On the other hand, we find among the lower animals and the higher plants a composition of homogeneous parts, which represents a higher stage of life-unity. The tree is an individual, but it is made up of a number of branches or individual sprouts, each of which consists in like manner of an axial stem with leaves attached. If we detach such a branch and plant it in the ground, it takes root and grows into an independent plant. So the coral-stem is made up of a number of individual animals or persons, each of which has its own stomach and mouth with a crown of tentacles. Each several coral-individual is equivalent to a single living polyp (actinia). Thus the stem (_cormus_) is a higher unity, both in the animal and the plant world. Even the herds of gregarious animals, the swarms of bees and ants, and the communities of human beings, are similar unities; with the difference that the individual persons or citizens are not physically connected, but held together by common interests. We can, therefore, distinguish three stages of organic individuality, one building upon the other--the cell, the person (or sprout), and the stem or state (cormus). Each higher unity represents an intimate union of lower individuals. Morphologically, in relation to their anatomic structure, the latter are independent; but physiologically, in respect of the life-unity of the whole, they are subordinated to the former. This relation is quite clear in the familiar examples I have quoted. But there are other organisms in which this is not so, and where the question of the real individuality is very difficult to answer. Thus, fifty years ago, we came to recognize floating animal-stems in the remarkable siphonophora, or social medusæ, which had hitherto been regarded as individual animals, or medusæ with a multiplicity of organs; further study proved that each of these apparent organs is really a modified medusa, and the whole united structure a stem. This example throws a good deal of light on the important question of association and division of labor. The whole floating siphonophoron is, physiologically considered (in respect of its vital activity), a harmoniously organized animal with a number of different organs; but from the morphological point of view (in respect of form and structure) each dependent organ is really an independent medusa.