60 FORREST SHREVE 



their physiological behavior in relation to environment. It is 

 for this reason that the ultimate questions of general distribu- 

 tion and those of habital distribution are so nearly the same. 

 It is for this reason also that the study of distribution in the old 

 sense of Grisebach and Hemsley and the study of vegetation in 

 the sense of the present school of ecologists have so much common 

 ground. We have been accustomed to feel that the evolutionary 

 and paleoclimatic factors have so much to do with general dis- 

 tribution, and that such a strong role is played by agencies of 

 dispersal, biotic factors, mechanical agencies and the like, that 

 the impossibility of completing our knowledge of the operation 

 of these factors, or the impossibility of experimentation in regard 

 to them, made it advisable to pass the subject on to posterity 

 for solution. We have turned to the study of vegetation and the 

 distribution of physically controlled plants as presenting more 

 tangible and more solvable problems. 



The natural communities of plants, considered either in a large 

 or in a small sense, vary greatly in the number of species which 

 they contain. Some of them present the extreme simplicity of 

 a pure stand, as the marshes of Spartina polystachya on the 

 Atlantic coast, many others are relatively simple, still others 

 are rather rich, and a great many are highly complex, containing 

 a large number of very dissimilar species, as is true of the fresh- 

 water marshes of the Atlantic coast, of such tropical rain-forests 

 as have been described by Warming and by Spruce, and of the 

 deserts of Arizona and the South African Karroo. In all of these 

 communities, even those which are rich in species, we have a 

 certain degree of similarity in the physiological requirements of 

 the species which are dominant, that is to say, of the species which 

 are most directly influenced by the climatic conditions. It is 

 only, however, by speaking in the very broadest terms that we 

 can allude to these associated plants as being similar in their 

 life requirements. In a more exact analysis of the relation of 

 these plants to their physical surroundings we find them highly 

 unlike. In fact we have no warrant for supposing that two 

 plants which grow side by side and attain the same height are 

 subjected to the same constellation of conditions. I may men- 



