132 



THE CHAIN OF LIFE. 



of land-plants in che Devonian and Carboniferous. Another 

 curious illustration of the diminishing necessity for air-breathing 

 to the fishes, is the change of the tail from the unequally-lobed 



or heterocercal form, which pre- 

 vailed in the Palaeozoic, to the 

 more modern equally-lobed 

 (homocercal) style in the Meso- 

 zoic. The former is better 

 suited to animals which have 

 to rise rapidly to the surface 

 for air, and is still continued 

 in some modern fishes, which 

 for other reasons need to ascend 

 and descend, or to turn them- 

 selves in the water; but the 

 homocercal form is best suited 

 to the ordinary fish, whether 

 Ganoids or Teleosts (Fig. 122). 

 It is curious also to find the 

 beginning of the dominancy of the ordinary fish to coincide with 



Fig. 



119. 



-Tooth of a Tertiary Shark 

 (Carcfiarodon). 



Fig. 120. — A Liassic Ganoid (/?«/<"<///«). Restored, — After-Nicholson. 



tnat of the broad-leaved exogenous trees in the later Cretaceous, 

 and to precede immediately the appearance of the mammals on 

 the land ; all these changes being related to the purer air, the 



