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Show 28 ISLAND LIFE. [PART I. f d. I mot11s Urania is confined to the same ~enus o 1urna 1 · 1 • ~wo countries. A· somewhat similar but better known Illus-tration is afforded by the two genera of ostriches, one confined to Africa and Arabia, the other to the plains of temperate South America. General jeat~tres of Overlapping and Disoontinumts Areas.- These numerous examples of discontinuous genera and families form an important section of the facts of animal dispersal which any true theory must satisfactorily account for. In greater or less prominence they are to be found all over the world, and in eyery group of animals, and they grade imperceptibly into those cases of conterminous and overlapping areas which we have seen to prevail in most extensive groups of species, and which are perhaps even more common in those large families which consist of many closely allied genera. A sufficient proof of the overlapping of generic areas is the occurrence of a number of genera of the same family together. Thus in France or Italy about twenty genera of warblers (Sylviadrn) are found, and as each of the thirty-three genera of this family inhabiting temperate Europe and Asia has a different area, a great number must here overlap. So, in most parts of Africa at least, ten or twelve genera of antelopes may be found, and in South America a large proportion of the genera of monkeys of the family Cebidm occur in many districts; and still more is this the case with the larger bird families, such as the tanagers, the tytant shrikes, or the tree-creepers, so that there is in all these extensive families no genus whose area does not overlap that of many others. Then among the moderately e?Ctensive families we find a few instances of one or two genera isolated from the rest, as the spectacled bear, Tremarctos, found only in Chili, while the remainder of the family extends from Europe and Asia over North America to the mountains of Mexico, but no further south; the Bovidre, or hollow-horned ruminants, which have a few isolated genera in the Rocky mountains and the islands of Sumatra and Celebes; and from these we pass on to the cases of wide separation already given. Restricted Areas of Families.-As families sometimes consist of single genera and even single species, they often present CHAP. n.] THE ELEMENTARY FACTS OF DISTRIBUTION. 29 ~xamples of very restricted range; but what is perhaps more Interesting are those cases in which a family contains numerous species and sometimes even several genera, and yet is confined to a .n~rrow area. Such are the golden moles (Chrysochloridre) cons1stmg of two genera and three species, confined to extratropical South Mrica; the hill-tits (Liotrichidre )1 a family of eleven genera and thirty-five species almost wholly limited to the Himalayas, but with a few straggling species in the Malay countries; the Pteroptochidrn, large wren-like birds, consisting of eight genera and nineteen species, almost entirely confined to temperate South America and the Andes; and the birds-ofparadise, consisting of nineteen or twenty genera and about thirty-five species, almost all inhabitants of New Guinea and the immediately surrounding islands, while a few, doubtfully belonging to the family, extend to East Australia. Among reptiles the most striking case of restriction is that of the rough-tailed burrowing snakes (Uropeltidrn), the five genera and eighteen species being strictly confined to Ceylon and the southern parts of the Indian Peninsula. The Distribution qf Orders.-When we pass to the larger groups, termed orders, comprising several families, we find comparatively few cases of restriction and many of world-wide distribution; and the families of. which they are composed are strictly comparable to the genera of which families are composed, inasmuch as they present examples of overlapping, or conterminous, or isolated areas, though the latter are com-paratively rare. Among mammalia the Insectivora offer the best example of an order, several of whose families inhabit areas more or less isolated from the rest; while the J\-Iarsupialia have six families in Australia, and one, the opossums, far off in America. Perhaps, mor:e important is the limitation of some entire orders to certain well-defined portions of the globe. Thus the Proboscidea, comprising the single family and genus of the ~lephants, and the Hyracoidea, that of the Hyrax or Syrian coney, are confined to parts of Africa and Asia; the Marsupials to Australia and America; and the Monotremata, the lowest of all mammals-comprising the duck-billed Platypus and the |
