Of course, the difficulty of classifying vanishes in the higher forms; the lowest plants are allied to the lowest animals, but the higher the plant the more it diverges from the animal, which is evidence that in the succession of life the highest plants do not pass into the lower animals. Descent is not lineal, but lateral; the relations between the two kingdoms are represented by two lines starting from a common point and spreading in different directions. Even the “lower” and “higher” are relative terms; the organization of the amœba is as complete for its purpose, as is that of the man for his purpose, the modification in the complex forms being due to the division of functions which are performed in every part by the simple forms. Although the foregoing and numberless other facts, together with the law of continuity, alike forbid the drawing of any hard and fast lines, and involve the conclusion, to borrow Professor Huxley’s words, “that the difference between animal and plant is one of degree rather than of kind, and that the problem whether, in a given case, an organism is an animal or a plant may be essentially insoluble,” there exists, exceptions notwithstanding, a broad distinction in the mode of nutrition. “All things the world which fill Of but one stuff are spun,” and this stuff, the basis of all life, the formative power, is a semi-fluid, sticky material, full of numberless minute granules in ceaseless and rapid motion, to which the name “protoplasm” (Gr. _protos_, first; _plasma_, formed) has been given. It consists of four of the elementary substances, carbon, hydrogen, oxygen, and nitrogen, complexly united in the compound called _protein_, which is closely identical with the albumen or white of an egg. These are the _essential_ elements, but a few others enter into the chemistry of life, with slight resulting differences in the _incidental_ elements in animals and plants. As water is necessary to all vital processes, a very large proportion enters into living matter.