If an organism advances through a homogeneous medium with one end always foremost, that end, being exposed to forces unlike those to which the other end is exposed, may be expected to become unlike it; and supposing this to be the only constant contrast of conditions, we may expect an equal distribution of the parts round the axis of movement--a radial symmetry. If, in addition to this habitual attitude of the ends, one surface of the body is always uppermost and another always lowermost, there arise between the top and bottom dissimilarities of conditions, while the two sides remain similarly conditioned. Hence it is inferable that such an organism will be divisible into similar halves by a vertical plane passing through its axis of motion--will have a bilateral symmetry. We may presume that this symmetry will deviate but little from double bilateralness where the upper and under parts are not exposed to strongly-contrasted influences; while we may rationally look for single bilateral symmetry of a decided kind, in creatures having dorsal and ventral parts conversant with very unlike regions of the environment: as in all cases where the movement is over a solid surface. If the movement, though over a solid surface, is not constant in direction, but takes place as often on one side as on another, radial symmetry may be again looked for; and if the motions are still more variously directed--if they are not limited to approximately-plane surfaces, but extend to surfaces that are distributed all around with a regular irregularity--an approach of the radial towards the spherical symmetry is to be anticipated. Where the habits are such that the intercourse between the organism and its environment, does not involve an average equality of actions and reactions on any two or more sides, there may be expected either total irregularity or some divergence from regularity. The like general relations between forms and incident forces are inferable in the component parts of animals, as well as in the animals as wholes. It is needless, however, to occupy space by descriptions of these. Let us now pass to the facts, and see how they confirm, _à posteriori_, the conclusions here reached _à priori_. CHAPTER XIV. THE GENERAL SHAPES OF ANIMALS.