160 ELEMENTS OF BIOLOGY. 



more than one or two classes of the deposits which must 

 have been formed in every great period. We may have 

 the deep-sea deposits of the period, or the littoral accumu- 

 lations, or the sediments which were laid down in its rivers 

 and lakes ; but we very seldom, if ever, obtain all of these. 

 We can therefore rarely expect to acquire a complete 

 knowledge of even the aquatic animals alone of any period. 

 4. Lastly, we have every reason to believe that the life of 

 vast periods of the earth's history will ever remain to us 

 wholly, or almost wholly, unknown, in consequence of the 

 fact that the deposits of these periods have been subjected 

 to such change that all traces of their contained fossils have 

 been destroyed. 





