THE PHYLOGENESIS OF VERTEBRATES 47 



Jossil representative among adult animals, the only clew is 

 chat furnished by embryology. It will be seen that in a few 

 cases, notably in the history of the transition from fins to 

 walking limbs between fishes and amphibians, both records are 

 unsatisfactory, and in such cases the only hope of a definite 

 solution lies in the future discovery of some extinct form which 

 may bridge the gap and thus furnish a clew by which the two 

 discontinuous threads may be united. 



