CURRENTS OF AIR AND OF WATER 



149 



make their way to North America against 

 the prevailing westerly winds. 



As we have seen (p. 146), hurricanes 

 are potent possible forces for over-water 

 transport of organisms. The inner cyclonic 

 wind closest to the hurricane center angles 

 rather sharply upward, and the rate of as- 

 cent probably increases when the cloud 

 zone is reached. The central updraft carries 

 some thousands of feet into the air. The 

 main hurricane wind probably has relatively 

 Uttle Ufting power, but it carries along 

 gusts, in the fonn of small secondary whirls, 

 that may pick up hve shrimp, for example, 

 along with masses of sea water and carry 

 them aloft until the slackening force of the 

 wind releases them to fall back into the sea, 

 sometimes still ahve. These secondary 

 whirls lift and carry palm leaves and other 

 heavy debris for long distances. 



Zoologists usually underestimate the hft- 

 ing and carrying power of winds, partly 

 because, until relatively recent times, men 

 were confined to the ground level and, fur- 

 ther, because men are large animals not 

 readily swept literally from their feet. Small 

 animals have a much higher ratio of surface 

 to weight than do larger ones. This rela- 

 tionship has already been noted in connec- 

 tion with the radiation of heat (Bergmann's 

 rule, p. 119) and with the flotation of 

 plankton (p. 133). Weight increases in pro- 

 portion to the cube of the length, modified 

 by an appropriate factor, and surface simi- 

 larly increases as its square. An animal that 

 weighs an ounce— an adult house mouse, for 

 example— has about fourteen times the ratio 

 of surface to weight as does man. 



Another phase of these possible compari- 

 sons is important: The pressure exerted 

 against a given object varies as the square 

 of the wind's velocity. A gale of 100 miles 

 per hour exerts sixteen times the force of 

 a wdnd that is blowing one-fourth that rate. 

 These values pyramid upon each other in a 

 formidable fashion. A mouse exposed to the 

 full force of a gale of the strength just given 

 would have to meet 14 times 16 or 224 

 times the amount of carrying power as 

 would a man in a wind of 25 miles per 

 hour. The smaller the animal to be com- 

 pared with man, the higher the ratio. It is 

 difficult for those who are not accustomed 

 to handUng aeroplanes in strong winds to 

 appreciate the force such winds exert. Rel- 

 atively heavy smaller animals such as 

 earthworms, tadpoles, frogs, salamanders, 



small fishes, and mollusks are known to 

 have been carried to new locations by wind 

 storms (McAtee, 1917; Gudger, 1921; Dar- 

 lington, 1938a). 



"Dust devils" of tlae dry lands are small 

 whirlwinds, with the conelike apex near the 

 ground, that are made visible by the dust 

 they carry. They also pick up plants and 

 animals such as mice, and even those as 

 big as a kangaroo rat (Dipydumys) . The 

 vertical component of large "dust devils" 

 exceeds a speed of 25 miles per hour (Ives, 

 1947). 



Darlington (1938a) summarizes evidence 

 indicating that the foundation stock of 

 many animals of the Greater Antilles 

 (Cuba, Hispaniola, Jamaica, and Puerto 

 Rico) may have been transported by air 

 from Central America. To be sure, the 

 majority of these storms pass from the 

 islands, to Florida, but there are enough 

 that move from Central America to the 

 islands to satisfy the requirements made 

 by the observed distribution. Three fur- 

 ther considerations support this possibility: 



(1) The more violent, more eflficient 

 right-hand half of the hurricane flows 

 from Central America toward the Antilles. 



(2) This may carry plant debris of con- 

 siderable size which could, in turn, car- 

 ry many different kinds of animals, even 

 those that cannot withstand long exposure 

 to sea water— for example, mites from the 

 forest floor. (3) The geological evidence 

 suggests that the water gap was once nar- 

 rower than at present, although it does not 

 present conclusive evidence of a former 

 land-bridge connection. Certainly the 

 known evidence is not notably inconsistent 

 with the theory that the land animals of 

 these larger islands of the West Indies have 

 descended from waifs deposited after aerial 

 or, in some instances, raft transport from 

 Central America. 



OCEAN CURRENTS 



Temperature relations in the oceans are 

 one of the significant influences in estab- 

 lishing and maintaining ocean currents. 

 These relations can be specifically related 

 to oceanic conditions by an apphcation of 

 the theorem of Bjerknes, which Sverdrup, 

 Johnson, and Fleming (1942) state as fol- 

 lows: "If within a thermal circulation, heat 

 shall be transformed into mechanical 

 energy, the heating must take place at a 

 greater depth (and therefore at a greater 



