160 ORIGIN OF LIFE IN AMEEICA 



continental basin, as they are of such importance in tracing 

 the geological history of the fauna. The Mississippi lies 

 wholly within the boundaries of the United States, and drains 

 more than two-fifths of their area. Originating in Lake Itasca 

 in Minnesota, the Mississippi receives during its long course 

 four great tributaries, the Missouri, Ohio, Arkansas and Red 

 River, and a large number of smaller ones. The two prin- 

 cipal groups of animals inhabiting this great river system are 

 the fishes and fresh-water mussels. Some of the fishes are 

 able to live in brackish water, others spend part of their 

 lives in the sea, so that they are not of such extreme im- 

 portance from a zoogeographical point of view as the fresh- 

 water mussels. 



Fresh-water mussels, or Naiades as they have been called, 

 all die quickly if immersed in salt water or if removed to thie 

 land. Their distribution being world-wide, they have been 

 looked upon by some naturalists as among the best indicators 

 of former changes of land and water over the globe. Others 

 have urged that the wide range of these mussels may be due to 

 accidental conveyance by birds or fishes. It was thought 

 that the eggs or the newly-hatched fry of the mussels had 

 been thus transported. Many species immediately after their 

 fry has been hatched from the eggs, develop booklets on the 

 temporary shell, by which the young mussels can attach 

 themselves to foreign objects. It has been argued that such 

 larval moUusks might become attached to the feet of aquatic 

 birds and be carried by them in their flight from the fresh 

 waters of one region to those of other regions and there be set 

 free. Theoretically, such an accidental transport would seem 

 quite a passible one from time to time, certainly much more 

 likely than a similar conveyance of th.e fry by fishes from one 

 river system to another. In a country like North America, 

 where millions of migratory birds pass annually north and 

 south, and to some extent east and west, the effects of a con- 

 veyance such as suggested should be clearly discernible in the 

 composition of the North American fresh -water mussel fauna. 

 Yet although there are over four hundred different kinds of 

 fresh-water mussels in the Mississippi drainage area, some 

 of them having existed there almost unchanged since Cre- 

 taceous times, the fauna to the east and west of that area is 



