286 ANIMAL LIFE 



parent in the fact that the plateau is fringed with cataracts 

 which fishes can not ascend. Each stream has a canon 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 

 Eiver itself, evidently needs 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 Eiver. 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 Eiver 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 zoogeography 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 



