382 PALEOZOIC TIME. 



It is important to observe, in this connection, that the length of the 

 Age of Invertebrates, or Silurian age, as just shown, was at least 

 four times that of either the Devonian or the Carboniferous. 



The following are some of the principles bearing on the progress of 

 life, which have been exemplified in Paleozoic history. 



(1.) The earlier species were aquatic, and all of them marine. 



Protozoans, Radiates, Mollusks, and the water-Articulates, comprise 

 all known species of animals, and Sea- weeds all the fossil plants, to 

 the close of the Lower Silurian ; and the Upper Silurian adds only 

 Fishes, or aquatic Vertebrates, and terrestrial Cryptogams. In all 

 divisions of the kingdoms of life, the species made for the water are of 

 inferior grade. As already stated, there were probably exceptions, in 

 the existence of Lichens and Fungi even before the Silurian, and of 

 Insects and Spiders before the Devonian ; but direct proof of this is 

 wanting. 



(2.) Many of the earlier types were comprehensive types, that is, they 

 combined the characteristics of two or three groups of the same or 

 later time. Thus, the Brachiopods, the most common of all the kinds 

 of life, combined characteristics of both the Mollusks and the Worms, 

 and so decidedly that a recent writer, Mr. E. S. Morse, takes the 

 ground that they are more closely related to Worms than to Mollusks. 



Crinoids, and especially the Cystids, combine the flexible arms of 

 Starfishes with much of the box-like structure and other characters of 

 Echini. Trilobites have intermediate characters between those of 

 Entomostracans and those of Tetradecapods, although apparently be- 

 longing to the former of these groups. 



Neuropterous Insects of the Devonian and Carboniferous eras were 

 in general not purely Neuropters, but combined characters of Orthop- 

 ters also, showing it in their wings and other parts, one even having a 

 stridulating arrangement, which at present is peculiarly the property of 

 Orthopters. 



Ganoid fishes are well called Reptilian fishes by Agassiz, they hav- 

 ing the teeth of the ancient Labyrinthodont Reptiles, a cellular air- 

 bladder approximating to a lung, and a flexible articulation between 

 the head and neck — points not known among the ordinary Osseous 

 fishes. 



The Cephalaspids, the earliest Ganoids, were intermediate in some 

 respects between Ganoids and the Sharks, the other fishes of the De- 

 vonian. 



The Amphibian Reptiles of the Carboniferous were mainly if not 

 wholly Labyrinthodonts, species that, along with the ordinary charac- 

 ters of the Amphibians, had the scaly skin, strong teeth, etc., of 

 Lizards, or true Reptiles. 



The Lepidodendrids of the Coal era, while true Acrogens, have the 



