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Ch. XL.] 



MIGEATION OF SPECIES. 



301 



of the river Zaire, in Africa, tlie late Dr. Eobert Brown 

 found that thirteen species were also met with on the oppo- 

 site shores of Guiana and Brazil. He remarked that most 

 of these plants were found only on the lower parts of the 

 river Zaire, and were chiefly such as produced seeds capable 

 of retainin g their vitality a Ion g time in the currents of the 



Dr. J. Hooker informs me that after an examination 

 of a great many insular floras, he has found that no one of 

 the large natural orders is so rich in species common to other 



ocean. 



countries, as 



the Leguminosce. 



The seeds in this order, 



which comprises the largest proportion of widely diffused 

 littoral species, are better adapted than those of any other 



plants for water-carriage. 



Mr. Darwin has made a series of experiments to ascertain 

 the number of days for which the seeds and fruits of various 

 plants could be immersed in salt water without injury, and 

 he found that out of 87 kinds, 64 germinated after they had 

 been 28 davs in salt-water, and some survived an immersion 



According to the average rate at which oceanic 

 currents run, he came to the conclusion that a large number 



of 37 days. 



nearly 1,000 miles 



of seeds might be carried uninjured for 



across the sea.* 



Currents and winds in the arctic regions drift along ice- 

 bergs covered with an alluvial soil, on which pine-saplings 

 and a variety of herbaceous plants are seen growing, all of 



some 



■With respect to mar 



familiar rvxuxx ..xx^. >.^^ 



> 



ic ice-island may be strandea. 



Dis])ersion of marine plants.— 



etation, the seeds, being in their native element, may remain 

 immersed in water without injury for indefinite periods, so 

 that there is no diflaculty in conceiving the diffusion of species 

 wherever uncongenial climates, contrary currents, and other 

 causes do not interfere. All a 

 the floating sea-weed ; 



Elung from the rock on ocean's foam to sail, 



Where'er the surge may sweep, the tempest's breath prevail. 



I have before called attention (p. 386) to the interesting 

 fact that one-fifth of all the algse found in the antarctic 

 regions in 1841-3, by Dr. J. Hooker, were of species common 



* Origin of Species, chap. xi. 



