September ii, 1890] 



NATURE 



48 



migrated to their present habitat after the upheaval of the great 

 sea which in Eocene times stretched from the Atlantic to the 

 Indian Ocean, tnaking Southern Africa an island continent like 

 Australia. The original fauna of Africa, of which the lemur is 

 the distinctive type, is still preserved in Madagascar, which then 

 formed part of it. 



The fish fauna is naturally the most conclusive evidence as to 

 the true line of separation betwaen Europe and Africa. We 

 find the trout in the Atlantic region, and in all the snow-fed 

 rivers falling into the Mediterranean ; in Spain, Italy, Dalmatia ; 

 it occurs in Mount Olympus, in rivers of Asia Minor, and even 

 in the Lebanon, but nowhere in Palestine south of that range, in 

 Egypt, or in the Sahara. This fresh-water salmonoid is not 

 exactly the same in all these localities, but is subject to consider- 

 able variation, sometimes amounting to specific distinction. 

 Nevertheless, it is a European type found in the Atlas, and it is 

 not till we advance into the Sahara, at Tuggurt, that we come 

 to a purely African form in the Chromidse, which have a wide 

 geographical distribution, being found everywhere between that 

 place, the Nile and Mozambique. 



The presence of newts, tailed batrachians, in every country 

 round the Mediterranean, except again in Palestine, Egypt, and 

 the Sahara, is another example of the continuity of the Mediter- 

 ranean fauna, even though the species are not the same 

 throughout. 



The Sahara is an immense zone of desert which commences on 

 the shores of the Atlantic Ocean, between the Canaries and 

 Cape de Verde, and traverses the whole of North Africa, Arabia, 

 and Persia, as far as Central Asia. The Mediterranean portion 

 of it may be said roughly to extend between the 15th and 30th 

 degrees of north latitude. 



This was popularly supposed to have been a vast inland sea in 

 very recent times, but the theory was supported by geological 

 facts wrongly interpreted. It has been abundantly proved by 

 the researches of travellers and geologists that such a sea was 

 neither the cause nor the origin of the Libyan Desert. 



Rainless and sterile regions of this nature are not peculiar to 

 North Africa, but occur jn two belts which go round the world 

 in either hemisphere, at about similar distances north and south 

 of the equator. These correspond in locality to the great inland 

 drainage areas from which no water can be discharged into the 

 ocean, and which occupy about one-fifth of the total land 

 surface of the globe. 



The African Sahara is by no means a uniform plain, but forms 

 several distinct basins containing a considerable extent of what 

 may almost be called mountain land. The Hoggar Mountains 

 in the centre of the Sahara are 7000 feet high, and are covered 

 during three months with snow. The general average may be 

 taken at 1500. The physical character of the region is very 

 varied ; in some places, such as at Tiout, Moghrar, Touat, and 

 other oases in or bordering on Morocco, there are well-watered 

 valleys, with fine scenery and almost European vegetation, 

 where the fruits of the north flourish side by side with the palm 

 tree. In others there are rivers like the Oued Guir, an affluent 

 of the Niger, which the French soldiers, who saw it in 1870, 

 compared to the Loire. Again, as in the bed of the Oued Rir, 

 there is a subterranean river, which gives a sufficient supply of 

 water to make a chain of rich and well-peopled oases equal in I 

 fertility to some of the finest portions of Algeria. The greater 

 part of the Sahara, however, is hard and undulating, cut up by 

 dry water-courses, such as the Igharghar, which descends to the 

 Chott Melghigh, and almost entirely without animal or vegetable 

 life. 



About one-sixth of its extent consists of dunes of moving sand, 

 a vast accumulation of detritus washed down from more northern 

 and southern regions — perhaps during the glacial epoch — but 

 with no indication of marine formation. These are difficult and 

 even dangerous to traverse ; but they are not entirely destitute of 

 vegetation. Water is found at rare but well-known intervals, 

 and there is an abundance of salsolaceous plants which serve as 

 food for the camel. This sand is largely produced by wind 

 action on the underlying rocks, and is not sterile in itself, it is 

 only the want of water which makes it so. Wherever water does 

 exist, or artesian wells are sunk, oases of great fertility never fail 

 to follow. 



Some parts of the Sahara are below the level of the sea, and 

 here are formed what are called chotts or sebkhas, open depres- 

 sions without any outlets, inundated by torrents from the southern 

 slopes of the Atlas in winterand covered with a saline efflorescence 

 in summer. This salt by no means proves the former existence 



NO. 1089, VOL. 42] 



of an inland sea ; it is produced by the concentration of the 

 natural salts, which exist in every variety of soil, washed down 

 by winter rains, with which the unevaporated residue of water 

 becomes saturated. 



Sometimes the drainage, instead of flooding open spaces and 

 forming chotts, finds its way through the permeable sand till it 

 meets impermeable strata below it, thus forming vast subter- 

 ranean reservoirs where the artesian sound daily works as great 

 miracles as did Moses' rod of yore at Meribah. I have seen a 

 column of water thrown up into the air equal to 1300 cubic 

 metres per diem ; a quantity sufficient to redeem 1800 acres of 

 land from sterility and to irrigate 60, coo palm trees. This seems 

 to be the true solution of the problem of an inland sea ; a sea of 

 verdure and fertility caused by the multiplication of artesian 

 wells, which never fail to bring riches and prosperity in their 

 train. 



The climate of the Sahara is quite different from that of what 

 I have called the Mediterranean region, where periodical rains 

 divide the year into two seasons. Here, in many places, years 

 elapse without a single shower ; there is no refreshing dew at 

 night, and the winds are robbed of their moi-ture by the im- 

 mense continental extents over which they blow. There can be 

 no doubt that it is to these meteorological, and not to geological, 

 causes that the Sahara owes its existence. 



Reclus divides the Mediterranean into two basins, which, in 

 memory of their history, he calls the Phoenician and the Cartha- 

 ginian, or the Greek and Roman seas, more generally known 

 to us as the Eastern and Western Basins, separated by the island 

 of Sicily. 



If we examine the submarine map of the Mediterranean, we 

 see that it must at one time have consisted of two enclosed or 

 inland basins, like the Dead Sea. The western one is separated 

 from the Atlantic by the Straits of Gibraltar, a shallow ridge, 

 the deepest part of which is at its eastern extremity, averaging 

 about 300 fathoms ; while on the west, bounded by a line from 

 Cape Spartel to Trafalgar, it varies from 50 to 200 fathoms. 

 Fifty miles to the west of the Straits the bottom suddenly sinks 

 down to the depths of the Atlantic, while to the east it descends 

 to the general level of the Mediterranean, from 1000 to 2000 

 fathoms. 



The Western is separated from the Eastern Basin by the 

 isthmus which extends between Cape Bon in Tunisia and Sicily, 

 known as the " Adventure Bank," on which there is not more 

 than from 30 to 250 fathoms. The depth between Italy and 

 Sicily is insignificant, and Malta is a continuation of the latter, 

 being only separated from it by a shallow patch of from 50 to 

 100 fathoms ; while to the east and west of this bank the depth 

 of the sea is very great. These shallows cut off the two basins 

 from all but superficial communication. 



The configuration of the bottom shows that the whole of this 

 strait was at one time continuous land, affording free communi- 

 cation for land animals between Africa and Europe. The 

 palaeontological evidence of this is quite conclusive. In the caves 

 and fissures of Malta, amongst river detritus, are found three 

 species of fossil elephants, a hippopotamus, a gigantic dormouse, 

 and other animals which could never have lived in so small an 

 island. In Sicily, remains of the existing elephant have been 

 found, as well as the Elephas antiqtius, and two species of 

 hippopotamus, while nearly all these and many other animals of 

 African type have been found in the Pliocene deposits and 

 caverns of the Atlantic region. 



The rapidity with which such a transformation might have 

 occurred can be judged by the well-known instance of Graham's 

 Shoal, between Sicily and the island of Pantellaria ; this, owing 

 to volcanic agency, actually rose above the water in 1832, and 

 for a few weeks had an area of 3240 feet in circumference and a 

 height of 107 feet. 



The submersion of this isthmus no doubt occurred when thf 

 waters of the Atlantic were introduced through the Straits of 

 Gibraltar. The rainfall over the entire area of the Mediterranean 

 is certainly not more than 30 inches, while the evaporation is at 

 least twice as great ; therefore, were the Straits to be once more 

 closed, and were there no other agency for making good this 

 deficiency, the level of the Mediterranean would sink again till 

 its basin became restricted to an area no larger than might be^ 

 necessary to equalize the amount of evaporation and precipita 

 tion. Thus not only would the strait between Sicily and Africa 

 be "again laid dry, but the Adriatic and ^^gean Seas also, and a 

 great part of the Western Basin. 



The entire area of the Mediterranean and Black Seas has been 



