208 MORPHOLOGICAL DEVELOPMENT. 



mechanical, partly physiological, which the organs exercise 

 on one another, become the chief causes of their changes of 

 figure and arrangement; 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, throughout t the Vertebrata, the alimentary system, 

 with the exception of its two extremities, is asymmetrically 

 arranged, the respiratory system, which occupies one end of 

 the body, generally deviates but little from bilateral sym- 

 metry, 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 treatment. 



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 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, can- 

 not be rejected without raising the demand for some other 

 interpretation. 



