Shelter (Continued) 

 as protection from passing 



predators, 247 

 choosing of in vegetation according 



to strata by mammals, 248 

 corrplation between length of foot- 

 span and kind of vegetation in 



seeking, 248-249 

 factors in choice of by birds in 



same general type of vegeta- 

 tion, 248 

 from cold, foliage as, 247 

 from sun, vegetation as, 247 

 from wind, vegetation as, 247 

 in vegetation of deciduous forests 



for warblers, 248 

 kinds of cover, described, 247 

 lack of occurrence of species when 



required kind missing, 249 

 nests as, for protection of eggs and 



young against weather, 248 

 nightfall as, 247 

 required by animals as protection 



against weather and enemies, 



247 

 various uses of trees as, 299 

 Sigmoid Curve: 

 consistent presence of as describer 



of ecesis in new communities, 



161 

 intrinsic growth rate as limited to 



early stages of, 220 

 plotting of, 160-161 

 Soil (see also Erosion; Forests; 



Humus; Sand; Soil Types): 

 acidity of, 165 

 alluvium, defined, 163-164 

 and carbon cycle, 167 

 and nitrogen cycle, 166-167 

 classification of horizons in, 170 

 clay, porosity of after addition of 



organic matter, 164 

 decomposition of humus in, 165 

 effect of ant movements on struc- 

 ture of, 164 

 effect of burrowing by small mam- 

 mals on structure of, 164 

 effect of crayfish excavations on 



structure of, 164 

 effect of dead roots on, 164 

 effect of earthworms on structure 



of, 164 

 effect of temperature on formation 



of, 168-169 

 effect of water on formation of, 



168-169 

 effect of wind on formation of, 168 

 effects of animal burrowing on 



erosion of, 165 

 elements in required by animals, 



167 

 excretory matter in composition 



of, 165-166 



Soil (Continued) 



factors in depth distribution of 

 organisms in, 171 



formation of hardpan in, 170-171 



freezing and thawing of in arctic 

 tundra, 315-316 



harmful effects of excess of ele- 

 ments in, 168 



horizon, defined, 170 



horizons, and occurrence of 

 organisms, 171 



humus formation and carbon- 

 dioxide content of, 165 



humus formation and oxygen 

 content of, 165 



immature profiles of as found in 

 early serai stages, 170 



in grass tundra, 316-317 



in marine habitats with sandy and 

 muddy bottoms, 364 



listing of factors in formation of, 

 163 



loess, defined, 163-164 



mature profiles of characteristic 

 of late serai stages of succes- 

 sion, 170 



minerals in as determiners of 

 chemical composition and 

 structure of, 163-164 



moderation of temperature of by 

 humus, 165 



moisture content in, 165 



mull layer of humus in, 171 



mor layer of humus in, 171 



nature of in alpine tundra, 316 



non- nitrogenous substances in 

 fresh litter in, 166 



of desert as influencing niche 

 segregations of small mam- 

 mals, 339 



of tropical biomes and rainfall, 

 340-341 



profile, defined. 170 



reactions of large terrestrial 

 animals on structure of, 164- 

 165 



residual, defined, 163-164 



sandy, porosity of after addition 

 of organic matter, 164 



sequence of events in layering of, 

 170 



subdivisions of "A" horizon of, 

 170 



till, defined. 163-164 



wind erosion of, 168 

 Soil Types [see also Soil): 



alluvial, 172 



chernozemic, 172 



desertie, 172 



latosolic, 172 



mountain, 172 



podzolic, 172 



saline, 172 



Soil Types (Continued) 



tundra, 172 

 Solar Radiation (see Light) 

 Speciation (see also Taxonomy) 



and adaptive radiation, 266 



and development of polyploidy, 260 



and segregation of related species 

 into different regions or com- 

 munities, 255-256 



as adaptive process which explains 

 niche occupation, 256 



as occurring with organisms isola- 

 ted during glaciation, 286-288 



as result of competition in animal 

 community, 183 



as result of isolation of mountain 

 regions, 313 



conditions of occurrence among 

 forest- edge forms, 297 



conditions of occurrence of, 26 



confusion of with elevation of sub- 

 species into species, 258 



encouragement of through split of 

 Arcto- tertiary forest into 

 western and eastern sections, 

 307-308 



ethological isolation as step in, 

 258-259, 259 



factors influencing period of time 

 required for, 267 



first- generation hybridization as 

 indication of non- occurrence of, 

 263-264 



genetic isolation as step in, 259 



geographic isolation as necessary 

 factor in, 260 



mechanical isolation as step in, 259 



nature of when geographic barriers 

 are only partially effective, 259 



occurrence of as result of catas- 

 trophes and cycles, 262 



occurrence of through natural 

 evolutionary selection, 264-265 



period of time required for, 266- 

 267 



possibility of genetic drift as 

 significant factor in, 262 



possible occurrence of when rate 

 of gene glow is slow, 259-260 



process of, defined, 257 



repeated mutations of genes and 

 possible occurrence of, 263 



resulting from natural selection 

 of hybrids as superior to 

 parents, 264 



size of habitat as related to, 265 



suimnary of process of, 260 

 Stimulus: 



and behavior responses to, 12 



and drives, 14 



and kineses, 12 



and taxes, 12 



and tropisms, 12 



442 Index 



