THE CAMBRIAN PERIOD. 303 



nizes sufficiently the fact that a great evolution of ancestral forms must 

 apparently have taken place in a common body of water, and in close 

 association with one another, for their defensive coverings are good 

 evidences that they had lived and battled together long enough to 

 evolve these and all the structures, habits, activities, and dependencies 

 that go with them. 



