988 ANIMAL LIFE 
drainage basin with another. An example of this is found 
in Lava Creek in Yellowstone Park. Above Undine and 
Wraith Falls, both insurmountable, are found an abun- 
dance of trout. A marsh dry in summer connects Lava 
Creek with Black Tail Deer Creek, a tributary of the 
Yellowstone and without waterfall. From the Yellow- 
stone through this creek and marsh the trout find their 
way into Lava Creek. In California numerous anomalics 
have been noted, as the occurrence of Tahoe trout in 
Feather River and in the Blue Lakes of Amador, which arc 
on the other side of the main crest of the Sierra Nevada 
from Lake Tahoe, and the occurrence of the Whitney 
golden trout in Lone Pine Creek, another similar instance. 
In each case naturalists have found the man who actually 
carried the species across the divide. If this matter had 
been investigated a generation later, these cases would have 
been unexplainable anomalies in geographical distribution. 
Real causes are almost always simple when they are once 
known. 
The ways in which species may cross barriers in a state 
of Nature are as varied as the creatures themselves, and far 
more varied than the actual barriers. By the long-con- 
tinued process of adjustment to conditions with the inces- 
sant destruction of the unadapted, the various organisms 
have become so well fitted to their surroundings that the 
casual observer may well suppose that each inhabits the 
region best fitted for it. Men have even thought that the 
conditions of life have been fitted to the creatures them- 
selves, so perfect is this relation. 
155. Character of barriers to distribution —Taking the 
animal kingdom as a whole, the two great barriers modify- 
ing distribution are the presence of the sea and changes in 
temperature. Jt is only in rare cases that any land ani- 
mals can cross either of the great oceans, and these rare 
cases relate chiefly to the arctic regions. For this reason 
the land faune of Africa, South America, and Australia 
