LIFE ON THE EARTH. Ill 



the Trout, and other valued creatures, to rivers which 

 have never yet been visited by them. 



The results arrived at by tracing some of the 

 successive groups of freshwater life, from the earliest 

 date to their eventual distribution, are veiy instruc- 

 tive in regard to what may, with reference to human 

 ideas, be termed the Theory of Creation. Taking 

 first the molluscous classes and remembering that 

 Brachiopoda ahd Monomyaria are absent from fresh 

 waters, we may fix our attention on the Dimyarian 

 family of UnionidaB and Cycladse. They occur fossil 

 according to the following scale: 



Caenozoic^-Unio, and Cyclas, not very plentiful, with Gas- 

 teropoda and land-plants. 



Mesozoic Wealden Unio and Cyclas, with Cyprides 



and Gasteropoda and land- 

 plants. 



Purbeck Unio and Cyclas, with Cyprides 



and Gasteropoda and land-plants 

 (Cycadacese). 



Yorkshire Oolitic. Unio not common, with land- 

 plants (Cycadacese, &c.) 



Palaeozoic Coal-formations. Unio, several species, common, 



with land-plants, Lepidoden- 

 dron, &c. Cyprides, <kc. 

 Upper Old Red. Unio, in one locality, with Ferns. 



Here we have the remarkable fact of decided 

 affinity in the successive groups of freshwater bi- 



