In this species of polyp a special form of individual has developed with a quite definite position in the colony, and furnished with a special instinct or reflex mechanism which is directly useful only to the crab, and has therefore, in a sense, arisen for its advantage. This can quite well be explained through natural selection, for indirectly these polyps are also of use to the colony, inasmuch as they protect their valuable partner, and thus render it possible for the hydroid colony to make the partnership of use to the hermit-crab as well as to itself. This mutual arrangement thus satisfies the requirement which, from the selectionist point of view, must be made in regard to all that is new--that it must be useful to its possessor. If it be asked what service the hermit-crab renders to the polyp colony in return, the answer is that, as in the symbiosis with sea-anemones, the hermit-crab carries the polyps to their food, which is also its own. Hermit-crabs eat all sorts of animal food, living or dead, which they find on the sea-floor, and the remains of their meal fall to the share of the polyps. Once, without special intention, I laid a hermit-crab with its polyp colony in a flat vessel of sea-water beside a bright green living sponge. After some time the majority of the polyps had become bright green; they had filled themselves with the green cells of the sponge. I do not know how else we should picture to ourselves the origin of symbiotic instincts in such lowly animals except through the transmission and augmentation of variations in the instincts of the two partners--variations which made their possessors more capable of survival. Mollusc shells, ever since there were any, must have served as a foundation and point of attachment for polyp colonies; as a matter of fact, we find to-day on mollusc shells many kinds of polyp colonies which show no special adaptation to a life of partnership with hermit-crabs. From such indifferent associations a symbiotic one must gradually have been evolved in some instances, through the preservation and augmentation of every useful variation, both of instincts and reflex actions, as well as of form and structure. I shall not attempt to trace the course of this evolution in detail, but it is obvious that the development of defensive polyps, and of their instinct to defend the crustacean, can be interpreted neither as due to any direct influence nor as due to the effect of use, but only to the utility of this arrangement, the beginnings of which--polyps with stinging-cells--were already present. Their augmentation and perfecting must be referred entirely to natural selection. It is the same with adaptations which do not refer directly to the crustacean partner, but rather to the disposition of the polyps on the shell. The spinous personæ which protect the softer polyps from being crushed by being rolled about on the pebbles by the waves cannot possibly be regarded as the direct result of this crushing.