78 BIOLOGICAL LECTURES. 



evolution. There is good reason to believe that even in Plio- 

 cene time the outlines of the continents were very different 

 from the present, some areas now below the sea being then 

 above it, while other tracts then beneath oceanic waters have 

 since been raised into dry land. We know that Miocene 

 geography differed still more greatly from that of to-day, and it 

 is not therefore unreasonable to suppose that in the Cretaceous 

 period large parts of the modern oceans were land, and large 

 parts of the modern continents were portions of the ocean, 

 the continental connections being totally different from what 

 they are now. In short, the interchange we believe is in the 

 frequent interchange of small portions of oceans and conti- 

 nents till, in the course of time, the accumulated changes have 

 accomplished great geographical mutations." 



