When we come to the Exodus the impossibilities of the narrative are even more obvious. The robust common-sense of Bishop Colenso, sharpened by a mathematical education, has reduced many of these to the convincing test of arithmetic. The host of Israelites who left Egypt is said to have comprised 603,550 fighting men above the age of twenty; exclusive of the Levites and of a mixed multitude who followed. This implies a total population of at least 2,500,000, who are said to have wandered about for forty years in the desert of Sinai, one of the most arid wildernesses in the world, destitute alike of water, arable soil, and pasture, and where a Bedouin tribe of even 600 souls would find it difficult to exist. They are said to have been miraculously fed during these forty years on manna, a sweetish, gummy exudation from the scanty foliage of certain prickly desert plants, which is described as being "as small as the hoar frost," and as being so imbued with Sabbatarian principles, as to keep fresh only for the day it is gathered during the week, but for two days if gathered on a Friday, so as to prevent the necessity of doing any work on the Sabbath. Bishop Colenso points out with irresistible force the obvious impossibilities in regard to food, water, fuel, sanitation, transport, and other matters, which was involved in the supposition that a population, half as large as that of London, wandered about under tents from camp to camp for forty years in a desert. No attempt has ever been made to refute him, except by vague suppositions that the deserts of Sinai and Arabia may then have been in a very different condition, and capable of supporting a large population. But this is impossible in the present geological age and under existing geographical conditions. These deserts form part of the great rainless zone of the earth between the north tropical and south temperate zones, where cultivation is only possible when the means of irrigation are afforded by lakes, rivers, or melting snow. But there are none of these in the deserts of Sinai and Northern Arabia, and therefore no water and no vegetation sufficient to support any population. No army has ever invaded Egypt from Asia, or Asia from Egypt, except by the short route adjoining the Mediterranean between Pelusium and Jaffa, and with the command of the sea and assistance of trains to carry supplies and water. And the account in Exodus itself confirms this, for both food and water are stated to have been supplied miraculously, and there is no mention made of anything but the present arid and uninhabited desert in the various encampments and marches. In fact, the Bible constantly dwells on the inhospitable character of the "howling wilderness," where there was neither grass nor water. Accordingly reconcilers have been reduced to the supposition that ciphers may have been added by copyists, and that the real number may have been 6000, or even, as some writers think, 600.