THIRD OBSTACLE 177 



During this period of free larval existence it is at the 

 mercy of winds and currents ; these and its own move- 

 ments carry it far and wide from the place of its birth, 

 and thus it becomes distributed over large areas. What 

 is true of the oyster is true also of thousands of other 

 kinds of organisms, and the peopling of the sea by slow- 

 moving or stationary animals has been accomplished 

 chiefly, if not wholly, in this manner. But obviously no 

 new forms can have been introduced into existing river 

 systems through the agency of free swimming larvae, for 

 these fragile and feeble forms can by no means make 

 headway against the steady flow of a river ; indeed, as a 

 matter of observation, larvae are not known to swim 

 against currents, but rather along with them. And thus 

 the method which has been most potent in disseminating 

 organisms through the sea has been wholly inoperative in 

 transferring them to a freshwater habitat. 



Further than this, if some slow-moving animal should 

 manage in the adult state to work its way from the 

 mouth of a river for a short distance up-stream, it could 

 seldom succeed in permanently establishing itself so long 

 as it passed through a free larval stage, for its larvae 

 would be carried out to sea, where they would perish or 

 resume the ancestral habit. 



Swift-moving animals such as fish are, of course, more 

 advantageously situated, since they can rapidJy travel a 

 long way against a rapid stream, and may easily find 

 some sheltered recess or quiet bend wherein their young 

 may be brought to maturity. 



Thus to difference in climate and in composition we 

 now add a third obstacle, quite as efficient as either of 

 the others, perhaps more so ; and these, which must 

 frequently act together, sufficiently bar the way against 

 any general immigration from the sea. But, again, 

 success engenders success, and the exclusion of some 



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