280 THE STUDY OF ANIMAL LIFE CHAP. 



lives where pools are scarce, the young, after breathing 

 by gills for a time within the mother, are born as lung- 

 breathers ; also in some species of tree-frogs (Hylodes), 

 which live in situations where water is scarce, the gilled 

 stage is suppressed. 



The development of the frog (fig. 90) should be studied 

 by every student of natural history. The eggs are 

 fertilised as they are being laid. The division of the ovum 

 . can be readily observed. In its early stages the tadpole 

 is somewhat fish-like, for instance in its two-chambered 

 heart and in its circulation ; and the mouth reminds 

 one at first of a lamprey's. Its gills are more like the 

 external gills of mud-fishes than the gills of ordinary 

 fishes, and there are two sets of them. When the tadpole 

 is about two months old, it breathes by lungs as w r ell 

 as gills, and comes to the surface to take gulps of air. 

 It is then physiologically like one of the mud-fishes or 

 double-breathers (Dipnoi). Towards the third month- 

 the time varying with the species and the temperature- 

 the gills entirely disappear, the tail is all but absorbed, 

 and a minute lung-breathing frog climbs out of the water. 

 The larva feeds first on its legacy of yolk, then on fresh- 

 water plants, then on small animals or even on its own 

 relatives ; then it fasts, absorbing its tail ; and finally it 

 becomes an insect-catching frog. 



7. Reptiles. Fishes and Amphibians are closely allied ; 

 so Reptiles are linked to Birds, and more remotely to 

 Mammals also. Those three highest classes Reptiles, 

 Birds, and Mammals are very different from one another, 

 but they have certain characters in common. Most of 

 them have passed from the water to dry land ; none of 

 them ever breathe by gills ; all of them have two 

 embryonic birth-robes amnion and allantois which 

 are of great importance in early life. Compared with the 

 other Vertebrates, the brains are more complex, the 

 circulation is more perfect, the whole life has a higher 

 pitch. As symbols of mammal, bird, and reptile, take 

 the characteristic coverings of the skin hair, feathers, 

 and scales. Hair typifies strength and perhaps also 

 gentleness ; feathers suggest swift flight, the beauty 



