In arid areas the colors are usually distinctly paler and grayer, in
the humid districts they are darker and browner. Other conditions also
effect these changes among members of the same species, as is shown
in some of the most arid and desert plains of the southwestern United
States, where mammals living among dark-colored lava beds are darker
than those found, sometimes within a few rods, on paler adjoining
soil. Complete isolation under the same climatic and other conditions
sometimes produces marked changes, as is well illustrated by the
difference between the Abert and Kaibab squirrels on the two sides of
the Grand Canyon in Arizona (see page 448).
The different forms of a species occupying areas under varying
conditions are commonly termed geographic races. They grade
imperceptibly into one another along the border between their ranges,
step by step with the gradations of the climatic and other conditions
which have produced their differences.