248 Prof. Hughes, On the relation of the appearance [May 5, 



convenient grouping is obviously to bracket together locally con- 

 tinuous deposits, i.e. all the sediment which was formed from the 

 time when the land went down and accumulation began to the 

 time when the sea bottom was raised and the work of destruction 

 began. 



In the accompanying table (Plate VI.) I have given the rocks 

 of Great Britain classified on this system, and bearing in mind 

 that waste in one place must be represented by deposit elsewhere, 

 I have represented the periods of degradation by intervals esti- 

 mated where possible by the amount of denudation known to 

 have taken place between the periods of deposition in the same 

 district. 



It is obvious that when the dry land goes down there is an end 

 of exclusively terrestrial life over that region, and when the sea 

 bottom has been upheaved there can be no more exclusively 

 marine forms over that area till it goes down again, and when the 

 one comes up or the other goes down it will be invaded from 

 adjoining areas by those forms of life for which it from time to 

 time becomes adapted. But they may not be the same as those 

 that inhabited it before. 



Supposing then a submergence along the axis of the Medi- 

 terranean were to move south, so that Africa would by degrees 

 sink, being always encroached upon by a deep sea creeping over it 

 from the north, the land sinking on the south and rising on the north, 

 so that Europe followed, extending on the north side of the sea, as 

 Africa was then swallowed up on the south. First, we might imagine 

 that the Alpine plants, which according to Hooker still linger in 

 the high mountains of Morocco, would never cast their seed and 

 grow from year to year so as to get across the equator and they 

 Avould all perish. Whether the Black Sea fish and Caspian seals 

 could get away round by France or would all disappear might be 

 difiicult to answer. When the tropical part of Africa was sub- 

 merged its snakes, its lions, its elephants might hold their own 

 till the Cape of Good Hope was reduced to an island too small for 

 them. 



But we have assumed that there would be land on the north 

 side of this sea, and such forms as could migrate and adapt them- 

 selves to the climate w^ould follow the receding sea. The monkeys 

 from Gibraltar and from India would take the place of the gorilla and 

 chimpanzee of the Tropics. The rhinoceros of Sumatra and the 

 Asiatic elephant might replace their African cousins. The kite 

 and the kestrel, the dolphin and the tunny, the lion and the tiger 

 might still be there, but the ostrich and the giraffe would have no 

 representatives. 



How many genera, how many species would be common to the 

 Old and New Africa, whether we searched its blown sand or its 



