THE NORTH AMERICAN SPIDER FAUNA 257 



number of species from comparable ecological zones; rather, they 

 are essentially equal in faunal wealth, indicating that similar biologi- 

 cal areas usually support quite similar kinds and numbers of animals. 

 The following totals are only approximations of the present known 

 numbers: In Palearctica there are about 3500 species, of which 557 

 occur in the British Isles, 450 in Spain, 688 in Switzerland, 341 in 

 Norway and Sweden, 391 in Greece, 1335 in France, and 3100 in 

 all Europe; in Nearctica, about 2500 species, of which 50 occur in 

 Greenland, 249 in Alaska, 497 in the Georgia region, 600 in New 

 York State, and 650 in New England. France has long been a center 

 of arachnology, so it is not surprising that its spider fa-una is so 

 well known. This is also true of the British Isles, from which the 

 number of known species is near the real total for the area. Inten- 

 sive studies of American and Oriental spiders will ultimately bring 

 the faunas of comparable regions to parity with Europe. 



More than a hundred Amercan spiders are seemingly identical 

 with species from Europe, showing no differences that indicate they 

 have changed sufficiently to be called subspecies. Most of these are 

 long-established residents that have lived in this zone around the 

 world for thousands of years. Through the medium of the balloon- 

 ing threads, boreal spiders have been able to keep quite intimate 

 contact with their own kind in Siberia and Alaska. Nearly one out 

 of ten of the boreal American spiders occurs on the Eurasian land 

 mass, and the percentage will rise as the faunas are more thoroughly 

 studied. Aranea raji and nordmanni are as typical of America as of 

 Europe, and the American derivation of these or any of the other 

 boreal types is just as arguable as a European origin. Most boreal 

 spiders are anciently American, and have not come in through 

 accidental introduction by man. 



The ubiquitous spiders do not represent any special group, but 

 single species from a number of different genera have become 

 specialized to live all over the world without regard to differences 

 in climate. The most successful is Theridion tepidariorum, a house 

 spider which is the most common and most widespread of all 

 spiders. The remaining cosmopolitan species are far less generally 

 distributed, and are often rare or entirely lacking even in apparently 

 suitable regions. Some others spiders have become widely dissemi- 

 nated around the world in the tropical belts, but do not extend as 

 far north as the temperate zones. Many of these tropicopolitan 

 species are found in the southern portions of the Holarctic Realm. 



The cosmopolitan and tropicopolitan spiders have probably 



