110 LIFE ON THE EARTH. 



separate the denizens of lakes and rivers from those 

 of the lagoons, bays, and currents of the sea. 



This fact, which is observed in all countries, is 

 connected with another of much significance, the 

 comparative fewness of the freshwater races, and 

 their great affinity over large tracts of the earth. 

 Unionidse and Cycladeo are the prevailing family of 

 Dimyarian Mollusks ; Paludinadse and Limnseadee, 

 reinforced by Ancylus, Melania, Neritina, &c. the 

 most frequent of Gasteropoda. How these fresh- 

 water genera have been so widely diffused as in fact 

 we find them ; how some particular species have be- 

 come common to so many rivers and lakes, are ques- 

 tions of much interest. For as neither the animals 

 nor their ova could pass through the salt water, 

 and retain vitality, we must ascribe to changes in 

 physical geography and the accidents of mixed oc- 

 cupation of a country, the transference from one 

 river to another, of the germs of life ; or regard the 

 existing fresh waters, now unconnected, as formerly 

 somehow connected; or suppose a creation of the 

 same species or the same genera at many separate 

 points. Rejecting this last, we may venture to prefer, 

 as a general explanation, the transfer of the germs 

 of life by natural events, specially by birds carrying 

 spawn from one river to another, even as now we 

 are transferring by experimental means the Crawfish, 



