All the parts of the organisation are to a certain extent connected or correlated together; but the connexion may be so slight that it hardly exists, as with compound animals or the buds on the same tree. Even in the higher animals various parts are not at all closely related; for one part may be wholly suppressed or rendered monstrous without any other part of the body being affected. But in some cases, when one part varies, certain other parts always, or nearly always, simultaneously vary; they are then subject to the law of correlated variation. Formerly I used the somewhat vague expression of correlation of growth, which may be applied to many large classes of facts. Thus, all the parts of the body are admirably coordinated for the peculiar habits of life of each organic being, and they may be said, as the Duke of Argyll insists in his ‘Reign of Law,’ to be correlated for this purpose. Again, in large groups of animals certain structures always co-exist; for instance, a peculiar form of stomach with teeth of peculiar form, and such structures may in one sense be said to be correlated.