater in the Eocene, and so on till we reach a time when it embraced the whole surface of the earth, bestowing everywhere an equal temperature, feebly oscillating between certain limits. This climatic equality, which, according to Saporta, reaches at least as far back as the time of the coal, would probably cease at the epoch of the inferior chalk. Such is the fact established by examination of the flora of different ages.
Let us proceed to details. The Quaternary epoch, contrary to the opinion of the majority of geologists, was not, in France, and probably also in other countries, a period of universal cold. The term
