128 ECOLOGICAL FOUNDATIONS OF ZOOGEOGRAPHY 



and reptiles have no cosmopolitan families, though Siluridae, Ranidae, 

 and Colubridae are very widely distributed. Numerous world-wide 

 genera of air-breathing vertebrates are to be found among the birds, 

 witness: Turdus, Hirundo, many birds of prey, owls, and numerous 

 water birds. Canis and a few genera of bats are the only genera of 

 mammals which have such a range. Species of such wide distribution 

 are not uncommon among the marine invertebrates. Among terrestrial 

 animals, there are a few cosmopolitan arthropods,* but among verte- 

 brates the only cosmopolitan species are a few birds, such as Pandion 

 haliaetus and Asio accipitrinus. 



Species with a wide range usually belong to genera with a still 

 wider distribution. 21 Thus the leopard, puma, and lion belong to the 

 more widespread genus Felis, and the widely distributed wolf and 

 fox to the almost cosmopolitan genus Canis. The genera to which the 

 raven (Corvus corax), the peregrine falcon {Falco peregrinus) , and 

 the ringdove (Columba oenas) belong are likewise widespread and 

 rich in species. The same may be observed among reptile and amphib- 

 ian genera (Lacerta, Bufo, Rana) , and among invertebrates such as 

 the land and fresh-water mollusks {Helix, Limnaea). Species of 

 monotypic or oligotypic genera, in contrast, usually have a narrow 

 range. Among the American moles the genera with the most restricted 

 distribution are the least varied. 22 Other examples among mammals 

 are the panda (Aelurus) and binturong {Ar otitis) , and among birds 

 two Brazilian species of Eurypyga, Mesites with a single Madagascan 

 species, and Didunculus with a single Samoan species. 



Eigenmann 23 found in the course of his researches on South American 

 fishes that in a given river system, even so vast as that of the 

 Amazon, the widespread genera have several times as many species 

 as the genera confined to the system in question. This indicates that 

 a genus with a small range has fewer species because there is less 

 available space for them. The examination of the conditions which 

 make possible a wide range will cast some light on this problem. 

 It is precisely the especially vital and adaptable genera which are 

 differentiated into numerous species and which inhabit a wide range. 

 Such genera are in the minority. Exact statistics show that among the 

 most widely distinct families of plants and animals the monotypic 

 genera are by far the most numerous, followed by those with two 

 species, then by those with several or many. 24 



* A few arachnids (Theridium tepidariorum, Th. rufipes, Tegenaria derhauni) ; 

 a few insects (Pyrantels cardui, Celerio lineata, Heliothis armigera, Nomophila 

 noctuella) 20 among Lepidoptera, and Cercyon nigriceps among Coleoptera; and 

 the myriapods Scolopendra morsitans and S. subspinipes. 



