MORPHOLOGICAL DIFFERENTIATION IN ANIMALS. 185 



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 move- 

 ment 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 sym- 

 metry may be again looked for; and if the motions are still 

 more variously directed — if they are not limited to approxi- 

 mately-plane surfaces, but extend to surfaces that are dis- 

 tributed all around with a regular irregularity — an approach 

 of the radial towards the spherical symmetry is to be antici- 

 pated. Where the habits are such that the inter- 

 course 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 irregu- 

 larity 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, a posteriori, the conclu- 

 sions here reached a priori. 



