FAMILIES OF ROCKS 



such highly developed forms of land-life that they prob- 

 ably had some primitive ancestors in the Cambrian. No 

 real fish have been found in the Cambrian rocks, but 

 they appeared in the next era (Ordovician). It is the 

 trilobite which is the characteristic animal of the Cam- 

 brian times. They were crustaceans ; they had eyes ; and 

 they gave the promise of development; but there is no 

 reason for believing that they were as high in the order 

 of creation as the commonest lobster of the sea-shore. 

 Nothing remains to us of them except their bony structure, 

 but we believe that they could both swim and walk on the 

 sea-bottom ; that some were swift of movement, and that 

 they acquired the habit of moulting their shell. They 

 may have been sociable animals, for the shells of trilo- 

 bites are sometimes found together in large numbers, 

 occasionally closely packed, " spoon fashion," and though 

 these may be moulted shells, we are warranted in sup- 

 posing that the early trilobites lived in colonies, hunted 

 for food, and made war like their descendants millions 

 of years after. What were the actual conditions of life 

 in this world of Cambrian days we do not know posi- 

 tively. The first beginnings of life, the simple one-celled 

 plants, may have first dwelt in the deep ocean. The land 

 was barren, its lakes unfitted to support life. On the 

 other hand, it is equally likely that the first beginnings 

 of life may have been the simple plants growing in inland 

 waters and gradually spreading down to the sea. We 

 do not know, but it is most probable that life began 

 in some great body of water, where plants and insignifi- 

 cant animals grew "together, perhaps fought together, 



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