8 THE HABITAT 
entirety upon mechanical analyses, and in some cases are too closely related 
to be useful. The line between them can nowhere be sharply drawn. In- 
deed, the variation within one class is so great that soils have frequently 
been referred to the wrong group. Thus, Cassadaga sand (gravel 22 per 
cent, sand 43 per cent, silt 21 per cent, clay 10 per cent) is more closely re- 
lated to Oxnard sandy loam (26-37-18-12) and to Afton fine sandy loam 
(28-43-18-8) than to Coral sand (61-29-3-4), Galveston sand (6-91- 
1-1), or Salt Lake sand (84-15-1-0). Elsinore sandy loam (8-38- 
35-10) is much nearer to Hanford fine sandy loam (9-36-33-14) than to 
Billings sandy loam (1-60-22-11) or to Utuado sandy loam (48-23- 
19-8). The soil types are much more confused, and for ecological pur- 
poses at least are entirely valueless. Lake Charles fine sandy loam has 
the composition, 1-34-52-9; Vernon fine sandy loam, 1-37-54-7, while 
many other so-called types show nearly the same degree of identity. 
117. The chemical nature of soils. The effect of alkaline and acid sub- 
stances in the soil upon water-content and the activities of the plant is 
far from being well understood. It is generally recognized that salts and 
acids tend to inhibit the absorptive power of the roct-hairs. In the case 
of saline soils, this inhibitive effect seems to be established, but the action 
of acids in bogs and swamps is still an open question. It is probable that 
the influence of organic acid has been overestimated, and that the curious 
anomaly of a structural xerophyte in a swamp is to be explained by the 
stability of the ancestral tvpe and by the law of extremes. Apart ‘from the. 
effect which excessive amounts of acids and-salts may have in. reducing 
the chresard,(the chemical character of the soil is powerless to produce 
structural modification in the plant.) Since Thurmann’s researches there 
has been no real support of the contention: that the cheimical properties of. 
the soil, not its physical nature, are the decisive factors in the distribution 
and adaptation of plants. It is not sufficient that the vegetation of a 
silicious soil differs from that of a calcareous one.. A soil.can modify the: 
plants upon it only though its water-content, or the solutes it contains. 
Hence, the chemical composition of the original rock is immaterial, except 
in so far as it modifies these two factors. Humus, moreover, while an 
important factor in growth, has no formative influence beyond that which 
it exerts through water-content. 
PHYSIOGRAPHY 
118. Factors. The physiographic factors of a definite habitat are altitude, 
exposure, slope, and surface. In addition, topography is a general though 
less tangible factor of regions, while the. dynamic forcés of weathering, 
