among animals, there is always either a true skin or an outer coat analogous to one. Wherever aggregates of the first order have united into aggregates of the second and third orders--wherever they have become the morphological units of such higher aggregates--the outermost of them have grown unlike those lying within. Even the Sponge is not without a layer that may by analogy be called dermal. This lapse of the relatively homogeneous into the relatively heterogeneous, first showing itself, as on the hypothesis of evolution it must do, by the rise of an unlikeness between outside and inside, goes on next to show itself, as we infer that it must do, by the establishment of secondary contrasts among the outer parts answering to secondary contrasts among the forces falling on them. So long as the whole surface of a plant remains similarly related to the environment, as in a _Protococcus_, it remains uniform; but when there come to be an attached surface and a free surface, these, being subject to unlike actions, are rendered unlike. This is visible even in a unicellular _Alga_ when it becomes fixed; it is shown in the distinction between the under and upper parts of ordinary _Fungi_; and we see it in the universal difference between the imbedded ends and the exposed ends of the higher plants. And then among the less marked contrasts of surface answering to the less marked contrasts in the incident forces, come those between the upper and under sides of leaves; which, as we have seen, vary in degree as the contrasts of forces vary in degree, and disappear where these contrasts disappear. Equally clear proof is furnished by animals, that the original uniformity of surface lapses into multiformity, in proportion as the actions of the environment upon the surface become multiform. In a Worm, burrowing through damp soil which acts equally on all its sides, or in a _Tænia_, uniformly bathed by the contents of the intestine it inhabits, the parts of the integument do not appreciably differ from one another; but in creatures not surrounded by the same agencies, as those that crawl and those that have their bodies partially inclosed, there are unlikenesses of integument corresponding to unlikenesses of the conditions. A snail’s foot has an under surface not uniform with the exposed surface of its body, and this again is not uniform with the protected surface. Among articulate animals there is usually a distinction between the ventral and the dorsal aspects; and in those of the _Arthropoda_ which subject their anterior and posterior ends to different environing agencies, as do the ant-lion and the hermit-crab, these become superficially differentiated. Analogous general contrasts occur among the _Vertebrata_. Fishes, though their outsides are uniformly bathed by water, have their backs more exposed to light than their bellies, and the two are commonly distinct in colour. When it is not the back