168 MORPHOLOGICAL DEVELOPMENT. 



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 witli 

 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 ap- 

 proach 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, a posteriori, the con- 

 clusions here reached a priori. 



