“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.” [Illustration: SALT-BUSH IN THE HEART OF THE SAHARA KILLED OUT BY CHOKING SANDS] [Illustration: DISINTEGRATING ROCK IN A REGION OF TASSILI] Again, in a further paper in _Novitates Zoologicæ_, March 1924, dealing with my second expedition, Dr. Hartert adds: “More than ever it is clear that the ornis of Aïr is tropical, as a country where Sunbirds, Barbets, Glossy Starlings, etc., live has a tropical ornis, though there are a number of palæarctic species, to which now a few must be added. On the other hand, these striking tropical families like Sunbirds, Glossy Starlings, Emerald Cuckoos, Hornbills, Barbets, are absent from the Ahaggar (Hoggar) Mountains, and the almost lifeless desert between Aïr and Ahaggar forms the boundary between the palæarctic and tropical African faunas.” From all this it is clear that Aïr maintains many tropical influences that penetrate northward, like a wedge, far into the Sahara, although its surroundings are foreign to like conditions. For instance, regarding the last remark, if we draw longitudinal lines 200 miles or so clear of either side of the Aïr Mountains, immediately those lines leave the southern shores of the Sahara, about latitude 15°, they enter desert where all tropical influence ceases.