180 



REPRODUCTION 



and among Vertebrates it was the Amphibians that began it. 

 At present Amphibia form but a small and inconspicuous 

 Class of Vertebrates and they show no particular enterprise. 

 But in Carboniferous times, when our coal-measures were 

 being laid down, their ancestors must have shown a wonderful 

 spirit of adventure. They are clearly derived from a fish-like 

 stock, possibly from one of the lung-fishes, in which the 

 swimming-bladder has been 

 converted into a respiratory 

 organ. Even at the present 

 time certain Amphibian gen- 

 era may be permanently 

 aquatic, and with very few 

 exceptions their larvae are 

 always cradled in the water. 

 Whatever their ancestry was 

 in the sea, they must have 

 come on land via the fresh 

 water. All the lung-fishes live 

 in rivers and the Amphibia 

 live in fresh water or lay 

 their eggs there, never in the 

 sea. It is common enough 

 among animals who have 



changed their habitat during ^^^^ ^^- }' Tadpoles soon after hatching 

 . *^ . ° clinging to water- weeds. 2. ladpole with 



their lifetime to return, when two pairs of external giUs. 3 and 4. Tad- 

 producing their young, to poles with operculum forward and form- 

 the place where th^v wpre i^?-. P- Tadpole with weU-developed 



they 



were 



hind legs. 



born. Land-crabs, for in- 

 stance, do this, and their eggs are always hatched in the 

 sea, and conversely turtles who have taken to a marine 

 habitat return to the land to lay their leathery eggs in the 

 hot, sandy beaches. The amphibian tadpoles have many fish- 

 like characters. They breathe by gills, they cannot move their 

 tongue, they swim by the flexure of their body and tail, they 

 have a two-chambered heart ; but gradually all these change. 

 The Amphibia are the lowest vertebrates which possess 

 fingers and toes. They can grasp an object, they can even 

 burrow and put food into their mouth. In the adult form they 



