"None of the observations are more in point, as bearing on the doctrine of what Hooker terms 'creation by variation,' than the great extent to which the internal characters and properties of plants, or their physiological constitution are capable of being modified, while they exhibit externally no visible departure from the normal form. ...When several of these internal or physiological modifications are accompanied by variation in size, habits of growth, colour of the flowers, and other external characters, and these are found to be constant in successive generations, botanists may well begin to differ in opinion as to whether they ought to regard them as distinct species or not."