SUMMARY OF MORPHOLOGICAL DEVELOPMENT. 217 



they present are those between the part in advance and the 

 part behind, and between the upper part and the under part ; 

 while there is complete correspondence between the two 

 sides. Externally the IQtenesses and differences among 

 limbs, and internally the Hkenesses and differences among 

 vertebrae, are expressible in terms of this same law. 



And here, indeed, we may see clearly that these truths are 

 corollaries from that ultimate truth to which all phenomena 

 of Evolution are referable. It is an inevitable deduction 

 from the persistence of force, that organic forms which have 

 been progressively evolved must present just those funda- 

 mental traits of form which we find them present. It cannot 

 but be that during the intercourse between an organism^ and 

 its environment, equal forces acting under equal conditions 

 must produce equal effects ; for to say otherwise, is, by im- 

 plication, to say that some force can produce more or less 

 than its equivalent effect, which is to deny the persistence of 

 force. Hence those parts of an organism which are, by its 

 habits of Kfe, exposed to like amounts and Uke combinations 

 of actions and reactions, must develop alike ; ' while unlike- 

 nesses of development must as unavoidably follow unlike- 

 nesses among these agencies. And this being so, all the 

 specialities of symmetry and unsymmetry and asymmetry 

 which we have traced, are necessary consequences. 



