28 



ISLAND LIFE. 



[part I. 



genus of diurnal motlis, Urania, is confined to the same 

 two 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 features of Overlapping and Discontinuous 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 every 

 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 over- 

 lapping 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 (Sylviadse) 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 

 Jarge proportion of the genera of monkeys of the family Cebidee 

 occur in many districts ; and still more is this the case with the 

 larger bird families, such as the tanagers, the tyrant 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 extensive 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 Bovidse, or hollow-horned ruminants, which have 

 a few isolated genera in the Eocky 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 



