DISTRIBUTION OF FLORA AND FAUNA. 121 



RELICTS NOT SHOWING ASIATIC AFFINITIES. 

 Among animals, there are not only members of this fauna 

 showing close Asiatic affinities, but there are other relicts which 

 do not show such a relationship. Jordan ('88, p. 168) has 

 given a number of genera of lowland fish which he considers 

 archaic forms, or members of an ancient fauna. The mud min- 

 now (Umbra), the pirate perch (Aphredoderus), the pygmy sun- 

 fishes (Elassowa), and the blind fishes (Amblyopsidae), are ex- 

 amples of such forms. 1 Among butterflies LibytJica is of this type 

 also, and Taclioptct-yx among dragon-flies, and doubtless many 

 other insects are. 







Two CENTERS OF DISPERSAL IN SOUTHERN UNITED STATES. 



That the South was a retreat for life during the Ice Age is too 

 well known to be discussed here. On account of the great 

 amount of endemism and relicts in the Southeast it seems fair to 

 conclude that quite an abundant and diversified fauna and flora 

 lived there during the Ice Age, although the contrary opinion is 

 often expressed. In south temperate North America there seem 

 to be two very distinct and powerful centers of dispersal, or 

 centers of " adaptive radiation," as Osborn ('oo, p. 49) might call 

 them. One, to which we have been calling attention, is in the 

 Southeast, and the other in the Southwest on the arid plateau 01 

 Mexico and Southwestern United States. This latter area, or 

 distinct center of dispersal, has a very large number of indigenous 

 forms, as is shown by Merriam's system of zoogeographical areas 

 ('93, p. 401). This is undoubtedly the area where the charac- 

 teristic arid fauna (and flora) 1 of North America has originated 

 or has had its center of dispersal. The great amount of ende- 

 mism in this area has suggested that this may have retained much 

 of its present arid condition for a considerable period of time, 

 possibly preceding and during the Ice Age ; and one condition 

 of its present richness may be due to its greater distance from 

 the ice-sheet which destroyed many forms in Southeastern United 

 States. The heavy rainfall and forests of Southeastern United 

 States should be contrasted with the arid plains and deserts of the 

 Southwest and the corresponding difference in the types of ani- 

 mals originating in these two regions, noted. 



'Gray, '81, p. 62, nnd I '.ray, xo, p. 713. 



