PROGRESS OF LIFE. 595 



Under Vertebrates, Fishes appear to have culminated in the Tertiary period, 

 when the highest Sharks existed and were in great numbers ; Amphibians, in 

 the Triassic period; True Reptiles, at the close of the Jurassic or commencement 

 of the Cretaceous; Birds, in the present era; Mammals, Man excluded, in the 

 Post-tertiary. 



4. The earliest types representing a group often comprehensive types. — 

 Comprehensive types have been explained to be those which em- 

 brace, along with the characteristics of the group to which they 

 belong, others of another group ; and usually at their first appear- 

 ance this other group is not yet in existence. Examples are men- 

 tioned on pp. 395, 500. 



They are in part intermediate types between two groups, although 

 never occupying the middle point, as they always belong funda- 

 mentally to one of the two. They are often more normal or regular 

 in structure than later species of the class or group, so far as this is 

 consistent with the above law, — the more abnormal or less typical 

 forms being of later origin. The earliest Mammals included Mar- 

 supials, in accordance with the first law, — species which have, along 

 with the Mammalian structure, some of the characteristics of Birds 

 and Reptiles, and which were therefore fit inhabitants of the Rep- 

 tilian age. With them there appear to have been species of In- 

 sectivores, one of the more typical groups among the Microsthenes 

 (p. 423), and none of the inferior abnormal Edentates. In the 

 Mammalian age the earliest species were certain peculiarly typical 

 Pachyderms and Carnivores (p. 527). The low Edentates, or Sloth 

 tribe, appeared at a still later epoch. Two grand facts connected 

 with comprehensive types are, hence, their partly intermediate posi- 

 tion, and their comparatively normal or regular structure. 



The idea of comprehensive types was first recognized by Agassiz in the Ganoid 

 fishes. It was afterwards brought out by Owen in an article on the Labyrin- 

 thodont Reptiles; and also by Bronn. Agassiz, as stated on p. 203, has called 

 them synthetic types. Bronn named them complex types (Complications- 

 Typen), an objectionable name, as they are not complex, but, on the con- 

 trary, often in their very nature simpler than the later groups which were 

 foreshadowed. 



5. The earliest types, as shown by Agassiz, sometimes having certain 

 characteristics which may be styled embryonic, being such as are presented 

 by embryos or young individuals of the tribe at the present time. 

 This principle flows out of the general truth that there is a degree 

 of parallelism between the grades of species in a group and the 

 successive stages in the embryonic development of an individual 

 animal in that group. Thus, the early Ganoids had a cartilagi- 

 nous vertebral column like the young of modern Gars, and the 



