CH. i] COSMOPOLITAN GENERA. 9 



pedidce (Grebes). Only one family of mammals are really 

 cosmopolitan, namely, the Vespertilionidce. There are 

 several families of butterflies and moths which are in 

 this position and nearly all the families of beetles. The 

 Helicidce alone of the terrestrial Mollusca are universally 

 distributed, and among earthworms the two families I/wm- 

 bricidce and Cryptodrilidce. It is a significant fact that of 

 these families by far the majority are winged creatures, 

 which are naturally less restricted by barriers. The earth- 

 worms form an apparent exception which is not really so ; 

 for it is highly probable that the universal range of these 

 families is due to human agency. Again, it is important 

 to notice that of the cosmopolitan birds at least eight are 

 largely aquatic in habit or frequent the margins of streams 

 and lakes. The conditions of life for aquatic birds are 

 more similar in different parts of the globe than those 

 ■of purely land birds. Moreover all these families are 

 numerous in genera and species. 



Cosmopolitan genera are fewer in proportion. Among 

 birds we have Hirundo (Swallow), Pandion (Osprey), Strix 

 ■(Barn Owl), Rallus, Porzana (Rails), Oallinula, Fulica 

 (Coots), Numenius (Stilt Plovers), Limosa (God wits) and 

 several other Scolopacidae, Gharadrius (Plovers), Ardea 

 (Heron), Nycticorax (Night-heron), Anas (Duck), Sterco- 

 rarius (Skua), Larus (Gull), Sterna (Tern), Puffinus (Puffin), 

 Procellaria, Fulmarus (Petrels), Pelecanus (Pelican), Pha- 

 lacrocorax (Cormorant), Podicipes (Grebe). Among butter- 

 flies the genera Pyrameis (Painted Lady), Polyommatus 

 {Blues), Pieris (Whites), Papilio (Swallowtail), Pamphila 

 and Hesperia (Skippers) are universally distributed. 



