28 ASSOCIATION 



relation between soil and climate, is merely one of extent, 

 arising out of the fact that a certain correspondence of soil 

 and climate prevails over a larger or smaller area, as the 

 case may be. Truly edaphic formations are possible only 

 where the soil moisture is so great as to make water loss 

 entirely negligible as a factor capable of producing xero- 

 phytic modification or a tendency toward it. This is the 

 case only in hydrophytic situations. Hydrophytic species 

 are cosmopolitan ; they are much the same the world over, in 

 a very moist or in a very dry climate, in heat or in cold. 

 Water loss is normally never great enough to seriously 

 threaten the water supply. With mesophytes and xero- 

 phytes, it is very different. Here there is constantly a limit 

 to the water supply, and the production of mesophytic or 

 xerophytic vegetation depends upon the nearness with which 

 the water loss approaches the water supply. Hydrophytic 

 formations might, then, be regarded as truly edaphic, in a 

 sense, though mesophytic and xerophytic formations cannot 

 properly be called climatic. Any distinction at all on such 

 grounds is at present gratuitous. As is true of all funda- 

 mental problems in phytogeography, it must await the 

 general employment of quantitative methods. While our 

 knowledge of soil factors is in an extremely elementary con- 

 dition, we have no exact understanding whatsoever of the 

 sum of physical factors, which we term climate. According 

 to Schimper's idea, as well as in the general opinion, 

 edaphic or soil factors are thought to vary greatly, while 

 climatic ones are thought to be constant over vast areas. 

 The investigations of the author during the last five years 

 indicate conclusively that this is erroneous, and that soil 

 factors are relatively less variable and climatic ones more 

 variable, as would be expected. The whole question awaits 

 the labors of many investigators through terms of years. 



