PHYSICAL FEATUKES. xiii 
themselves in its zoology have almost invariably selected the winter months for 
their journeys seems largely to account for the very meagre knowledge we, as yet, 
possess of its Reptiles. 
The few species that have been recorded from the Nile Valley as far south as 
Khartum, Kordofan, and Sennaar are incidentally noticed, so that a still wider horizon 
is brought into view. 
Physical Features of the Areas treated op in this Work. 
The area lying approximately between 16° and 17° 40' N. lat. is a zone in which an 
annual struggle for the mastery takes place in May and June between the northerly 
and southerly winds. The wind from the latter direction follows the sun in its course 
to the north of the Equator and carries along with it rain-clouds brought up from the 
Indian Ocean ; but as it approaches this zone it encounters opposition from the north 
wind that blows up the Nile Valley and over its deserts, and is ultimately neutralized 
by it. The climatic conditions over this debatable zone vary annually, more or less, 
according to the relative strength of the two winds. The immense area to the south 
of this zone, as far as the Equator, is characterized by the periodical fall of rain, 
generally between the months of April and October, and as the Equator is approached 
the rain becomes more and more torrential. The northern limit of these periodical 
rains may be placed about the junction of the Atbara with the Nile. However, even 
so far south as Khartum, where the rain begins rarely before June or July, and lasts 
to September, the rainfall is frequently very deficient. These periodical rains, where 
fully developed, exercise a remarkable influence on the character of the flora, and 
consequently also on the fauna. To the north of 17° 40', on the other hand, as far 
almost as the head of the Delta of the Nile, there is a complete absence of periodical 
rains ; indeed the entire region is practically rainless, and thus the sparse flora and 
the fauna differ entirely from those of the southern portion of the region of 
periodical rains. When rain does fall north of 17° 40', it is only as fitful local 
storms of short duration after long intervals of absolute drought, which may extend to 
years. These storms are generally violent and attended by high wind, thunder and 
lightning, the rain being precipitated in torrents, filling the empty khors and flooding 
the plains. Whilst the area may be thus generally characterized, the portion of it 
lying near to the shores of the Red Sea, traversed by high mountains, is more highly 
favoured, as storms of rain more frequently break along their heights. 
The lateral extent of Upper Nubia is clearly defined on the east by the more or less 
forest-clad slopes of the promontories of the Abyssinian plateau, and on the west by 
the main stream of the Nile. Its southern half falls within the so-called Eastern 
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