SAPORTA'S WORLD OF PLANTS. 451 



if these latitudes at this time were disposed as they are now — that is, 

 if the earth's axis has not been displaced — all the earth received more 

 heat, and the line of the tropics must have risen toward the north. 

 The difference of the Miocene period may be valued at 25° or 30° of 

 latitude — that is, we must at present descend 40° or 45° to find the 

 temperature which then existed in Greenland. 



The study of the more ancient floras brings new proof of this phe- 

 nomenon of the extension of heat into the higher latitudes, and con- 

 ducts us finally to that equality of climate of which we have spoken. 

 " We are forced to conclude, hoAvever," remarks Saporta, " that when 

 we reach the time of the coal and the most remote period in the his- 

 tory of organic beings, if there has been no change in the relations of 

 the heat that falls upon our globe, there have doubtless been other 

 changes profound enough to impress upon it a very different aspect 

 from that which it has since presented, and to create conditions of 

 existence about which we can form no idea." "We are, in fact, igno- 

 rant of the conditions in which living beings first made their appear- 

 ance and were developed. There have been many hypotheses about 

 it, but the facts on which they rest are yet neither sufficiently numer- 

 ous nor convincing. For the settlement of this question we must 

 await the future. 



As to the cause of climatic equality over all the earth in the Pri- 

 mary and Secondary epochs, we are equally in the dark. All the 

 explanations that have been given have been successively rejected. 

 The displacement of the axis of the earth, the inclination of that axis 

 on the orbit of our planet, the precession of the equinoxes, etc., are 

 some of the hypotheses put forth on this subject. We can not here enu- 

 merate them all ; but there is one on which Saporta insists, not because 

 it explains everything, but because it agrees more or less with the cele- 

 brated cosmogony of Laplace, and accords with the phenomena of the 

 primitive world as revealed by science. This hypothesis was put forth 

 some years ago by M. Blaudet. We know that, according to the 

 theory of Laplace, the entire solar system was originally an immense 

 nebula which has since condensed little by little, and successively given 

 off rings of cosmical matter which have become planets. The central 

 star is hence more and more reduced, has become more dense, more 

 luminous and more ardent, until it has attained the dimensions and 

 properties of our actual sun. In other words, if we could trace back- 

 ward the course of the ages, we should find the sun progressively aug- 

 menting in volume, but its heat and light would diminish in intensity 

 in the same proportion. We do not know what sun lighted the earth 

 when life first appeared upon it, but, from the theory of Laplace, we 

 may suppose that it was much larger than ours. 



Such conditions, however, would explain many phenomena. This 

 great sun, occupying a good part of the horizon, would give a twilight 

 so luminous and so prolonged as perhaps to annul the night. Sending 



