SUMMARY OF MORPHOLOGICAL DEVELOPMENT. 235 



branch, all sides of the leaf, and all sides of the flower, where 

 these parts are similarly conditioned on all sides. So, too, is it 

 with animals which move about. The most marked contrasts 

 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 likenesses and differences among limbs, and 

 internally the likenesses 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 life, exposed to like amounts and like combina- 

 tions of actions and reactions, must develop alike; while 

 unlikenesses of development must as unavoidably follow 

 unlikenesses among these agencies. And this being so, all 

 the specialities of symmetry and unsymmetry and asymmetry 

 which we have traced, are necessary consequences. 



