DISTRIBUTION OF FLORA AND FAUNA. I2j 



often plants and animals taken from their indigenous country to 

 a new one, require a very favorable habitat in order to live. 

 Hollick ('93, p. 196) and Cowles ('01, p. 105) have called atten- 

 tion to certain plants of more general distribution in the South, 

 which in the North occur only under very favorable or restricted 

 conditions. This change of habit and habitat with divergence 

 from the center of dispersal is a subject worthy of special atten- 

 tion. If Osborn's law of adaptive radiation is of general applica- 

 tion, we should expect just such a change in habitat as has been 

 noticed to accompany the divergence from the original home. 



The eighth deserves some special attention on account of the 

 present active interest taken in the study of individual variation, 

 to which reference will be made later. The direction of biogeo- 

 graphical affinities and convergence of lines of dispersal, as has 

 been shown, first called my attention to the Southeast. 



OUTLETS OR HIGHWAYS OF DISPERSAL FROM THE SOUTHEAST. 



Next in importance to the centers of origin themselves are the 

 highways from and to these regions. From the Southeast we 

 have several very well-marked outlets, as follows : 



1. The Mississippi Valley, and valleys of tributary streams: 

 the leading outlet. 



2. The Coastal plain, leading along the Atlantic seaboard north- 

 ward, and along the gulf coast to Florida, and the Southwest. 



3. The southern Appalachians and adjacent plateaus, forming 

 an outlet to the North. 



These are the principal paths of dispersal ; two are lowland, 

 one is upland. The lowland highways have functioned not only 

 for the Southeast, but also for the fauna of tropical origin, as 

 Webster ('98, p. 72) has shown to be the case for the chinch- 

 bug, which has pushed into the United States via Florida and the 

 West Indies and also from Mexico. Of course, these outlets 

 may serve also as inlets from other regions. 



Aquatic life has been greatly favored by the Mississippi River 

 system. From the Southeast, the Tennessee River has un- 

 doubtedly been the leading outlet ; and next, perhaps, the Cum- 

 berland River. Formerly the ancient Appalachian River (/. c., 

 the Tennessee above Chattanooga plus the Coosa-Alabama River) 



