An authority says: "The names _species_ and _genus_ are merely relative and the same common term may, in one case, be the species which is predicated of an individual, and in another case the individual of which a species is predicated. Thus the individual, George, belongs to the logical species Man, while Man is an individual of the logical species Animal." Jevons says: "It is desirable to have names by which to show that one class is contained in another, and accordingly we call the class which is divided into two or more smaller ones, the _genus_, and the smaller ones into which it is divided, the _species_." _Animal_ is a _genus_ of which _man_ is a _species_; while _man_, in turn, is a _genus_ of which _Caucasian_ is a _species_; and _Caucasian_, in turn, becomes a _genus_ of which _Socrates_ becomes a species. The student must avoid confusing the _logical_ meaning of the terms _genus_ and _species_ with the use of the same terms in Natural History. _Each class is a "genus" to the class below it in extension; and each class is a "species" to the class above it in extension._ At the lowest extreme of the scale we reach what is called the _infima species_, which cannot be further subdivided, as for instance "Socrates"--this lowest species must always be an individual object, person or thing. At the highest extreme of the scale we reach what is _summum genus_, or highest genus, which is never a species of anything, for there is no class higher than it, as for instance, "being, existence, reality, truth, the absolute, the infinite, the ultimate," etc. Hyslop says: "In reality there is but one _summum genus_, while there may be an indefinite number of _infimae species_. All intermediate terms between these extremes are sometimes called _subalterns_, as being either genera or species, according to the relation in which they are viewed." Passing on to the classification of the character of the _intension_ of terms, we find: _Difference_, a term denoting: "The mark or marks by which the species is distinguished from the rest of the genus; the specific characteristic." Thus the color of the skin is a _difference_ between the Negro and the Caucasian; the number of feet the _difference_ between the biped and the quadruped; the form and shape of leaves the _difference_ between the oak and the elm trees, etc. Hyslop says: "Whatever distinguishes one object from another can be called the _differentia_. It is some characteristic in addition to the common qualities and determines the species or individual under the genus." _Property_, a term denoting: "A peculiar quality of anything; that which is inherent in or naturally essential to anything." Thus a _property_ is a distinguishing mark of a class. Thus black skin is a _property_ of the Negro race; four feet a _property_ of quadrupeds; a certain form of leaf a _property_ of the oak tree. Thus a _difference_ between two species may be a _property_ of one of the species.