THE POPULAR SCIENCE MONTHLY.
younger and more highly developed competitors solely in virtue of their singular combination of desert-resisting qualities.
Now, it must at once strike everybody as a curious circumstance in the geography of animals that the existing cameloids should be so strangely distributed—one group of them in the desert region of Asia and Africa, the other group away across the world among the snowy slopes of the Andes of South America. What can be the meaning of so quaint a freak of distribution? Why should the two surviving cameloid tribes be thus separated from one another by half the earth's surface, and by many deep seas and shadowy mountains—one in the Old World and one in the New; one in the desert and one in the uplands; one in the northern hemisphere and one in the southern? Clearly, the answer suggested alike by geological facts and by analogies elsewhere, is simply this: we have here, as it were, two little surviving biological islands, colonies of an ancient race which once covered both worlds alike with its numerous members. Time was when the camels or their allies were of cosmopolitan distribution. They ranged, no doubt, the Eocene plains of all the great continents. But they are an ancient and in many respects an undeveloped ungulate form, which has become extinct elsewhere in the intermediate regions through the fierce competition of the higher ruminants, and has lingered on only under special circumstances in two remote corners of the world—in the deserts of Arabia and in the Andes of Peru.
The llamas and alpacas, as the lower and less specialized type of the two, explain best the true systematic position of the family. For South America, as everybody knows, is in many respects a very antique biological province. Less ancient in its life-forms than Australia, that world of living mesozoic fossils, it yet retains in many places the scattered remnants of its extremely old-fashioned fauna. There is reason to believe, indeed, that the circumpolar continent—Europe, Asia, and North America—was once for many ages continuous, while Australia, South Africa, and the South American peninsula formed separate islands in a wide and winding southern sea. Hence the higher life-forms developed rapidly in the broad and varied northern land-mass, while more antiquated types continued to live on, uninfluenced by their competition, in the three isolated southern provinces. Of these three, Australia alone still remains a great island; but South Africa has been joined to the Mediterranean world by a gradual upheaval of the Saharan area; while the Isthmus of Panama, still later in date, apparently, has formed a great natural bridge by which some of the North American land-animals have been able to invade the comparatively unpeopled tropical realms of the low southern species. In both cases, however, many of the low local