5IO NATURAL SCIENCE. sept.. 



He now advances certain arguments against the wholesale inter- 

 change of oceanic and continental areas, but the force of his reasoning 

 is greatly weakened by a misconception which appears to underlie 

 the whole of it. He seems to think that those who oppose the theory 

 of the permanence of oceans must believe in the possibility of a 

 brand new continent rising whole and complete from the ocean 

 depths while an older continent was compelled to vanish beneath 

 the waters in order to preserve the terrestrial balance. I am not 

 aware that anyone has ever propounded such an audacious hypothesis. 

 I have never seen it seriously suggested that an entire continent has 

 been contemporaneously submerged, nor has any modern geologist 

 contemplated " the building up of a continent the size of Africa from 

 the mean depth of the ocean." Certainly Dr. Wallace is not 

 warranted in imputing such crude ideas either to Dr. Blanford or to 

 myself, and every geologist will admit that " the remarkable 

 parallelism and completeness of the series of geological formations in 

 all the best known continents indicates that none of them have risen 

 from the ocean floor during any portion of known geological 

 history." 



Those who oppose the doctrine of permanence say that the 

 present continents are the outcome of a long series of geo- 

 graphical mutations, and I would add that each phase was 

 an episode in a long process of geographical evolution. There 

 is good reason to believe that even in Pliocene time the out- 

 lines 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 dif- 

 ferent from what they are now. In short, the interchange we believe 

 in is the frequent interchange of small portions of oceans and con- 

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

 accomplished great geographical mutations. 



Against this view of interchange it does not seem to me that 

 either the first or the third of the considerations mentioned by Dr. 

 Wallace are very powerful objections. With respect to the second 

 argument, that there are no irregularities on the ocean floor corre- 

 sponding to those on land, I would point out (i) that it applies most 

 completely to the deeper and consequently to the oldest portions of 

 the ocean floor, (2) that we really as yet know very little about the 

 details of sub-oceanic contours, (3) that long-continued deposition 

 must tend to obliterate any pre-existent irregularities, (4) that such 

 inequalities are likely to have been much smaller than those shown 

 in continental areas, because the continents are probably those 



