THE ORDOVICIAN PERIOD. 343 



relative fact that there was no distinct break in the succession of life. 

 All reasonable doubt that the continuity between the two periods was 

 real and not merely apparent is thus removed, for had any great inter- 

 val been left unrecorded by the deposits, it must have made itself 

 manifest by a break in the continuity of the life. There seems ample 

 warrant, therefore, for the view that the vast period from the beginning 

 of the Cambrian to the close of the Ordovician was one long eon of 

 progressive development and of expansion of life, and that its division 

 into two nominal periods is artificial rather than natural. 



Edaphic, Provincial, and Cosmopolitan Development. 



Nevertheless, the life progress was not uniform in all regions, though 

 in its general aspect it was cosmopolitan. It varied much as the con- 

 tinental evolution varied, and largely as the result of it. The variations 

 assumed three general phases : (1) adaptation to the immediate physical 

 environment, particularly the nature and depth of the sea-bottom 

 (edaphic adaptation); (2) modification by self -evolution within re- 

 stricted areas where isolated from other regions by barriers (provincial 

 evolution), and (3) modification toward a universal type through the 

 intermigration of the life of various regions when barriers were removed 

 (cosmopolitan development). 



(1) Edaphic modification — Areas in which the bottom was rocky, 

 or sandy, or muddy, or calcareous, had their appropriate life, as did 

 also the tracts of shallow and deep water, and the areas dominated 

 by other special conditions. The assemblages adapted to these special 

 conditions were not entirely distinct from one another, for there were 

 not a few forms that were indifferent to these conditions, particularly 

 the floating and free-swimming species; but the aspect of the fauna, 

 particularly the sea-bottom portion of it, was locally modified. These 

 modifications are well shown in the Ordovician fossils, but they are 

 of too intricate a nature to be set forth specifically here. 



(2) Provincial modifications. — Although for the greater part the 

 sea was spread widely upon the face of the continent in Ordovician 

 times, and the facilities for the intermigration and mingling of faunas 

 were usually excellent, there is evidence of considerable separation 

 into zoological provinces. This was probably due in the main to barriers 

 interposed by gentle warpings of the sea-bottom which introduced 

 emergent tracts and tracts of excessive depth; but probably also in 



