The logical distinction between genus and species can, of course, apply to any objects whatsoever. Thus, for example, the genus knife may be divided into several species, such as penknife, table knife, bread knife, razor, etc. We might define the penknife as a knife small enough to be carried in the pocket, with foldable, sheathable elements, characterized by the feature that one blade at least is always suitable for sharpening pencils and erasing ink spots. We are quite sure in which features all the members of a certain species agree, notwithstanding possible individual characteristics, say a monogram on one penknife, or a corkscrew in another.