104 



a hundred miles or so in the location of their boundaries are 

 scarcely noticeable. 



But to map the vegetation of a country or state of average 

 size satisfactorily requires much experience and rare judgment. 

 One individual does not usually live long enough to examine 

 every square mile of such an area, and it is therefore necessary 

 to collate the work of several persons, whose points of view 

 may vary considerably. And even if the exact location of every 

 plant was known, the different types of vegetation often inter- 

 grade in all sorts of ways, so that it may be impossible to say 

 within a few miles just where one ends and another begins. 

 Again, where the transition from one type to another is complete 

 within a few yards they may occur in such small patches that 

 they cannot be indicated separately on a map of a whole state 

 or larger area, and no two persons might agree on how to general- 

 ize them into larger categories. Another great difficulty, in 

 populous regions like much of the eastern United States and 

 Europe, is reconstructing the vegetation that has been destroyed 

 or greatly modified by civilization. 



For these reasons very few vegetation maps of the United 

 States have been made. Dr. Shreve's is the best and most 

 detailed thus far published. He takes the correct position that 

 vegetation should be mapped for itself alone, completely ignoring 

 all environmental factors, but clearly recognizes the dif^culties 

 of such an undertaking. His map, on a scale of i : 9,600,000, 

 shows 18 generalized types of vegetation, indicated by different 

 colors,* about equally divided between the eastern and western 

 halves of the country. There are about seven types of desert 

 and semi-desert, three mainly grass-land, two of open park-like 

 forests, and six of ordinary forests. Brief descriptions, averaging 

 about seven lines each, are given in the text. 



Nothing is said about the normal frequency of fire, which the 

 reviewer has found to vary greatly in different regions and dif- 

 ferent types of vegetation, and depends on the nature of the 



* The engravers unfortunately did their work rather poorly. Some of the 

 colors meant to be different are too much alike, and some meant to be the same 

 are different; and there are no numbers or other symbols (as on the government 

 soil maps, for instance) to assist the reader in identifj'ing them. 



