THE GENERAL SHAPES OF ANIMALS. 191 



become the chief causes of their changes of figure and ar- 

 rangement ; and these influences are complex and indefinite. 

 One general fact may, indeed, be noted — the fact, namely, 

 that the divergence towards asymmetry which generally 

 characterizes the viscera, is marked among those of them 

 which are most removed from mechanical converse with the 

 environment, but not so marked among those of them which 

 are less removed from such converse. Thus while, through- 

 out the Vertebrata, the alimentary system, with the exception 

 of its two extremities, is asymmetrically arranged, the re- 

 spiratory system, which occupies one end of the body, ge- 

 nerally deviates but little from bilateral symmetry, and the 

 reproductive system, partly occupying the other end of the 

 body, is in the main bilaterally symmetrical : such deviation 

 from bilateral symmetry as occurs, being found in its most 

 interiorly-placed parts, the ovaries. Just indicating these 

 facts as having a certain significance, it will be best to leave 

 this part of the subject as too involved for detailed treat- 

 ment. 



Internal structures of one class, however, not included 

 among . the viscera, admit of general interpretation — struc- 

 tures which, though internal, are brought into tolerably- 

 direct relations with the environing forces, and are therefore 

 subordinate in their forms to the distribution of those forces. 

 These internal structures it will be desirable to deal with 

 at some length ; both because they furnish important illustra- 

 tions enforcing the general argument, and because an inter- 

 pretation of them which we have seen reason to reject, 

 cannot be rejected without raising the demand for some other 

 interpretation. 



Vdl. IL 9 



