All creatures on our earth, since they have bodies formed from material constituents, are subject to the physical laws which govern matter; and the life of all is determined, so far as their bodies are concerned, by the physical conditions which foster, or depress, or destroy life. Plants need soil, moisture, sunshine, and certain constituents of the atmosphere. Their distribution over the earth’s surface depends not only upon the greater or less extent to which these things, essential to their existence, are present, but also upon the configuration of the earth’s surface (continents and oceans), upon the greater or less elevation above sea level of parts of it, upon such forces as winds and ocean currents (occasionally also upon volcanoes), upon the interposition of arid deserts between moister regions, or upon the flow of great rivers. The flora of each country is the resultant (until man appears upon the scene) of these natural conditions. [Sidenote: Natural Conditions of Life] We know that some plants are also affected by the presence of certain animals, particularly insects and birds. Similarly, animals depend upon these same conditions which regulate their distribution, partly directly, partly indirectly, or mediately through the dependence of the animal for food upon the plants whose presence or absence these conditions have determined. It would seem that animals, being capable of moving from place to place, and thus of finding conditions suitable for their life, and to some extent of modifying their life to suit the nature around them, are somewhat more independent than plants are, though plants, too, possess powers of adapting themselves to climatic surroundings; and there are some--such, for instance, as our common brake-fern and the grass of Parnassus--which seem able to thrive unmodified in very different parts of the globe. [Sidenote: Man the Servant of Nature]