"Natural Law," p. 233.
Certain fungi seem to afford an exception to this. The above is, I believe, true as a theoretical action of plants and animals in protoplasmic form. But practically, in all higher developments of either kind, other distinctions come into play; e.g., that plants can make use of inorganic matter, gases, and water, and elaborate them into organic matter. Animals cannot do this, they require more or less solid food—always requiring "complex organic bodies which they ultimately reduce to much simpler inorganic bodies. They are thus mediately or immediately dependent on plants for their subsistence" (Nicholson, "Zoology," 6th ed. p. 17). It is perhaps with reference to this that in the Book of Genesis the Creator is represented as giving plant life to the service of man and animals—while nothing is said of the preying of Carnivora and Insectivora on animal life.
At the risk of repetition I will remind the reader that nature contains nothing like a progressive scale from plant to animal. It is never that the highest plant can be connected with the lowest animal as in one series of links. The animal kingdom and the plant kingdom are absolutely apart. Both start from similar elementary proteinaceous structures; and both preserve their development upwards—each exhibiting some of the features of the other. It is at the bottom of each scale that resemblance is to be found, not between the top of one and the lowest members of the other.
The reader may find this admirably put in Wallace, "Contributions to the Theory of Natural Selection," p. 302.