136 ECOLOGICAL FOUNDATIONS OF ZOOGEOGRAPHY 



flora of a given biotope. Particular modifications of a biotope, differing 

 from the typical conditions in recurrent minor respects, may be char- 

 acterized as "facies" (originally a geological term with a similar 

 application) . These bear the same relation to the typical biotope as do 

 subspecies and varieties to a species. 



As species are combined into genera and these into families and 

 orders, so may biotopes be grouped according to their resemblances into 

 biochores. Thus the biotope rock desert combines with sandy desert 

 and stony desert into the biochore "desert." This in turn may be 

 united with the biochore ice desert under the superbiochore wasteland. 

 The biotopes mud beach, sand beach, gravel beach and shingle beach 

 (boulder beach) belong together as the subbiochore "depositing shore," 

 which with the subbiochore eroding shore makes the biochore seacoast. 

 This together with the sea bottom from deeper water makes the super- 

 biochore benthal. 



The biochores and superbiochores are finally combined into still 

 higher groupings, which may be called biocycles. The biosphere may 

 be divided into three such biocycles: ocean, fresh water, and land. 

 Each of these represents special exclusive habitat conditions to the 

 animals inhabiting it, and they are thereby fundamentally distinct. 

 This is reflected in the complete difference in their animal populations. 

 Certain animals enter more than one biocycle, like salmon or eel in 

 the ocean and in fresh water, amphibia in fresh water and on land, 

 and some birds occur in all three, but these are so obviously exceptions 

 that they "prove the rule." 



Among the biochores differences in general aspect, the so-called 

 "habitus," appear with differences in vertical or horizontal position. 

 Vertically arranged provinces are called strata or layers; horizontally 

 distinguished ones, zones. Thus, independently of other subdivisions 

 of the biosphere, we have depth and altitude strata and climatic 

 zones. The depth strata of the sea are those of different degrees of 

 pressure, motion, and light (see Chapter XII), and on land the alti- 

 tude strata are of two sorts, those based primarily on stratification of 

 plants as in a forest and those based on physiographic features such 

 as lowland, hills, mountains, and high mountains, with their subdivi- 

 sions. The customary climatic zones are the polar, the temperate, the 

 subtropical, and the tropical, and since these are represented in moun- 

 tains in the altitude strata it is customary to call these zones also. 



All these divisions are independent of the historically based faunal 

 regions. The biotopes and biochores include analogous provinces whose 

 faunae are similar on account of the influence of similar habitat con- 

 ditions, and though they may be of joint origin are not necessarily so. 



