CHAPTER IX 



CONDITIONS OF EXISTENCE OF LAND ANIMALS 



I. Introduction 



Man being a land animal, it is natural that he should be more familiar 

 with the conditions of existence of land animals than with those of aquatic 

 forms. The reader will recognize that the primary divisions into which 

 land animals may be divided are (a) those living exposed to the atmos- 

 phere on the surface of the soil and of plants and animals, and (b) those 

 out of direct contact with the atmosphere, in the soil, in wood, and in the 

 tissues of living plants and animals. The solid substances in and upon 

 which animals live are called materials for abode (55, 115) and, aside from 

 soil, materials are just as varied as are the living and decaying bodies of 

 plants and animals. For this reason, an adequate discussion of such 

 materials for abode would require a separate treatise. Since the laws 

 governing the physical conditions surrounding animals living hidden 

 away, for example in the bodies of living and dead organisms, are little 

 known, we will pass directly to a discussion of the conditions of existence 

 of animals living in soil and exposed to atmosphere. 



II. Soil (116) 



Because of its importance in agriculture, the relation of plants to soils 

 has been much studied. The laws governing plants in their relation 

 to soils apply in the main to soil-inhabiting animals, all the various 

 properties of soils being of some importance in this connection. 



1. TEXTURE 



The texture of soils is of importance to animals because of the vary- 

 ing difficulty with which they may burrow into it, and the ease with 

 which their burrows are maintained when once dug. Particular animals 

 prefer soils of a particular texture, some preferring rock, some sand, etc. 



2. WATER 



Most subterranean animals are submerged in water during rains. 

 The amount of water which they encounter in the soil at other times is 

 determined to a large extent by their relation to the water table (57), and 

 by the character of the soil. The water-holding power of different soils 

 is different. It increases with the decrease in size of the soil particles and 



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