Page:Popular Science Monthly Volume 74.djvu/586

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hole so charmingly described by Mrs. Austin. Every trout-fisherman has seen the roots of alder and willow growing from the bank into the stream. We are accustomed to having our house-sewers stopped up by the roots that will grow into them, though there is much more room outside. But the formative influence of water is not so obvious. Yet when we think of the creatures, plants and animals, of arid regions and of well-watered ones, we perceive certain differences. When we realize that water is formed, or is the surrounding medium, in almost every chemical reaction which takes place outside the living body, and in every chemical reaction within the living body, its importance is evident enough. The shape, size, structure and covering of every animal and plant are influenced by the ease with which water may be obtained and held. All land animals and plants lose water from their bodies by evaporation; submersed aquatics do not. Land animals and plants ordinarily get water from the earth, from depressions in its surface or from its soil, and only through those limited parts of their whole bodies which touch the water; but aquatics can take it in through their entire surface. If a land plant or animal takes in water only through its roots or its alimentary canal, there must be some system for distributing the water to all the parts of the body; but this is not necessary in aquatics. The differences in structure and form between land and water organisms is, then, partly due to their relations to water. The differences between the tadpole and the frog, between the submersed and floating leaves of the water-buttercup, between the swimming sperm of moss and fern and the wind-or insect-borne pollen of the higher plants, these differences are in their relations to water, in the degrees in which the formative influence of water has been unopposed by other factors.

Before turning to other formative influences, we should realize that the force of gravity acts constantly, night and day, uniformly, age after age, and it is impossible either to eliminate it in experiments or to conceive of its operation ever being or having been interrupted in nature. Water also is constant and uniform and unavoidable; for until water ceases to be a necessary component of the living protoplasm of the plant and animal body, until it becomes something else than hydrogen and oxygen in the proportion of two to one, we can not conceive of its being eliminated, by exclusion or substitution, in experiment, or of its absence in nature. If water is absent, life is absent. This is not true, however, of other influences, material or energetic, which affect form and substance, as well as the direction of growth or movement. These influences may be temporary.

Light is not a necessary condition of active life. It comes and goes, day and night. In the extreme northern mid-summer the sun never sets; the winter is dark and gloomy. In spite of the lack of light, however, life goes on in the winter darkness provided sufficient