The reason of this uncertainty is that the _thing_ Species does not exist: the term expresses an _abstraction_, like Virtue, or Whiteness; not a definite concrete reality, which can be separated from other things, and always be found the same. Nature produces individuals; these individuals resemble each other in varying degrees; according to their resemblances we group them together as classes, orders, genera, and species; but these terms only express the _relations of resemblance_, they do not indicate the existence of such _things_ as classes, orders, genera, or species.[12] There is a reality indicated by each term—that is to say, a real relation; but there is no objective existence of which we could say, This is variable, This is immutable. Precisely as there is a real relation indicated by the term Goodness, but there is no Goodness apart from the virtuous actions and feelings which we group together under this term. It is true that metaphysicians in past ages angrily debated respecting the Immutability of Virtue, and had no more suspicion of their absurdity, than moderns have who debate respecting the Fixity of Species. Yet no sooner do we understand that Species means a relation of resemblance between animals, than the question of the Fixity, or Variability, of Species resolves itself into this: Can there be any _variation in the resemblances_ of closely allied animals? A question which would never be asked. No one has thought of raising the question of the fixity of varieties, yet it is as legitimate as that of the fixity of species; and we might also argue for the fixity of genera, orders, classes; the fixity of all these being implied in the very terms; since no sooner does any departure from the type present itself, than _by_ that it is excluded from the category; no sooner does a white object become gray, or yellow, than it is excluded from the class of white objects. Here, therefore, is a sense in which the phrase “fixity of species” is indisputable; but in this sense the phrase has never been disputed. When zoologists have maintained that species are variable, they have meant that _animal forms are variable_; and these variations, gradually accumulating, result at last in such differences as are called specific. Although some zoologists, and speculators who were not zoologists, have believed that the possibility of variation is so great that one species may actually be _transmuted_ into another, _i.e._, that an ass may be developed into a horse,—yet most thinkers are now agreed that such violent changes are impossible; and that every new form becomes established only through the long and gradual accumulation of minute differences in divergent directions.