ECOLOGICAL SUCCESSION. 343 



several strata, there is much overlapping of adjoining strata 

 corresponding to overlapping of single characters in taxonomy. 



4. Habitat or Horizontal Aspects. 



Ecological classification is not only vertical but also horizontal. 

 Some of the roughly horizontal aspects of classification, such as 

 division into aquatic (marine and fresh water) and terrestrial 

 (forest, prairie, etc.) are major aspects of classification long in 

 use. Animals invade several of the minor recognizable horizontal 

 conditions less often than several of the vertical. Still such over- 

 lappings are common, and as has already been noted, since 

 overlapping of single characters is common in taxonomy, it 

 cannot be urged as argument against ecological classification. 



(a) Large and Small Divisions. Considering terrestrial com- 

 munities, the largest are those that occupy areas of relatively 

 uniform climate (climatic, Schimper, '03: major; Adams, '08; 

 Shelford, 'n 3 ). The smaller those of similar soil (including 

 water) (edaphic, Schimper, '03), of similar degrees of exposure to 

 wind, sun, etc. due to the topography (local or secondary, 

 Adams, '08; Shelford, 'n 3 ). Each area when considered as an 

 environment includes the vegetation, which is usually clearly 

 different in growth-form in each area occupied by a different 

 animal community. 



Both local and climatic communities may be subdivided into 

 still smaller but easily recognizable subdivisions. Their number 

 is closely related to severity of climate, particularly with refer- 

 ence to moisture, and is greatest in the desert and smallest in 

 the rain forest. For example, in dealing with forest development 

 in the preceding paper (Shelford, 'I2 1 ) we noted that in the earlier 

 stages, conditions were dominated by the presence of bare sand 

 open forest communities. The later stages were closed forest 

 with the soil all covered and the character of the forest and of 

 the animal communities quite independent of soil and dependent 

 upon climate. Still each of these sets of conditions was divisible 

 into several subdivisions, the sand dominated habitat into the 

 cottonwood, the pine, and the black-oak habitats, each with a 

 different rate of evaporation and each with different soil condi- 

 tions along with recognizable differences in communities. The 



