If we look, now, at the map, and picture that the Sahara is, broadly speaking, a vast sheet of sand with a few island mountains, it will suffice in dealing generally with its boundaries of the past.
It is my belief that the Sahara is increasing in size, and I think there are many conditions that go to prove it. Wherefore I ask you, in the first place, to conceive that the sand in the desert has steadily risen, with consequent result that the shores have become appreciably less. The belt that has been so engulfed all around the margin, or wherever the surface was shallow, may be taken to represent the regions that are to-day pre-Saharan, though, so far as I am aware, such pre-Saharan areas are seldom more than vaguely referred to, and have not been geographically defined.
I will take, as an example, the southern area of the Sahara, because I have visited it more than once and know that region best. Not vastly distant from the shore there is the mountainland of Aïr, standing high above the surrounding country. Let us suppose that, before the Sahara commenced to fill up and change, this particular mountainland was not surrounded by sand, but was a part of a fertile foreland, and that the bushland of the Western Sudan, with its tropical fauna and vegetation and rainy season, either jutted out as a wedge or stretched right across Africa about the 20th degree of latitude, or 5 degrees farther north than obtains, with any solidity, at the present time.[5] If that was the case intimate problems that I have had to contend with would be logically explained.
My primary work in the Sahara was that of a field naturalist, and the following extracts from Dr. Hartert’s paper in Novitates Zoologicæ, May 1921, regarding my first journey, have bearing on one of the problems that I wish to deal with:
“The best zoogeographical boundary, apart from the oceans, has hitherto been the Sahara, a wide belt of poorly inhabited and unexplored country. As long as we knew very little about it, this was a very simple question—north of the Sahara palæarctic, south of it Ethiopian. This contention, however, was bound to be shaken to some extent when the Sahara (as it is marked on maps) became zoologically explored. Until the second decade of this century the Great Desert had only been touched by zoological collectors on some of its borders.
“Looking at any map, a somewhat large mountainland, Aïr, or Asben, catches the eye in the middle of the Sahara, on older maps and in textbooks called an ‘Oasis,’ which is, however, a most misleading name for a mountainous country with desert tracks and valleys, towns and villages, and mountains rising up to about 2,000 m. in height.
“Zoologically Aïr remained absolutely unknown until Buchanan’s expedition. We knew already, from Barth’s Travels, that Aïr has tropical vegetation, that some valleys are fertile and contain good water, that ostriches, lions, giraffes, birds were seen by him, that near Agades he observed monkeys and butterflies. Jean, in 1909, in his book, Les Touaregs du Sud-Est, l’Aïr, mentions lions in the mountains of Timgue and Baguezan, foxes, hyenas, cats, antelopes, monkeys, but he adds that giraffes do not now exist in the country, and that the ostrich is not found north of Damergu.
“Meagre as these statements are, they proved that the fauna of Asben is chiefly, if not entirely, tropical. This is borne out by Buchanan’s collections. Of the birds nearly all—apart from migrants—may be called tropical species or subspecies. The mammals are on a whole Sudanese, and not found in Algeria proper. The Lepidoptera are essentially Saharan, many forms being similar to those found by Geyr and myself in the Sahara between the Atlas and Tidikelt, and the Hoggar Mountains.
“The boundary between the palæarctic and tropical fauna may therefore be regarded as fairly fixed to about the 20th degree of latitude, though it is, of course, not a hard-and-fast dividing line, there being many exceptions—even among birds, which form the main basis of these notes.”
