We are aware that the world is made up of various classes and conditions of men.  How did we learn this?  Let us go back to the time when our minds were a blank, when we were babes and sucklings, when we had not perceived that men exist, much less that mankind is infinitely complex.  A baby comes slowly to understand that all objects in the universe are divisible into two classes, human and non-human, and that a member of the former may be separated from the others and regarded as an individual.  It has reached the initial stage of its knowledge on the subject; it has the basic idea, that of the individual human being.  As soon as it can speak, it acquires a designating term—­not of course the sophisticated human being, but the simpler man.  It uses this word in the generic sense, to indicate any member of the human race; for as yet it knows nothing and cares nothing about differences in species.  With increasing enlightenment, however, it discerns five species, and distinguishes among them by swelling this branch of its vocabulary to five words:  man (in the sense of adult male), woman, boy, girl, baby. (To be sure, it may chance to have acquired a specific term, as boy or baby, before the generic term man; but if so, it has attached this term to some particular individual, as the grocer’s boy or itself, rather than to the individuals of a species.  Its understanding of the species as a species comes after its understanding of the genus.) As time passes, it divides mankind into yet further species by sundry other methods:  according to occupation, for example, as doctors, chauffeurs, gardeners; to race or color, as white men; negroes, Malays, Chinese; to disposition, as heroes, gift-givers, teasers, talkers; and so on.  It perceives moreover that species are made up of sub-species.  Thus instead of lumping all boys together it begins to distinguish them as big boys, little boys, middle-sized boys, boys in long trousers, boys in short trousers, barefoot boys, schoolboys, poor boys, rich boys, sick boys, well boys, friends, enemies, bullies, and what not.  It even divides the sub-species.  Thus it classifies schoolboys as bright boys, dullards, workers, shirkers, teachers’ favorites, scapegoats, athletes, note-throwers, truant-players, and the like.  And of these classes it may make yet further sub-divisions, or at least it may separate them into the individuals that compose them.  In fine, with its growing powers and experience, it abandons its old conception that all persons are practically alike, and follows human nature through the countless ramifications of man’s status, temperament, activities, or fate.  And it augments its vocabulary to keep pace, roughly at least, with its expanding ideas.  In thought and terminology alike its growth is from genus to species.