But the time comes when we have to act as our own examiners, and when we have to give an account to ourselves of such words as _genus_ and _species_. Some people write, indeed, as if they had seen a _species_ and a _genus_ walking about in broad daylight; but a little consideration will show us that these words express subjective concepts, and that, if the whole world were silent, there would never have been a thought of a _genus_ or a _species_. There are languages in which we look in vain for corresponding words; and if we had been born in such a language, these terms and thoughts would not exist for us. They came to us, directly or indirectly, from Aristotle. But Aristotle did not invent them, he only defined them in his own way, so that, for instance, according to him, all living beings would constitute a _genus_, men a _species_, and Socrates an _individual_.
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 known 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_.