LIFE ON THE EARTH. 23 



the sea and the free wanderers of the air. While 

 some genera are of very wide occurrence in the sea, 

 as the little Spirula, and the huge Physeter ; while 

 some birds pass from arctic to temperate, and from 

 temperate to torrid zones, and some quadrupeds 

 annually migrate over large breadths of land in 

 quest of food, the far greater number appear to be 

 restrained by necessity or limited by choice to narrow 

 tracts and definite associations of life. 



It is not by conformity of climate or physical 

 conditions, or oceanic currents, that the few existing 

 genera of Brachiopoda are now allowed representa- 

 tives in almost all seas, for each particular species 

 of these genera is usually limited to one small area, 

 or zone of ocean. Rhynchonella psittacea to the 

 circumpolar seas ; Terebratula vitrea to the Medi- 

 terranean ; and Waldheimia australis to the shore 

 of New Holland 1 . 



Reptilia in a general sense depend for their 

 distribution on favourable climate; but when we 

 examine any certain group, as the Crocodilidse, one 

 species is found to belong to the Nile, another to 

 the Ganges, a third to the North American rivers. 

 Struthious birds wander over the dry lands of several 



1 See on questions of Distribution of Mollusca, Woodward's 

 Rudimentary Treatise on Recent and Fossil Shells, an excel- 

 lent work. 



