DISPERSION OF FRESH-WATER FISHES. 103 
The Killifishes have their greatest abundance in 
tropical America, which is perhaps the place of 
their origin. They are especially fishes of the 
brackish waters, rarely going far out to sea, 
Some of them ascend streams; and these fre- 
quent spring waters, and waters which are clear 
and cold. 
The two species of Mud-minnow are now very 
widely separated as to habitat, although very simi- 
lar to each other in structure. The one belongs 
properly to our Great Lake Fauna, the other to 
the streams of Austria. The two are probably re- 
mains of a past fauna, in which the group was 
more fully represented. Our Mud-minnow? is one 
of the most tenacious of life of all our fishes, and 
will often live for weeks in damp muck after the 
waters of a pond have evaporated. 
Of the five known species of Pzke, one is cos- 
mopolitan, being spread over northern Asia and 
Europe as well as America, while the other species 
are somewhat restricted in their range. The Com- 
mon Pike? is probably the parent stock of all; but 
whether originally American or not, we cannot 
say. The affinities of the Mud-minnow with the 
Pike are not remote, and doubtless forms between 
the two have existed. 
The Black-fish® of Alaska is another relative of 
the Mud-minnow and Pike. The single known 
species is found in Alaska and eastern Siberia. 
It too is probably an isolated relic of a disap- 
pearing group. 
1 Umbra limi Kirtland. 2 Esox luctus Linnzus. 
& Dallia pectoralis Bean. 
