120 LUTHER BURBANK 



So the American species made their way to the 

 region of the Gulf, and some of them even to the 

 southern continent. And when the ice sheet 

 finally receded, they were able to make their way 

 northward again, though never to their former 

 habitat; whereas Europe was treeless until the 

 plant life of Asia spread westward to repeople it. 



Such is the explanation that the paleobotanist 

 gives us of the fact that the indigenous vegeta- 

 tion of America to-day is closely similar to that 

 which stocked the subarctic regions of the entire 

 Northern Hemisphere in the geological period 

 known as the Mesozoic a period that seems 

 infinitely remote when measured in terms of 

 human history, yet which in the scale of time as 

 measured by the geologist is relatively recent. 



Such trees as the sequoia, we are told, are sur- 

 vivors of that ancient regime that chanced to find 

 hospitable shelter on the western slopes of the 

 Sierras. Similarly the tulip tree of the east, with 

 the blossoms that seem anomalous for a tree, 

 should be regarded as the souvenir of a past age 

 a lone representative of vast tribes that once 

 flourished in tropical luxuriance in regions that 

 now give scant support to moss and lichen and 

 stunted conifers. 



All in all, we are told, the remaining vegeta- 

 tion of to-day, varied though it seems, is but a 



