86 GROUPS OF ALLIED SPECIES [Chap. X. 



in the several formations of Europe and of the United 

 States ; and from the arnonnt of sediment, miles in 

 thickness, of which the formations are composed, we 

 may infer that from first to last large islands or tracts 

 of land, whence the sediment was derived, occurred in 

 the neighbourhood of the now existing continents of 

 Europe and North America. This same view has since 

 been maintained by Agassiz and others. But we do not 

 know what was the state of things in the intervals 

 between the several successive formations ; whether 

 Europe and the United States during these intervals 

 existed as dry land, or as a submarine surface near land, 

 on which sediment was not deposited, or as the bed of 

 an open and unfathomable sea. 



Looking to the existing oceans, which are thrice as 

 extensive as the land, we see them studded with many 

 islands ; but hardly one truly oceanic island (with the 

 exception of New Zealand, if this can be called a truly 

 oceanic island) is as yet known to afford even a remnant 

 of any palaeozoic or secondary formation. Hence we 

 may perhaps infer, that during the pakeozoic and 

 secondary periods, neither continents nor continental 

 islands existed where our oceans now extend ; for had 

 they existed, palaeozoic and secondary formations would 

 in all probability have been accumulated from sediment 

 derived from their wear and tear ; and these would have 

 been at least partially upheaved by the oscillations of 

 level, which must have intervened during these enor- 

 mously long periods. If then we may infer anything 

 from these facts, we may infer that, where our oceans 

 now extend, oceans have extended from the remotest 

 period of which we have any record ; and on the other 

 hand, that where continents now exist, large tracts of 



