326 PROFESSOR E. HULL, LL.D., F.R.S., F.G.8., ON THE 
present day. A few forms, such as the Machairodus and Mast- 
odon have become extinct; others have been somewhat 
modified, but the general change is not very great. During 
this epoch there is reason to believe that the continent of 
Africa was the abode of a very different and less varied fauna, 
which is now represented by that of the Island of Madagascar 
and the Seychelles at the present day. How then are we to 
accuunt for the presence in Africa of the numerous forms of 
Pachyderms. Felides, Antelopes, and Ruminants which people 
this great continent? The explanation which Wallace offers 
is clear and satisfactory. At the close of the Pliocene, or 
commencement of the Pleistocene, period a great migration 
of the Europasian animals took place into Africa, by which 
the aboriginal forms were exterminated or driven out, and 
replaced by the invading host of fiereer and more powerful 
animals.* But in order to render such an invasion possible, 
one or more highways had to be constructed across the 
barrier of waters, caused by the Mediterranean, the Gulf of 
Suez, and the Red Sea. Under existing conditions, it is clear, 
such a migration would be impossible except by one narrow 
passage, the Isthmus of Suez, which would have been quite 
insufficient for such an invasion as is now contemplated. It 
would appear, however, that the very same cause which forced 
on the southward migration gave rise to the formation of, 
at least, two causeways from Europe to Africa across the 
Mediterranean as it now exists. This was the increasing 
cold of the on-coming Glacial period, which made itself felt 
towards the close of that of the Phocene. The gradual lower- - 
ing of the temperature, owing to which the animals were 
driven southwards, caused the streams which feed the Mediter- 
ranean from the northward to become frozen, and ultimately 
to be converted into ice. As the snows accumulated on the 
Pyrenees, the Alps, the Carpathians, and the vast plains of 
Russia, the sources of the rivers feeding the Mediterranean 
from the north would be dried up, or become much reduced, 
and as a consequence its surface would be lowered. Mr. 
Wallace considers that there were three causeways thus pro- 
duced by which the animals passed over into Africa; one, 
the Straits of Gibraltar; another, between Italy, Sicily, the 
Maltese Islands, and Tunis; and a third, the Isthmus of Suez, 
which still remains. There may also have been a general 
elevation of the Mediterranean region, by which the connec- 
* Wallace, Zsland Life (1880), and Geographical Distribution of Animals 
(1876). 
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