306 



EVOLUTION AND ANIMAL LIFE 



zation for aquatic life, the true fishes must have been derived. 

 In the more primitive of these the air bladder retains the lung- 

 like structure characteristic of the Fringe-fins. But in the 

 more specialized forms this is reduced to a sac, at first with an 

 open tube, then to a closed sac without tube in the adult, and 

 finally in very many of the true fishes the air sac is altogether 

 lost. On the other hand, in the Amphibia, which were prob- 

 ably also derived from the Crossopterygia, the air bladder is 

 more highly specialized, fitting these animals for life outside 



the water, and the 

 fins give place to 

 fingers and toes as 

 befitting a terres- 

 trial habit. 



The amphibians 

 deposit their eggs in 

 damp places, and 

 the young are 

 hatched while the 

 external' gills are 

 still functional. 



Among the rep- 

 tiles, which mark the next stage of adaptation for terrestrial 

 life, the gills are absorbed before the animal leaves the egg. 

 The reptile is therefore no longer confined to the neighbor- 

 hood of the water for purposes of reproduction. 



The bird, derived from the reptile, and at first distin- 

 guishable solely by the possession of feathers, loses later 

 various reptilian traits and the group becomes one inhabit- 

 ing the air. 



From the reptiles again are derived the lowest mammals. 

 The Monotremes of Australia lay eggs as reptiles do, these, like 

 reptiles' eggs, being covered with a leathery skin. The higher 

 mammals hatch the eggs within the body, nourish them with 

 milk and, in general, care for them in a degree unknown within 

 the class of reptiles. The traits of external hair, warm blood, 

 double circulation of the blood from and to a two-chambered 

 heart, and other characters of the mammals become fixed with 

 time and the group diverges into a multitude of forms living 

 and extinct, the last, and on the whole the most specialized of 

 the series being Homo, the genus of man. 



FIG. 181. The flying dragon (Draco). (After Seeley.) 



