156 THE COMMON FROG. [chap. 



15. Respiration takes place through the introduc- 



tion of air by an action of swallowing. 



16. Development takes place through a metamor- 



phosis. 



17. Gills exist in the young. 



18. There is no corpus callosum. 



19 The ureters open into the oviducts. 



The course of our inquiry into the nature and affi- 

 nities of the Frog has not alone served to answer the 

 question with which this memoir opened. Incidental 

 bearings upon deep biological problems have come 

 before us more than once in its course, nor have all 

 the conclusions which seem to have forced themselves 

 upon us been totally negative. 



Thus we have met with several instances of the 

 independent origin of remarkably similar structures, 

 such as a shielded temporal fossa and elongated 

 tarsus, which together with structures like the tooth 

 of the Labyrinthodon, seem to be characters for the 

 existence of which neither the destructive agencies of 

 nature acting on minute oscillations of structure, nor 

 any sexual phenomena will account. 



Again in the life history of the Frog, considered 

 even purely by itself, we find a remarkable example 

 of spontaneous transformations due to innate powers 

 and tendencies. 



When, however, this process is considered in the 

 light derived from the curious phenomena of trans- 

 formation so enigmatically presented to us by the 

 axolotl, we have very strongly brought before us the 

 powerful action of internal tendencies lying dormant 



