ECOLOGY. 351 



prairie at the close of the Pleistocene to desert at the present under the 

 pressure of shifting climate. It not only seems possible to indicate the 

 successive changes in rainfall and the resulting changes in vegetation but also 

 to obtain a general idea of the time demanded for each. The detailed recon- 

 struction of grassland and climate is in full accord with the conclusions of 

 Merriam as to the former presence of a grazing fauna in the Mohave, and 

 hence of grassland, and those of Buwalda and Thompson as to lakes Mannix, 

 Mohave, and Little Mohave. 



The Method and Principle of Relicts, by F. E. Clements. 



The investigation of plant formations in the light of the changes effected 

 by climatic cycles has led to the recognition of relict groups and communities 

 as of universal occurrence, and this has been confirmed by the quantitative 

 study of the consequences of grazing. The concomitant study of migration 

 processes has contributed to the mass of evidence' that effective migration 

 at a distance is limited to carriage by water and by man and his domestic 

 animals, and that practically all extra- areal groups are to be regarded as 

 relicts of a former greater extension. Corroboration of this is furnished by 

 the current studies in experimental vegetation, which show that plants once 

 established can persist in the face of conditions that make their ecesis abso- 

 lutely impossible. Apart from grazing and physical conditions, competition 

 is such a vital factor in establishment that disturbance or climatic change 

 is almost universally a prerequisite to successful establishment. During 

 the intensive field work of the past 10 years, no case has been found of effective 

 distant migration other than by man and water. Even in the case of 

 fleshy fruits with exceptionally well-protected seed, such as the cedar, migra- 

 tion is practically always local. 



While forest and scrub have received much attention, this has been centered 

 on grassland, because of its peculiar climatic value and its ready response to 

 grazing. Forest relicts have been produced by fire, lumbering, and cultiva- 

 tion, and grassland relicts chiefly by grazing, though cultivation has been the 

 chief factor in the sub climax and true prairies. Under the serious over- 

 grazing widespread in the West, grasses have been able to persist only under 

 protection of some sort. This consists regularly of fencing, but it may be 

 afforded by a greater water-content, as in sand, valleys, and rocky slopes, or 

 by the steepness or exposure of the slope itself. Wet years invariably bring 

 into view grasses that have been persisting in a suppressed condition. Com- 

 petition on the part of weeds is a potent factor in destroying grassland, and 

 often the slightest barrier to the movement of weed-seeds will enable grasses 

 to persist. The best of all relict areas are railroad rights of way, followed by 

 fenced roadways, fenceways, cemeteries not sown to grass, hay-fields, rock 

 ridges, north slopes, sandy areas, depressions, and ditch edges. A fenced 

 roadway between two fields exhibits distinct differences with respect to 

 protection, water-content, and disturbance, and its analysis into field strip, 

 fence strip, road strip, ditch edge, and road edge has revealed a fairly definite 

 scale of adjustment to various sets of conditions. Species differ greatly in 

 their response to these, and hence serve as delicate indicators of processes 

 and changes no longer in evidence otherwise. 



