Ambrose Bierce has the answer...

MAN (noun)
An animal so lost in rapturous contemplation of what he thinks he is, as to overlook what he indubitably ought to be. His chief occupation is exterminating other animals and his own species, which, however, multiplies with such insistent rapidity as to infest the whole habitable earth, and Canada.

MONKEY (noun)
An arboreal animal which makes itself at home in genealogical trees.