The Problems of Plant Distribution 249 



analogies in the plants, although these may not be 

 at all closely related. For example, both the Cape 

 and California are characterized by a very large 

 number of showy bulbous plants, but those in Cali- 

 fornia are mostly of the Lily family, while in South 

 Africa it is the Iris family which is especially de- 

 veloped. The cacti and century-plants of our 

 Southwest are replaced in the drier parts of Africa 

 by the leafless Euphorbias and Aloes, which super- 

 ficially resemble to a remarkable degree the Amer- 

 ican cacti and agaves, but are not at all closely re- 

 lated to them botanically. It is probable that most 

 of the existing plant types were pretty well differ- 

 entiated in the later Tertiary and, as the fossil rec- 

 ords show, the flora was fairly uniform over the 

 Northern Hemisphere. At that period there is evi- 

 dence that many existing genera, which are now 

 restricted in their range, were widespread. In Eu- 

 rope, and even in Siberia and Greenland, there are 

 found remains of such genera as Sequoia, Tax- 

 odium, Liriodendron, Magnolia, Sassafras; and 

 laurels, and even palms abounded, all of which 

 have long since vanished from these regions, but 

 whose descendants still flourish in some more or 

 less isolated regions, where they have survived the 

 great readjustment of the flora, resulting from the 

 Glacial epoch. 



At the present day we may recognize a sub-polar 

 zone, north and south temperate, and tropical zones, 

 which of course are not absolutely denned. The 



