CHAP. IT THE ELEMENTARY FACTS OF DISTRIBUTION 



29 



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 ])roo{ 

 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 (Sylviada^) 

 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 Cebidse 

 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 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 Fctmilies. — As families sometimes 

 consist of single genera and even single species, they often 

 present examples 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 narrow area. Such are 

 the golden moles (Chrysochloridse) consisting of two 

 genera and three species, confined to extratropical South 

 Africa ; the hill-tits (Liotrichidae), a family of numerous 

 genera and species mainly confined to the Himalayas, but 

 with a few straggling species in the Malay countries and 

 the mountains of China ; the Pteroptochidae, large wren- 

 like birds, consisting of eight genera and nineteen species, 

 almost entirely confined to temperate South America and 



