986 ANIMAL LIFE 
parent in the fact that the plateau is fringed with cataracts 
which fishes can not ascend. Each stream has a cafion or 
deep gorge with a waterfall at its head, near the point 
where it leaves the hard bed of black lava for the rock 
below (Fig. 173). So for an area of fifteen hundred square 
miles within the Yellowstone National Park the streams 
were without trout because their natural inhabitants had 
never been able to reach them. When this state of things 
was discovered it was easy to apply the remedy. Trout of 
different species were carried above the cascades, and these 
have multiplied with great rapidity. 
The exception noted above, that of the Yellowstone 
River itself, evidently nceds explanation. An abundance 
of trout is found in this river both above and below the 
great falls, and no other fish occurs with it. This anomaly 
of distribution is readily explained by a study of the tribu- 
taries at the head waters of the river. When we ascend 
above Yellowstone Lake to the continental divide, we find 
on its very summit that only about an eighth of a mile of 
wet meadow and marsh, known as Two Ocean Pass (Fig. 
174), separates the drainage of the Yellowstone from that 
of the Columbia. A stream known as Atlantic Creek flows 
into the Yellowstone, while the waters of Pacific Creek on 
the other side find their way into the Snake River. These 
two creeks are connected by waterways in the wet meadow, 
and trout may pass from one to the other without check. 
Thus from the Snake River the Yellowstone received its 
trout, and from the Yellowstone they have spread to the 
streams tributary to the upper Missouri. 
This case is a type of the anomalies in distribution of 
which the student of zoégeography will find many. But 
each effect depends upon some cause, and a thorough study 
of the surroundings or history of a species will show what 
this cause may be. In numerous cases in which fishes have 
been found above an insurmountable cascade, the cause is 
seen in a marsh flooded at high water, connecting one 
