THE ORIGIN OF VERTEBRATED ANIMALS 187 



are three distinct cartilage bones in the shoulder 

 girdle — viz., coracoid, scapula, and pre-coracoid — 

 agreeing with the Anomodont reptiles. Some of 

 these extinct reptiles from the Permian (the top of 

 the Palaeozoic) and the Triassic deposits — Anomo- 

 dontia or Theromorpha — in the character of the 

 shoulder and hip girdles ; in the perforation of the 

 columella ; in the relation and number of the bones 

 of the hind foot, especially the astragalus and calca- 

 neum ; in the nature of the ribs and in other points, 

 are in many ways intermediate between mammals 

 and reptiles. They may possibly be the common 

 ancestors of amphibia, reptiles, and mammals. This 

 is a point of importance as showing a convergence 

 towards a common ancestor. 



We now enter on a second stage in our inquiry. 

 We have found evidence in embryology of the 

 descent of terrestrial Vertebrates from aquatic 

 ancestors, in the presence of gill-slits, and in the 

 actual transition shown us by Amphibians, such as 

 the frog and newt. The geological evidence of the 

 instability of land compared with the sea leads us to 

 think of this as of general application, and not con- 

 fined to Vertebrates, and that the origin of all the 

 great groups of animals is to be looked for in the 

 sea. 



So far, we are led to regard fish as the most 

 primitive of the five great groups of Vertebrates, 

 and to view the remaining four as descended from 

 fish or fish-like ancestors. Now, can we get any 

 (\_further ? What is there behind fish, and can we 

 trace the pedigree of Vertebrates further back ? 



