CHAPTER XVIII. 



THE PBDIGEEE OF MAN. 

 III. From the Primitive Fish to the Amniotic Animal. 



Comparative Anatomy of the Vertebrates. — The Characteristic Qualities of 

 the Double.noatrilled and Jaw-monthed : the Double-Nostrils, the Gill- 

 arch Apparatus, with the Jaw-arches, the Sivimming-bladder, the Two 

 Pairs of Limbs. — Relationship of the Three Groups of Pishes : the Pri- 

 mitive Fishes (^Selachii), the Ganoids (Ganoides), the Osseous Pishes 

 {Teleostei), — Dawn of Terrestial Life on the Earth. — Modification of 

 the Swimming-bladder into the Lungs. — Intermediate Position of the 

 Dipneusta between the Primitive Fishes and Amphibia. — The Three 

 Extant Dipneusta (Protopterus, Lepidosiren, Ceratodus) . — Modification of 

 the Many-toed Fin of the Fish into the Five-toed Foot. — Causes and 

 Effects of the latter. — Descent of all Higher Vertebrates from a Five-toed 

 Amphibian. — Intermediate Position of the Amphibians between the 

 Lower and Higher Vertebrates. — Modification or Metamorphosis of 

 Frogs. — DiflFerent Stages in Amphibian Metamorphosis. — The Gilled 

 Batrachians (Proteus and Axolotl) . — The Tailed Batraohians (Salaman. 

 ders and Mud-flsh). — Frog Batrachians (Frogs and Toads). — Chief 

 Group of the Amnion Animals, or Amniota (Reptiles, Birds, and 

 Mammals). — Descent of all the Amniota from a Common Lizard-like 

 Parent-form (^Protarrmion) . — First Formation of the Allantois and of the 

 Amnion. — Branching of the Amnion Animals in Two Lines : on the one 

 side, Reptiles (and Birds), on the other side. Mammals. 



" The imagination is an indispensable faculty ; for it is that which, by- 

 forming new combinations, occasions important discoveries. The naturalist 

 needs both the discriminating power of abstract reason, and the generalizing 

 power of the imagination, and that the two should be harmoniously inter. 



