This is the question for our present course of lectures; but, whatever the result of our inquiries, it will not invalidate the generality of the principle of association. If, contrary to our past opinions, these higher beings are not simple individuals, we must compare them with
Fig. 15.—Star-fish
those primordial individuals which by combination have produced other types, and which are still found at the base of each of the great divisions of the animal kingdom. Now, how have these individuals arisen?
The Hydras and analogous organisms reply. We can cut a Hydra into as many pieces as we like, and each piece, instead of dying, continues to develop and ends by becoming a complete Hydra, It follows that these different parts are independent of each other, like the polyps forming the lowest colonies. Each cell of the Hydra is a true individual, and the Hydras are a colony of these monocellular individuals as the Siphonophores themselves are colonies of Hydras. Aptitude to social life is communicated by heredity to these cells, as it is communicated to the polyps. Each cell, each polyp, detached from the colony, is a copy of it, and his after-development tends always toward its formation. At first all the members of a colony are equally apt to reproduce; then this function is localized like the others, and pertains to some individuals, or some parts, while sexual reproduction becomes more and more important. When the society reaches a certain degree of coherence, these different parts cease to live independently of the others, and can not be separated without danger of dying.
We see still more clearly in the Sponges their colonial nature. The spongarian individual is formed of two sorts of cellular individuals, the amoeba and infusorial flagellifere, of which we find analogues living, at liberty and in isolation (Fig. 16). The flagelliferous cells of sponges present exceptional features; they are provided with a nucleus and contractile vesicle, and their unique flagellum is surrounded by a mem-
