Of the

eye the white is pretty much the same in all creatures; but
what is called the black differs in various animals. Some have the rim
black, some distinctly blue, some greyish-blue, some greenish; and
this last colour is the sign of an excellent disposition, and is
particularly well adapted for sharpness of vision. Man is the only, or
nearly the only, creature, that has eyes of diverse colours.
Animals, as a rule, have eyes of one colour only. Some horses have
blue eyes.