GEOGRAPHICAL DISTRIBUTION. 113 



countless facts presented by the distribution of living things, in 

 present and past time, must be explicable in accordance with 

 any true theory, or, at least, never directly contradict it." * 



In studying the geographical distribution of organisms 

 the understanding of the nature of the conditions of environ- 

 ment can scarcely be overestimated. 



The Natural Conditions of Environment Nomenclature. 

 There are various conditions of environment which modify the 

 growth and life of organisms. Among the chief of these are : 

 The (i) medium air or water; (2) temperature or climate 

 and limits of annual temperature ; (3) in water the depth, the 

 purity, the salinity, the light, the motion ; (4) on land 

 secondarily altitude as affecting climate and temperature ; (5) 

 the other organisms, because all animal life appears to require 

 other animal or plant organisms for its own food, hence (50) 

 struggle for existence; also (5^) the amount of organic food 

 determines the growth of higher organisms which require the 

 food. Medium and Habitat are the names applied to the 

 immediate conditions in which the organisms live. Province 

 is the name of the region occupied by a group of organisms 

 which are naturally adjusted to each other. Zone is the name 

 of the tract of sea-bed between boundaries of depth, variously 

 determined. Flora is the name applied to all the plants, 

 naturally associated and adjusted to the conditions of environ- 

 ment, of a particular province or geographical area. Fauna is 

 the name cf the group of animals so associated and adjusted. 



Natural-history Provinces. The primary classification of 

 the conditions of environment as affecting organisms is con- 

 sidered under the terms Terrestrial (land plants and animals), 

 and Marine (those living in the ocean). It is found that 

 the present life of the globe is divided into numerous floras 

 and faunas, the boundaries of which are not absolutely fixed, 

 cither in species or in conditions ; but the areas are distinct 

 in some of their features, and the association of organisms 

 is peculiar for each, although some of them may be com- 

 mon to neighboring areas. These provinces, both marine 

 and terrestrial, differ in their outlines for different kinds of 



* Article " Distribution," (,th ed. Encyclo. Brit., vol. vn. p. 267. 



