SAPORTA'S WORLD OF PLANTS. 447 



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 in- 

 fluences. 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 al- 

 ways 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 repre- 

 sent 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 re- 

 mains inexplicable. It is this : the different climates of the earth have 

 not always been what they are now, either as to temperature or distri- 

 bution. We speak only of those epochs which have succeeded each 

 other since the time of the most ancient known plants. If we trans- 

 port oiirselves in thought to a time toward the end of the Tertiary 

 period, and then, leaving behind us the Quaternaiy 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 



