The problem of individuality, physical and mental, is one which obviously has great interest for philosophy. The unity and continuity of the ordinary human consciousness—the “ ego,” the “personality—give us the concrete standard by which we ordinarily judge other systems which have tended towards individuation. A comparative and evolutionary study of biological data, however, will provide us with many facts which throw a new light on the problem. They are often puzzling, but must be taken into account.