CHAPTER XII 



The North American Spider Fauna 



T 



JL.HE AREA COVERED IN THIS BOOK 



comprises much of the north temperate zone of the New World, 

 the Nearctic Realm. This is one of the natural biological land areas 

 of the world, and includes the part of North America north of the 

 humid tropical region of Mexico and Central America. It is a vast 

 land mass characterized over much of its surface by a climate that 

 may, with certain reservations, be termed temperate. The present 

 southern limit is a tropical climate that bars as effectively as an 

 ocean the southward extension of the northern faunas. To the 

 north, the faunas gradually become diminished as they approach the 

 pole, being greatly reduced where conditions of extreme cold per- 

 sist for long periods, and almost completely lacking on areas of 

 permanent glacial ice. 



The North American region has maintained its general form 

 and its separation from other major land areas of the world for vast 

 periods of time, probably since the Paleozoic. Its isolation has been 

 accomplished to the east and west by broad oceans, to the south by 

 the tropics and transitory ocean gaps, and to the north by Arctic 

 wastes. Whereas physical isolation has for the most part been com- 

 plete, there have been periodic joinings to adjacent land masses by 

 means of broad bands in the south and narrow bands in the north- 

 ern reaches. Animals have moved northward into North America 

 from centers in South America, and vice versa. Interchange of 

 faunas has been effected between Alaska and Siberia by the Bering 

 Strait land connection, a bridge believed always to have been a rela- 

 tively narow one but that allowed animals to pass in both directions 

 when climatic conditions were favorable. 



The result of this faunal intercourse during the Tertiary is re- 

 flected in the great similarity of the faunas of the temperate zones 

 of both the Old and the New World, which together constitute the 

 Holarctic Realm. Before the ice ages, the polar region probably 

 enjoyed a much milder climate, which made posible an intermin- 



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