726 HISTORICAL GEOLOGY. 



already had the lung for respiration, which is the characteristic feature of 

 all terrestrial Vertebrates. 



In rising from the nuiltiplicate structure of the Fish to the grade of 

 Amphibian, the Vertebrate type reached a fixed or normal limit in the 

 number of limbs, in the number of the bones of tlie fore and hind limbs, 

 including even the number of digits, but not in the number of articulations 

 of the digits. In the typical species of the old Carboniferous Amphibians 

 the fore limbs have the scapula, humerus, radius and ulna, wrist bones, and 

 the five fingers characteristic of the higher Vertebrates. 



Further, in rising to Amphibians, the method of progression, which is 

 urosthenic in Fishes, became podosthenic in the adult Amphibian. The 

 young Amphibian, or Tadpole, retains the urosthenic feature and the gills of the 

 Fish ; but in passing to the adult stage, when feet are developed, the higher 

 Amphibians lose both the tail and gills and have only feet for locomotion. 

 The tailed Amphibians are intermediate forms representing the stages of 

 progress. The absence of limbs in the Amphibian Snakes of the Carbon- 

 iferous is probably a case of degeneration. 



True Reptiles occur in the Permian. In this higher grade of Vertebrates 

 the fish-like features of gills, and of tails for locomotion, are absent in the 

 young state, and feet are throughout the locomotive organs. Besides, the 

 number of joints in the digits of the fore and hind feet of these terrestrial 

 Vertebrates has essentially the normal limit. But the teeth in the earlier 

 species are still multiplicate in number and in series. 



One prominent difference between the Eeptilian and Amphibian skeletons 

 is the existence in Amphibians of two occipital condyles for the articulation 

 of the skull with the first cervical vertebra, while in Keptiles there is but 

 one. In this feature Mammals, as early stated by Huxley, are more nearly 

 related to Amphibians than to Reptiles or Birds. 



REALITY OP THE PALEOZOIC WORLD. 



The term Paleozoic is not simply a name for a division of geological time. 

 It expresses a profound historical truth. It signifies the reality of a Paleo- 

 zoic character in the world's early life which was exhibited not only in the 

 very earliest of plants and animals, but also throughout the succession of 

 species, and so decidedly that the Paleozoic world stands out in bold contrast 

 with the Mesozoic. This truth has the greater importance inasmuch as 

 Paleozoic species were the earth's population for more than half of all post- 

 Archaean time. 



The truth of this statement is obvious after the review of Paleozoic life 

 on the preceding pages. Corals, Crinoids, Trilobites, Brachiopods and Mol- 

 lusks, even of their highest group, that of Cephalopods, commence in the 

 Cambrian and are prominent through the Paleozoic. Trilobites end near the 

 close of Paleozoic time. The prolific Brachiopods at its close lose nearly all 

 their Paleozoic genera; Crinoids drop their Paleozoic characteristics, and 



