168 THE EVOLUTION OF CONTINUITY 



elevation and subsidence of the ocean's bed in the distant 

 past. That elevation could bring about the evolution of a 

 new terrestrial type from a previously existing marine one. 



But the successive strata of the earth's crust do not reveal 

 to us in fossil forms any regular multiplications of Continuity, 

 and for the simple reason that as early as the Silurian Period 

 the main forms of living Continuity had already evolved. 

 It is difficult to believe otherwise than that the Archaean 

 rocks were once stratified, and that the alternate processes 

 of elevation and subsidence kept occurring during immense 

 stretches of time long before the Silurian Period. It is 

 indeed quite possible that as many strata preceded the 

 Silurian Period as have followed it. 



A little thought makes it clear that immense stretches 

 of time would be necessary for the evolution of the different 

 forms of Segmental Continuity, alone, which we know 

 existed in the Silurian Period ; and if we add to this the 

 time which must have been necessary for the evolution of 

 the Continuity -types lower than segmental, and the then 

 existing species within these types, we may conclude that 

 Life obtained on the globe at least as long before the Silurian 

 Period as from that Period to the present day. 



