28 Geographical Distribution of Animals. 



animals of a new country, there is a particular interest to a natural- 

 ist in ascertaining their peculiar geographical distribution, and their 

 true affinities with those of other countries. It is only by following 

 such a course, that we can hope to arrive at any exact results as to 

 their origin. In this respect the fresh-water animals have a pecu- 

 liar interest, as from the element they inhabit, they are placed under 

 exceptional circumstances. 



Marine animals, as well as those inhabiting dry land, seem to 

 have a boundless opportunity before them to spread over large parts 

 of the earth's surface, and their locomotive powers would generally 

 be sufficient to carry them almost anywhere ; but they do not avail 

 themselves of the possibility ; notwithstanding their facilities for loco- 

 motion, they for the most part remain within very narrow limits, 

 using their liberty rather to keep within certain definite bounds. This 

 tendency of the higher animals especially, to keep within well-ascer- 

 tained limits, is perhaps the strongest evidence that there is a natural 

 connection between the external world and the organised beings 

 living upon the present surface of our globe. The laws which regu- 

 late these relations, and those of geographical distribution in par- 

 ticular, have already been ascertained to a certain extent, and will 

 receive additional evidence from the facts recorded during our 

 journey. 



The fresh-water animals are placed in somewhat different circum^ 

 stances. Their abode being circumscribed by dry land, within limits 

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

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

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

 narrower limits than either terrestrial or marine types. Within 

 these limits again they are still further 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 fresh-water basin from one extreme of its 

 boundary to the other ; thus forming at various heights above the 

 level of the sea, isolated groups of fresh-water animals in the midst 

 of those which inhabit the dry land. These groups are very similar 

 in their circumscription 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 

 Archipelagos. 



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 



