{ Stable { (Heredity) { Temperature. } { { Chemical } { { substances in } { { solution in the } Germ-Cell { { Blastogenetic { fluids that } { { { bathe the } { { { germ-cells, as } { { { food, drugs, } { Unstable { { poisons, etc. } { (Variation) { } Environment { { Habitat } { { Temperature } { { Climate } { { Air } { Somatogenetic { Food } { Soil } { Water } { “Use” } { “Disuse” } Just in proportion as fertilized germ cells during the mitoses of ontogeny give origin, among the somatic cells, to other germ cells that are structurally, and therefore physiologically, like themselves, just to that extent do we have heredity; on the other hand, just to the degree that the new germ cells which are produced are unstable, to that degree also do we meet with variations. ENVIRONMENT. In zoölogy the environment of an organism means the sum-total of the conditions of life that surround and affect it, such as food, air, water, climate, etc. We have already stated that as far as evolution is concerned the structures of fundamental importance in an organism are the Germ-Cells; therefore, for our purposes, we will define _environment to be the sum-total of the conditions that directly or indirectly influence in any way the germ-cells_, by which variations in them may be produced, or through which stability may be maintained. There are two great classes of environmental factors that bring about variations in the germ-cells. One of these classes acts directly on the germ-cells, and is therefore called _blastogenetic_; the other acts indirectly through the body cells, and is therefore designated _somatogenetic_. Many of the blastogenetic factors bringing about structural changes in the delicate mechanism of germ-cells are entirely unknown, and are therefore designated as fortuitous. Many other causes, such as poisons in solution in the fluids that bathe them, can readily enough be appreciated.