Natural Selection, by which it is demonstrated that organized matter must move forward simply through the chemical and physical forces inherent in it, furnishes a key to all the phenomena of life, both animal and vegetable, which have ever appeared on the earth. Natural Selection, we are told, depends for its operation on the interaction of two processes or agencies, namely, Inheritance and Adaptation. Through Inheritance germs receive from their parents a plastic form which, as all development is a function of external physical conditions, is itself nothing more than a “manifestation of the remains of antecedent physical impressions.” This inherited form causes them to go forward in a predestined course, while through Adaptation there is a constant tendency to change that predestined form imposed upon them by their parents to one better suited to their changing physical conditions. According to the theory of Natural Selection, organic structures vary to meet the requirements of changed conditions; or, when existing circumstances are such that they are forced into new and unusual modes of life, they branch off into different directions; thus new varieties are formed, or possibly new species. Such portions of a group, however, as remain sheltered from conditions unsuited to their present line of development, retain their ancient forms. This change of structure by which organisms or portions of organic bodies are modified so as to perform more complicated functions, or those better suited to their environment, is denominated differentiation; hence the degree of differentiation attained by a structure determines the stage of development which it has reached. But to return to our single-celled animal—the simplest form of life on the earth. Except that by the action of the surrounding forces its surface has become somewhat hardened, this little animal is the same throughout, in other words, it is homogeneous. The hardening of the outer portion constitutes the first process of differentiation, and therefore the first step in the order of progress. Comparing the simplest form of life, the little carbon-sac found in the sea, with the germ from which animals and plants are derived, Haeckel says: Originally every organic cell is only a single globule of mucus, like a Moneron, but differing from it in the fact that the homogeneous albuminous substance has separated itself into two different parts, a firmer albuminous body, the cell-kernel (nucleus), and an external, softer albuminous body, the cell-substance or body (protoplasma).[3] [3] _History of Creation_, 1884, vol. i., p. 187.