272 PLANTS, SEEDS, AND CURRENTS 



sea-border by causing the precipitation of the moisture carried by 

 the prevalent south-west winds before they reach the land, so in 

 the past a cold current from the north swept along the northern 

 shores of the Bahamas and cut off the moisture carried by the north- 

 east trade-winds. From tljis point of view the seolian formation of 

 the Bahamas represents the New World's response in ancient times 

 to the influences producing arid sea-borders in warm latitudes (cold 

 currents and drying winds), but in a region of calcareous sands and 

 under insular conditions, an association of circumstances not repeated 

 on the same scale in any other part of the globe. 



At present, as Commander Campbell Hepworth informs me, the 

 influence of the Labrador or Arctic Current as a surface- stream may 

 be traced along the shores of the United States as far as the Florida 

 Straits during most of the year. But how little do we know of the 

 arrangement of the surface-currents in earlier times ; and what, we 

 may also ask, would be the behaviour of the Arctic Current if 

 the great Equatorial Current, from which the Gulf Stream takes its 

 rise, flowed in mass, as it probably once did, across the site of the 

 present Panama Isthmus into the Pacific? The effect of the 

 emergence of this isthmus was the birth of the Gulf Stream, and the 

 effect of the Gulf Stream was the dissipation to a large extent in 

 tropical latitudes of the cold current from the north, so that the 

 Bahamian region would no longer come under its influence. Before 

 the change we had here the same association of aridity with a cold 

 current that exists on the sea-border of Peru and also on the sea- 

 borders of South California and North Mexico, where the moisture- 

 laden winds, by passing over the cool waters of the Californian 

 Current, become drying winds when striking the continent and 

 produce the aridity of its sea-border. In the cases of the South 

 African Current and the west coast of South Africa and of the West 

 Australian Current and Western Australia we seem to have the 

 same association of conditions. [For further particulars of the 

 writer's views on the influence of the Humboldt Current on the 

 climate of the Pacific border of South America reference should be 

 made to pp. 490-6 and 500 of his work on Plant Dispersal (1905)]. 



I may add that the association of the birth of the Gulf Stream 

 with the elevation of the Panama Isthmus is by no means a new 

 idea. The various rearrangements of the land-areas in the tropics 

 of the New World adopted by some botanists, zoologists and geolo- 

 gists often involve the shutting off of the Gulf Stream from the 

 North Atlantic. However much Darwin's great authority may have 

 weighted the scale against them, the " extensionists " have obtained 

 results in these tropical regions that call for a re- valuation of their 

 testimony. 



The Standpoint of the Extensionists. — Reference may again 

 be made to the relations between the western Bahamas and 

 the adjacent islands of the Greater Antilles. Though depths of 2000 

 fathoms divide the eastern Bahamas from Hispaniola, much shallower 

 submarine connections exist between the Great Bahama Bank and 

 Cuba, and the possibility of a Cuban land-connection in times that 

 are past cannot be ignored. This leads one to refer to an alternative 



