160 
ORIGIN OF LIFE IN AMERICA 
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 the 
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 hooklets on the 
temporary shell, by which the young mussels can attach 
themselves to foreign objects. It has been argued that such 
larval mollusks 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 possible one from time to time, certainly much more 
likely than a similar conveyance of the 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 
