312 THE ".NDUCTIONS OF BIOLOGY. 



The confinement of different kinds of plants and different 

 kinds of animals, to the media for which they are severally 

 adapted, is the broadest fact of distribution. We have ex- 

 tensive groups of plants that are respectively sub-aerial and 

 sub-aqueous; and of the sub-aqueous, some are exclusively 

 marine, while others exist only in rivers and lakes. Among 

 animals, we similarly find some classes confined to the air 

 and others to the water ; and of the water-breathers, some 

 are restricted to salt water and others to fresh water. Less 

 familiar is the fact, that within each of these strongly con- 

 trasted media, there are further wide- spread limitations. In 

 the sea, certain organisms exist only between certain depths, 

 while other organisms exist only between other depths the 

 limpet within the littoral zone, and the Globigerma at the 

 bottom of the Atlantic ; and on the land, there are Floras 

 and Faunas peculiar to low regions, and others peculiar to 

 high regions. Next we have the well-known geographical 

 limitations, made by climate. There are temperatures that 

 restrict each kind of organism between certain isothermal 

 lines ; and hygrometric states that prevent the spread of 

 each kind of organism beyond areas having a certain hu- 

 midity or a certain dryness. Besides such general limita- 

 tions, we find much more special limitations. Some minute 

 vegetal forms occur only in snow. Hot springs have their 

 peculiar Infusoria. The habitats of certain Fungi are mines 

 or other dark places. And there are creatures unknown be- 

 yond the water contained in particular caves. After 

 these limits to distribution imposed by physical conditions, 

 come limits of a different class, imposed by the presence 

 or absence of other organisms. Obviously, graminivorous 

 animals are confined within tracts which produce plants fit 

 for them to feed on. Large carnivores cannot exist out of 

 regions where there are creatures numerous enough and 

 large enough to serve for prey. The requirements of the 

 sloth, limit it to certain forest-covered spaces ; and there can 

 be no insectivorous bats, where there are no night-flying 



