their present forms. Some of these are perhaps immigrants 

 from Northern Asia, where they still have their nearest rela- 

 tives. Still others are evidently modified importations from 

 the sea ; and of these some are very recent immigrants, land- 

 locked species which have changed very little from the parent 

 stock. 



We can say, in general, that in all waters not absolutely 

 uninhabitable there are fishes. The processes of natural selec- 

 tion have given to each kind of river or lake species of fishes 

 adapted to the conditions of life which obtain there. There is 

 no condition of water, of bottom, of depth, of speed of current, 

 but finds some species with characters adjusted to it. These 

 adjustments are, for the most part, of long standing ; and the 

 fauna of any single stream has, as a rule, been produced by 

 immigration from other regions or from other streams. Each 

 species has an ascertainable range of distribution, and within 

 this range we may be reasonably certain to find it in any suitable 

 waters. 



But every species has beyond question some sort of limit to 

 its distribution, some sort of barrier which it has never passed 

 in all the years of its existence. That this is true becomes evi- 

 dent when we compare the fish-faunae of widely separated rivers. 

 Thus the Sacramento, Connecticut, Rio Grande and St. John's 

 rivers have not a single species in common ; and with one or two 

 exceptions, not a species is common to any two of them. None 

 of these has any species peculiar to itself, and each shares a 

 large part of its fish-fauna with the water-basin next to it. It is 

 probably true that the faunae of no two distinct hydrographic 

 basins are wholly identical, while, on the other hand, there are 

 very few species confined to a single one. The supposed cases 

 of this character, some twenty in number, occur chiefly in the 

 streams of the South Atlantic States and of Arizona. All of 

 these need, however, the confirmation of further exploration. 

 It is certain that in no case has an entire river fauna originated 

 independently from the divergence into separate species of the 

 descendants of a single type. 



The existence of boundaries to the range of species implies, 

 therefore, the existence of barriers to their diffusion. We may 



