256 AMERICAN SPIDERS 



gling of faunas of the whole northern belt around the world. Over 

 a large part of North America the climate was subtropical, and 

 many tropical forms penetrated far into the north. During Floris- 

 sant time in the Oligocene, Colorado had a climate at least as mild 

 as that of our southeastern Gulf states, and had as part of its fauna 

 a silk spider (Nephiia) perhaps identical with the one now living 

 in Florida and some of our southern states. The spiders of America 

 and Europe were probably quite similar at that time, a conclusion 

 that has been reached through study of a very imperfect fossil 

 record, and perhaps not entirely valid. The similarity between these 

 faunas, however, is not a recent one. During the Paleozoic Era, the 

 same types of primitive spiders lived in Europe as those found in 

 the Carboniferous slates of Illinois. The much richer spider fauna 

 in Paleozoic Europe points to the derivation of American forms 

 from that or some other Eurasian center of origin. In the splendid 

 amber fauna of the Oligocene in Europe there is quite likely a 

 picture of the wealth and variety of our own American spider 

 fauna for the period, even though a close relationship on the basis 

 of existing fossils cannot be demonstrated. An interchange of new 

 types between the Old and the New World has gone on almost 

 continuously for more than four hundred millions of years, inter- 

 rupted for brief periods by transitory barriers which were not too 

 great for crossing by tolerant and enterprising spiders. 



The present faunal kinship between the temperate zones of the 

 Old and the New World is a striking one, which reflects itself in the 

 identity or close relationship of most of the genera and many of 

 the species. Unfortunately, the faunas of the two regions are not 

 well enough known to make possible explicit comparisons. The 

 number of known species of spiders from the entire Holarctic 

 Realm is around six or seven thousand, a number far below the real 

 total possibly no more than half of it. In Palearctica, only the 

 European spiders have been well studied; vast expanses of temper- 

 ate Asia are almost unknown. However, it is believed that the fauna 

 is a very homogeneous one, and is modified longitudinally only by 

 the character of the tropical genera and species, which press north- 

 ward into the temperate zone for varying distances. In Nearctica 

 only the spiders of the northeastern United States are well known; 

 knowledge decreases progressively as one leaves that area. 



One finds the same types of spiders in England and northern 

 Europe as occur in Siberia and Japan, and in Canada and the United 

 States. None of these regions shows a marked superiority in the 



