12 RELICS OF PRIMEVAL LIFE 



(6) Sponges^ Foraminifera and Animalcules of 

 simple organization (PROTOZOA). 



There are, it is true, some animals allied to the 

 mollusks and worms, which might be entitled to 

 form separate groups, though of minor importance 

 The position of the sponges is doubtful, and the 

 great mass of Protozoa may admit of subdivision ; 

 but for our present purpose these six great groups 

 or provinces of the Animal Kingdom may be held 

 to include all the humbler forms of aquatic life, 

 and they keep company with each other as far as 

 the Early Cambrian. If, in accordance with the pre- 

 vious statements, we choose to divide the earth's 

 history by the development of animal life rather 

 than by rock formations, and to regard each period 

 as presided over by dominant animal forms, -we 

 shall thus have an age of man, an age of mammals, 

 an age of reptiles and birds, an age of amphibians 

 and fishes, and an age of crustaceans and mollusks. 



It is only within recent years that the researches 

 more especially of Barrande, Hicks, Lapworth, 

 Linarrson, Brogger, and others in Europe, and 

 of Matthew, Ford and Walcott in America, have 

 enlarged the known animals of the Lower Cam- 

 brian to nearly 200 species, and below this we 



