THE BUILDING OF THE LAND 13 



American tropics. These are, to a Very consid- 

 erable extent, identical with plants found on the 

 Lower Keys. A little over a third of its flora is 

 temperate and warm temperate, having migrated 

 by land from the northward since the beginning 

 of the land elevation. Only a few of these hardier 

 northern plants occur on the keys. It is probable 

 that the Lower Keys formed a single island during 

 the time of this uplift, at which time the Miami 

 mainland was first elevated above the sea. For 

 a long time seeds and animals were carried north- 

 ward by the Gulf Stream and established simul- 

 taneously on both of these land bodies while the 

 present Upper Keys were only a living coral reef. 

 In all probability the present south shore of the 

 mainland was under water at that time and the 

 same is doubtless true of the present southwest 

 coast. Had the Upper Keys been elevated above 

 the sea at that time they would have proven a 

 rather effectual barrier to the landing of tropical 

 life along the old Miami shore. 



The Upper Keys, the extreme southern part of 

 the mainland, and the lower southwest coast are 

 inhabited by a common assemblage of plants, and, 

 to a considerable extent, of animals, which differ 



