THE FALL OP THE LEAF. 213 



the rhododendrons, the laurels, and the bay 

 trees, together with our own smaller holly, 

 box, and privet, hardly suffice to convey a 

 notion of the great southern forest trees, clad 

 all the year round in thick green, such as the 

 mangoes, the star-apples, and the sand-boxes. 

 Up to the beginning of the tertiary period, 

 however, large evergreens of what is now the 

 tropical type covered the whole of the world, 

 as far as the very Poles themselves. Green- 

 land and Spitzbergen then supported huge 

 forests of the same general character as those 

 which now spread over Brazil and the Malay 

 Archipelago. But from the first dawn of the 

 eocene onward, some combination of astro- 

 nomical and geographical causes, such as 

 those suggested by Dr. Croll and Mr. A. R. 

 Wallace, began to produce a general chilling 

 of the temperature at either Pole. Perhaps 

 the effect was wholly due, as Dr. Croll be- 

 lieves, to the eccentricity of the earth's orbit 

 and the precession of the equinoxes ; perhaps 

 it was further aided, as Mr. Wallace suggests, 



