o 



o CHARLES D. WALCOTT 



edly present, and, together with the minute Protozoa, must have formed 

 the primary food-supply.' 



The role assigned by Dr. W. K. Brooks to microscopic forms was 

 an important factor in Cambrian time, for the organisms found in the 

 rocks of that period were mainly carnivorous, and were adapted either 

 to straining minute organisms from the water, or to gathering them 

 up from the bottom. 



Uniform marine physical conditions over the submerged portions 

 of the continental platform in Lower Cambrian time are indicated by 

 the uniformity of the fauna on opposite sides of the present continent. 

 Whether this fauna was distributed between the east and the west to 

 the north of the central land-area, or south of it, is not definitely deter- 

 mined, yet the absence of Lower Cambrian rocks and fossils from the 

 collections made in the Arctic region, and the presence of closely allied 

 species in the Lower Cambrian rocks of Alabama and California, point 

 to the southern coast-line as the probable highway for the distribution 

 of the littoral fauna. Nothing that suggests the Lower Cambrian fauna 

 is known from South America; in this case, deep water may have been 

 the barrier. 



With the advent of Middle Cambrian time land- areas came into 

 existence on the northeast, forming barriers which so affected marine 

 conditions in relation to life that the Paradoxides fauna developed in 

 the Atlantic basin and the Olenoides fauna in the Appalachian region 

 south of the Champlain Valley. To the south and on all sides of the 

 central land-area the advancing seas forced the faunas to shift their 

 habhat and either to adjust themselves to the new conditions or to 

 perish. Local isolation for long periods led to the development of new 

 forms, and these, when the barriers were removed, contested and com- 

 peted for their position and life with other faunas until, by a process of 

 elimination of those least fit to survive, there was hastened the devel- 

 opment of a large and varied fauna. With the close of Middle Cam- 

 brian time more stable conditions returned, and the era of rapid 

 evolution was checked until the impulse of new conditions of environ- 



I W. K. Brooks, Studies from the Biological Laboratory, Johns Hopkins University, 

 Vol. V, 1893, pp. 136-38. On p. 137 Dr. Brooks says: "The simplicity and abun- 

 dance of the microscopic forms and their importance in the economy of nature show 

 that the organic world has gradually shaped itself around and has been controlled by 

 them." 



