206 FOOT-NOTES TO EVOLUTION. 
On the theory that each species occupies those places 
best suited to its life, this fact would represent a great 
oversight on the part of Mother Nature. But with this 
is the curious fact that the Yellowstone itself, both 
above and below its falls, is well stocked with trout and 
with no other fish. This is an anomaly of distribution, 
but this anomaly disappears when we examine the con- 
tinental divide at the head of the Yellowstone. At 
one point, as Dr. Barton W. Evermann, 
Dr. Oliver P. Jenkins, and others have 
shown, the Two-ocean Pass, only about an eighth of a 
mile of wet meadow and marsh, separates the drainage 
of the Yellowstone from that of the Columbia, From 
the Columbia the Yellowstone has therefore received its 
trout. No doubt every anomaly of distribution would 
become perfectly simple could we only know all the 
facts concerned in the case. 
The laws of geographical distribu- 
tion of animals reduce themselves to 
these very simple propositions: 
Every species of animal may be found in any part of 
the earth, unless: 
1. It has been unable to reach that region, through 
barriers of some sort, or, 
2. Having reached it, it is unable to maintain itself, 
in competition with other forms, or on account of the 
conditions of environment, or else, 
3. Having maintained itself, it has become so altered 
through natural selection as to become a species distinct 
from its ancestors. 
The primary barriers to distribution are the heights 
of the land and the depths of the sea—physical obstacles 
not to be crossed. Next in importance is the barrier of 
climate. With some forms of life this is absolute, for the 
palm and the banana are the index of the torrid zone as 
Two-ocean Pass. 
Laws of distribu- 
tion of animals. 
