

LIFE 377 





I 



an increase in numbers <>l individuals, and partly to better con- 

 diiinns of preservation. 



The general aspect of life was cosmopolitan, though it was not 

 he same everywhere. It varied with the physical evolution of the 

 continent, and largely as the result of it. The variations assumed 

 three general phases: (i) adaptation to the immediate physical 

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

 (2) modification by auto-evolution within areas isolated by barriers 

 (provincial evolution); and (3) modification toward a universal type 

 through intermigration (cosmopolitan development}. 



(1) Rocky, sandy, muddy, and calcareous bottoms had their 

 appropriate life, as did also tracts of shallow and deep water. The 

 faunas adapted to these special conditions were not altogether un- 

 like, for some animals, particularly free-swimmers, were indifferent 

 to them. 



(2) Although .the sea covered a large part of the continent, 

 affording facilities for the migration and mingling of faunas, there is 

 evidence of some separation into zoological provinces. This was 

 probably due partly (a) to barriers in the form of shoals, bars, and 

 spits, (b) to ocean-currents with their attendant differences in tem- 

 perature, and (c) to variations in the saltness of the waters. 



(3) Notwithstanding local and provincial modifications, the 

 progress of Ordovician life in the American continent seems to have 

 been, on the whole, in the direction of cosmopolitanism, especially 

 in the shallow water faunas of the great interior of the continent. 

 This was due, primarily, to the wide epicontinental seas, which 

 permitted free migration. 



The Ordovician system contains an exceptionally large number 

 of fossils of free-floating graptolites 1 (Fig. 343). Their remains are 

 mingled with the fossils of shallow-water life, showing that they 

 swam over the epicontinental seas freely. The Ordovician grapto- 

 lites are nearly identical in Europe, North America, and Australia. 

 The history of individual species was short, geologically speaking, 

 and hence the succession of species marks the progress of events in 

 all parts of the ocean. During the lifetime of the graptolites 

 (limited to the late Cambrian, Ordovician and Silurian), a score of 

 successive zones, each characterized by particular species, have been 



1 It is not universally agreed that all graptolites were floating forms at all 

 stages, but there seems to be little doubt that they usually were in their young 

 stages at least. 





