528 CORRELATION OF DISTRIBUTIONAL FEATURSE. 



humid northwestern region gives the moisture conditions both higher 

 and lower extreme values in the Transition than they exhibit in the 

 Alleghanian. With the same exception the moisture ratios have a 

 much narrower amplitude in the western zones than in the eastern 

 ones. They are clearly to be looked upon as expressing the most 

 important controlling condition in the Upper Sonoran and Lower 

 Sonoran Zones, but their amplitudes are relatively wide in the eastern 



zones. 



The moisture-temperature index is also of narrower amplitude in the 

 western zones than in the eastern ones. While it is sufficiently narrow 

 in the Transition Zone to be regarded as an expression of important 

 conditions in the limitation of this zone, ihis is not the case with the 

 other zones. In the three western zones this index has the same 

 minimum value, but increases in amplitude on passing southward, 

 so that its maximum values are progressively higher and consequently 

 its amphtudes increasingly wider. In the eastern zones the extremes 

 become progressively higher on passing from north to south, and the 

 amplitudes become greater. 



From the preceding discussion, and from considerations presented 

 in Part II, it appears that the system of hfe-zones worked out by 

 Merriam and now rather widely used in a descriptive way, especially 

 by the United States Biological Survey, will require much modification 

 before it may become at all satisfactory to a serious student of etio- 

 logical plant geography. It is extremely unfortunate that the actual 

 data on which this system was originally based, and on which its apph- 

 cations are based in current descriptions, do not exist in the published 

 literature. Neither Merriam nor any of his followers has thus far 

 attempted to present the actual basis for the system in form such that 

 a critical study of its good and bad features may be undertaken. 

 Perhaps this may be a main reason why the who.e subject of the 

 climatic relations of floral and faunal areas has received so little atten- 

 tion at the hands of students who are able and willing to undertake 

 the complex analyses which are involved in such a subject. The pub- 

 lication of the charts without the data on which they were based, 

 together with the general and official adoption of the system by the 

 United States Biological Survey, have given this important problem 

 the appearance of having been satisfactorily solved— of being a closed 

 subject. Those who have employed this zone system have either 

 refrained from any discussion of its good and bad characteristics, or 

 else they have merely taken the standpoint of advocates, and the lack 

 of the numerical data that are absolutely necessary for a critical study 

 has tended strongly to discourage such inquiries. Also, a sort of 

 authoritative atmosphere that seems to hang over government pub- 

 lications in general, together with the apparent authority and dog- 

 matism that invariably go with well-printed (and especially colored) 



