LIFE IN THE EARLY CAMBRIAN 21 



Apalachians, in New Brunswick, and in Newfound- 

 land, in the table-land of Colorado and in the 

 Rocky Mountains. In point of fact, a map of the 

 Northern Hemisphere at this period would show 

 only a limited circumpolar continent with some 

 outlying islands to the south of it, and shallows 

 stretching across the northern part of the areas 

 of the present Atlantic and Pacific Oceans. The 

 great ocean, however, thus extending over most 

 of the temperate and tropical parts of the North- 

 ern Hemisphere, was probably also more muddy 

 and shallow than that of modern times. The sur- 

 face temperature of this vast ocean was also, it is 

 probable, more uniform than that of the modern sea, 

 while even its profounder depths or abysses would 

 have more earth-heat than at present. Thus we 

 may, without hesitation, affirm that in this early 

 age the conditions for the introduction of swarming 

 marine life of low grade, and its extension over 

 the whole earth, were at a maximum. 



Let us inquire, then, what these old Cambrian 

 seas actually produced, more especially in the early 

 portions of that ancient and probably protracted 

 time. 



The most highly organized type of which we 



