No one would say that Aristotle had not a perfect right to define these terms, if those who use them in his sense would only always remember that they are thinking the thoughts of Aristotle, and not their own. The true way to shake off the fetters of old words, and to learn to think our own thoughts, is to follow them up from century to century, to watch their development, and in the end to bring ourselves face to face with those who first found and framed both words and thoughts. If we do this with _genus_ and _species_, we shall find that the words which Aristotle defined—viz., γένος and εἶδος—had originally a very different and far more useful application than that which he gave to them. γένος, _genus_, meant generation, and comprehended such living beings only as were believed to have a common origin, however they might differ in outward appearance, as, for instance, the spaniel and the bloodhound, or, according to Darwin, the ape and the man. εἶδος, or species, on the contrary, meant appearance, and comprehended all such things as had the same form or appearance, whether they had a common origin or not, as if we were to speak of a species of four-footed, two-footed, horned, winged, or blue animals. That two such concepts, as we have here explained, had a natural justification we may best learn from the fact that exactly the same thoughts found expression in Sanskrit. There, too, we find _g_âti, generation, used in the sense of _genus,_ and opposed to âk_ri_ti, appearance, used in the sense of _species_. So long as these two words or thoughts were used independently (much as we now speak of a genealogical as independent of a morphological classification) no harm could accrue. A family, for instance, might be called a γένος, the _gens_ or clan was a γένος, the nation (_gnatio_) was a γένος, the whole human kith and kin was a γένος; in fact, all that was descended from common ancestors was a true γένος. There is no obscurity of thought in this. On the other side, taking εἶδος or species in its original sense, one man might be said to be like another in his εἶδος or appearance. An ape, too, might quite truly be said to have the same εἶδος or species or appearance as a man, without any prejudice as to their common origin. People might also speak of different εἴδη or forms or classes of things, such as different kinds of metals, or tools, or armor, without committing themselves in the least to any opinion as to their common descent. Often it would happen that things belonging to the same εἶδος, such as the white man and the negro, differed in their εἶδος or appearance; often also that things belonged to the same εἶδος, such as eatables, differed in their γένος, as, for instance, meat and vegetables.