All this is clear and simple. The confusion began when these two terms, instead of being coördinate, were subordinated to each other by the philosophers of Greece, so that what from one point of view was called a _genus_, might from another be called a species, and _vice versâ_. Human beings, for instance, were now called a _species_, all living beings a _genus_, which may be true in logic, but is utterly false in what is older than logic—viz., language, thought, or fact. According to language, according to reason, and according to nature, all human beings constitute a γένος, or generation, so long as they are supposed to have common ancestors; but with regard to all living beings we can only say that they form an εἶδος—that is, agree in certain appearances, until it has been proved that even Mr. Darwin was too modest in admitting at least four or five different ancestors for the whole animal world.(15) In tracing the history of these two words, γένος and εἶδος, you may see passing before your eyes almost the whole panorama of philosophy, from Plato’s "ideas" down to Hegel’s _Idee_. The question of _genera_, their origin and subdivision, occupied chiefly the attention of natural philosophers, who, after long controversies about the origin and classification of _genera_ and _species_, seem at last, thanks to the clear sight of Darwin, to have arrived at the old truth which was prefigured in language—namely, that Nature knows nothing but _genera_, or generations, to be traced back to a limited number of ancestors, and that the so-called _species_ are only _genera_, whose genealogical descent is _as yet_ more or less obscure. But the question as to the nature of the εἶδος became a vital question in every system of philosophy. Granting, for instance, that women in every clime and country formed one species, it was soon asked what constituted a species? If all women shared a common form, what was that form? Where was it? So long as it was supposed that all women descended from Eve, the difficulty might be slurred over by the name of heredity. But the more thoughtful would ask even then how it was that, while all individual women came and went and vanished, the form in which they were cast remained the same?