APPENDIX A. 187 



or north and south slopes, may be found on the same sec- 

 tion of land, but a particular type of topography may 

 prevail over hundreds of square miles. 



The lines indicate the influences, and the arrows at 

 their ends the direction of influence ; and where there are 

 arrows at both ends of a line the influence is reciprocal. 

 For example, climatic factors evidently afl'ect animals, 

 while animals can hardly be said to have any influence on 

 climate. On the other hand, the relation between fire 

 and vegetation is reciprocal, for a forest fire could not 

 exist without vegetation growing thickly enough for it 

 to feed on.* Some lines representing very remote or un- 

 important influences are omitted, to avoid crowding the 

 diagram too much. 



Any two factors which are not directly connected are 

 supposed to be independent of each other, or nearly so. 

 Geology and latitude seem to be the two fundamental 

 independent factors, and within the limits of as small a 

 part of the earth's surface as Alabama, geology (i. e., the 

 structure of the earth's crust) is the more important. It 

 seems to influence nearly everything else directly or in- 

 directly, through either topography or soil. 



At some time in the not distant future it may be pos- 

 sible to devise formulas which will express the relative 

 abundance of certain trees, the percentage of evergreens, 

 or the yield of timber per acre, in terms of all these fac- 

 tors which can be expressed quantitatively. 



^Obviously fire would be impossible in a desert, and consequent- 

 ly desert vegetation does not need to have any protection 

 against it. 



