290 ZOOLOGICAL GEOGRAPHY. [PART lll. 
carnivora had reached it; and we consequently find there, no 
wholly terrestrial form of bird but the gigantic and powerful 
Aipyornis, well able to defend itself against such enemies. As 
already intimated, we refer the South American element in 
Madagascar, not to any special connection of the two countries 
independently of Africa, but to the preservation there of a 
number of forms, some derived from America through Africa, 
others of once almost cosmopolitan range, but which, owing to the 
severer competition, have become extinct on the African con- 
tinent, while they have continued to exist under modified forms 
in the two other countries. 
The depths of all the great oceans are now known to be so 
profound, that we cannot conceive the elevation of their beds 
above the surface without some corresponding depression else- 
where. And if, as is probable, these opposite motions of the 
earth’s crust usually take place in parallel bands, and are to 
some extent dependent on each other, an elevation of the sea 
bed could hardly fail to lead to the submergence of large tracts 
of existing continents; and this is the more likely to occur on 
account of the great disproportion that we have seen exists 
between the mean height of the land and the mean depth of the 
ocean. Keeping this principle in view, we may, with some 
probability, suggest the successive stages by which the Ethiopian 
region assumed its present form, and acquired the striking 
peculiarities that characterise its several sub-regions. During 
the early period, when the rich and varied temperate flora of the 
Cape, and its hardly less peculiar forms of insects and of low type 
mammalia, were in process of development in an extensive 
south temperate land, we may be pretty sure that the whole of 
the east and much of the north of Africa was deep sea. At a 
later period, when this continent sank towards the south and 
east, the elevation may have occurred which connected Mada- 
gascar with Ceylon; and only at a still later epoch, when the 
Indian Ocean had again been formed, did central, eastern, and 
northern Africa gradually rise above the ocean, and effect a 
connection with the great northern continent by way of Abys- 
sinia and Arabia. And if this last change took place with 
