26 ISLAXD LIFE 



All the phenomena presented by the species of a genus 

 are reproduced by the genera of a family, and often in a 

 more marked deOTee. Owino^, however, to the extreme 

 restriction of genera by modern naturalists, there are not 

 many among the higher animals that have a world-wide 

 distribution. Among the mammalia there is no such 

 thing as a truly cosmopolitan genus. This is owing to the 

 absence of all the higher orders except the mice from 

 Australia, while the genus Mus, which occurs there, is 

 represented by a distinct group, Hesperomys, in America. 

 If, however, we consider the Australian dingo as a native 

 animal we might class the genus Canis as cosmopolite, but 

 the wild dogs of South America are now formed into 

 separate genera by some naturalists. Many genera, 

 however, range over three or more continents, as Felis (tlie 

 cat genus) absent only from Australia; Ursus (the bear 

 genus) absent from Australia and tropical Africa ; Cervus 

 (the deer genus) with nearly the same range ; and Sciurus 

 (the squirrel genus) found in all the continents but 

 Australia. Among birds Turdus, the thrush, and Hirundo, 

 the swallow genus, are the only perching birds which are 

 truly cosmopolites ; but there are many genera of hawks, 

 owls, wading and swimming birds, which have a world-wide 

 rano^e. 



As a great many genera consist of single species there is 

 no lack of cases of great restriction, such as the curi'ous lemur 

 called the " potto," which is found only at Sierra Leone, 

 and forms the genus Perodicticus ; the true chinchillas 

 found only in the Andes of Peru and Chili south of 9° S. 

 lat. and between 8,000 and 12,000 feet elevation; several 

 genera of finches each confined to limited portions of the 

 higher Himalayas, the blood-jDheasants (Ithaginis) found 

 only above 10,000 feet from Nepal to East Thibet; the 

 bald-headed starling of the Philippine islands, the lyre- 

 birds of East Australia, and a host of others. 



It is among the different genera of the same family that 

 we meet with the most striking examples of discontinuity, 

 although these genera are often as unmistakably allied as 

 are the species of a genus ; and it is these cases that furnish 

 the most interesting problems to the student of distribution. 



