§ 104. There is a distribution of organisms in Space, and there is a distribution of organisms in Time. Looking first at their distribution in Space, we observe in it two different classes of facts. On the one hand, the plants and animals of each species have their habitats limited by external conditions: they are necessarily restricted to spaces in which their vital actions can be performed. On the other hand, the existence of certain conditions does not determine the presence of organisms that are fit for them. There are many spaces perfectly adapted for life of a high order in which only life of a much lower order is found. While, in the inevitable restriction of organisms to environments with which their natures correspond we find a _negative_ cause of distribution, there remains to be found that _positive_ cause whence results the presence of organisms in some places appropriate to them and their absence from other places equally appropriate or more appropriate. Let us consider the phenomena as thus classed. § 105. Facts which illustrate the limiting influence of surrounding conditions are abundant, and familiar to all readers. It will be needful, however, here to cite a few typical ones of each order. 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 extensive 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 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