28 
ISLAND LIFE. 
[part I. 
genus of diurnal moths, 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 
large proportion of the genera of monkeys of the family Cebidce 
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 w T hose 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, wffiile the 
remainder of the family extends from Europe and Asia over 
North America to the mountains of Mexico, but no further 
south; the Bovidae, or hollow-horned ruminants, which have 
a few isolated genera in the Bocky 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 
