396 THE INDUCTIONS OF BIOLOGY. 



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 ony 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 

 conspicuous is the fact that within each of these contrasted 

 media there are further widespread limitations. In the sea, 

 certain organisms exist only between certain depths, and 

 others only between other depths — the limpet and the mussel 

 within the littoral zone, and numerous kinds at the bottom 

 of the ocean; and on the land, there are Floras and Faunas 

 peculiar to low regions and others peculiar to high regions. 

 Next we have the familiar geographical limitations made by 

 climate. There are temperatures which restrict each kind of 

 organism between certain isothermal lines, and hygrometric 

 states which prevent the spread of each kind of organism 

 beyond areas having a certain humidity or a certain dryness. 

 Besides such general limitations 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 beyond the water contained in particular 

 caves. After these limits to distribution imposed by 



physical conditions, come limits 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. The great carnivores cannot exist out of regions 

 where there are creatures large enough and numerous enough 

 to serve for prey. The needs of the sloth limit it to certain 

 forest-covered spaces; and there can be no insectivorous bats 

 where there are no night-flying insects. To these dependences 

 of the relatively-superior organisms on the relatively-inferior 

 organisms which they consume, must be added certain 



