DISTEIBUTION OF OPHIDIANS. 
209 
waters or woods ; while the animals intended especially to 
inhabit plains should be there found in abundance ; and 
these views are confirmed by experience. We see in 
Africa, instead of deer, many species of antelopes, wan- 
dering in vast herds in open regions. Squirrels are there 
found in small numbers, and the species which are there 
met with generally depart from the true squirrels by their 
terrestrial habits. The great number of the Rodentia that 
people that continent almost all belong to terrestrial spe- 
cies ; many of them even live in open countries, and, being 
unprovided with means of defence. Nature has attended to 
their preservation, by developing their organs of locomo- 
tion, so as to make them true leapers ; and it is in this 
manner that these animals possess the faculty of escaping, 
by a sudden flight from the pursuit of their enemies. We 
observe the same fact in certain mammifera of the insecti- 
vorous order. — The Eeptiles of that part of the world af- 
ford still more striking examples of what we assert. 
Africa alone supports a greater number of land tortoises 
than all other parts of the world put together ; but the 
fresh- water tortoises are in such small numbers, that we 
only know a single species of Emys, and possibly one or two 
species of the genus Trionyx. Another observation worthy 
of notice, is the small number of Batrachians proper to this 
continent. There exist but a few toads, some species of 
Bombinator, as many of the frog, and one or two species 
of tree-frogs (Hyla.) The same fact presents itself as re- 
gards tree and aquatic serpents. The Dryiophis and the 
Homalopsis are entirely wanting ; and there exists only 
two species of the genus Dipsas, two of the Dendrophis, 
and one or two of the Tropidonotus. The most striking 
example, however, is the almost total absence of fish in the 
fresh waters of Southern Africa. — But the general ob- 
servations which we have made on the physical constitu- 
tion of Africa cannot be applied to every country of that 
part of the world. At the point of greatest breadth in 
that continent, the great plateau which occupies all the 
southern part, descends rapidly towards the desert plains 
of the north, and is prolonged, on one side, beyond the 
Quorra, in Upper Soudan ; whilst the terraces of that same 
s 
