188 WATCHED BY WILD ANIMALS 
acquaintance with the plains antelope I visited 
the Yellowstone region, thinking that I was well 
grounded in all antelope habits. One day I 
came upon a flock in a deep grassy forest bay 
in the edge of a dense woods. Thinking to get 
close I walked in behind them. To my amaze- 
ment they darted into the woods, dodging trees 
right and left like lightning, and hurdling fallen 
trees as readily as any deer or mountain sheep 
that I have seen. They well illustrated a 
phase of animal behaviour called ecology, or 
response to environment. 
The pronghorn or antelope is distinctly Ameri- 
can. Fossilized antelope bones have been 
found in western Nebraska that are estimated 
to be two million years old. This antelope 
family is not related to the African or Asiatic 
antelope, nor to any American mammal species; 
it is alone in the world. 
Many prehistoric species of animals that lived 
in the same scenes with the ancient ancestors 
of the antelope have been extinct for thousands of 
years. The rhinoceros, toothed birds, American 
horses, ponderous reptiles, and numerous other 
species failed to do what the antelope did—read- 
just to each radical change and survive. Climatic 
changes, new food, strange enemies, uplifts, sub- 
sidences, wild volcanic outpourings, the great Ice 
Age—over all these the antelope has triumphed. 
