mal. Each of these families of the great invertebrate kingdom has been bandied back and forth from the botanist to the zoölogist, and each has finally found its place in the animal world.
No purely empirical knowledge is sufficient to determine, among the lower forms of life, to which kingdom they should be referred. It is only by studying facts in their relations, by patiently observing the life-history, and by ascertaining the modes of nutrition and reproduction of each form, that its true place in the organic world has been determined.
Fig. 2.—Diagram of a Section of Hydroid.
It was, for many years, thought that, beyond the depth of 300 fathoms, organic life ceased to exist in the ocean. Forbes reached this zero of life in the Ægean Sea, and the fact ascertained for the Mediterranean was inferred for all other seas. The transmutation of inorganic into organic matter is only performed by vegetables, and then only under the controlling power of light. The distinction made by naturalists between the lowest forms of animal and vegetable life lies just here: vegetables convert the inorganic elements of earth, air, and water, into organized matter; animals rearrange this organized matter into animal tissue. It is well known, as no light penetrates
