HABITAT OF THE EARLY VERTEBRATES 409 



That some primitive animal aggregate far back in pre-Cam- 

 brian time should have found refuge from marine persecution or 

 competition in the sweet running water that entered the sea at 

 so many points, and should have evolved on lines in strict con- 

 formity with the dominant force of its new environment does 

 not seem improbable. 



If such were the origin of the vertebrate type, its subse- 

 quent history and the peculiar phases of fossilization previously 

 discussed are natural sequences. 



Distribution from river to river would be slow but inevitable, 

 without the aid of the bizarre agencies of water-fowl, whirlwinds, 

 etc., sometimes appealed to in modern instances. The degra- 

 dation of the land by streams involves inevitably much 

 piracy, and at the stage of capture the two streams are united 

 for a certain period ; and for a still longer period they relieve 

 each other of surplus waters in times of local floods which hap- 

 pen to affect one basin more than the other. Measured by the 

 time requisite for fish migration, these periods of continuous and 

 occasional communication are long. The event itself is, to be 

 sure, infrequent. But in the history of a river basin, the piracy 

 of some one or another of its numerous branches interlocking 

 with the branches of neighboring basins is probably not espe- 

 cially rare. In the next geological period the number of pira- 

 cies between the headwaters of the Mississippi, the St. Lawrence, 

 the Hudson Bay system, and the Mackenzie will certainly not 

 be few, and before the Cordilleran tract is base-leveled, it may 

 safely be affirmed that piracies at many points will have fur- 

 nished a migratory tract between the river systems of the interior 

 and those of the Pacific. 



Certain attitudes of the sea to the land develop lagoons 

 and sounds behind spits, fringing inlands, and barrier tracts, and 

 if the land be growing at the expense of the sea, the waters of 

 these lagoons and sounds often become wholly cut off from the 

 sea and so pass from the salt to the fresh condition, and thus 

 afford a means of migration from river to river near their mouths. 

 The attitudes which favor this kind of communication occur 



