FISHES OF LAKE SUPERIOR. 247 



which are often reduced to a narrow current of water, and being far- 

 ther, for the most part, prevented by structural peculiarities from 

 passing from the rivers into the ocean, they are confined within nar- 

 rower limits than either terrestrial or marine types. Within these 

 limits again they are still farther restricted ; the shells and fishes of 

 the head-waters of large rivers, for instance, being scarcely ever the 

 same as those of their middle or lower course, few species extending 

 all over any freshwater basin from one extreme of its boundary to the 

 other : thus forming at various heights above the level of the sea, 



^j cj 



isolated groups of freshwater animals in the midst of those which in- 

 habit the dry land. These groups are very similar in their circum- 

 scription to the islands and coral reefs of the ocean ; like them they 

 are either large or small, isolated and far apart, or close together in 

 various modes of association. In every respect they form upon the 

 continents as it were a counterpart of the archipelagoes. 



From their circumscription, these groups of lakes present at once 

 a peculiar feature in the animal kingdom, their inhabitants being en- 

 tirely unconnected with any of the other living beings which swarm 

 around them. What, for instance, is there apparently in common 

 between the fishes of our lakes and rivers, and the quadrupeds which 

 inhabit their shores, or the birds perching on the branches which 

 overshadow their waters ; or what connection is there between the 

 few hermit-like terrestrial animals that live upon the low islands of 

 the Pacific, and the fishes which play among the corals, or in the 

 sand and mud of their shores ? And nevertheless there is but one 

 plan in the creation ; freshwater animals under similar latitudes are 

 as uniform as the corresponding vegetation, and however isolated and 

 apparently unconnected the tropical islands may seem, their inhabi- 

 tants agree in their most important traits. 



The best evidence that in the plan of creation animals are intended 

 to be located within circumscribed boundaries, is farther derived from 

 their regular migrations. Although the Arctic birds wander during 

 winter into temperate countries, and some reach even the warmer 

 zones ; although there are many which, from the colder temperate 

 climates, extend quite into the tropics, there is nevertheless not one 

 of these species which passes from the northern to the southern hem- 

 isphere ; not one which does not return at regular epochs to the 



