It is easy to understand, therefore, that genuine 'symbiosis' may have arisen from parasitism. But that this is not the only path that leads to symbiosis is shown by the cases of animal symbiosis we have already discussed. [Illustration: FIG. 39. _A_, fragment of a Silver Poplar root, with an envelope of symbiotic fungoid filaments (mycelium); after Kerner. _B_, apex of a Beech root, with the closely enveloping mantle of mycelium; enlarged 480 times.] The partnership between polyps and hermit-crabs may have arisen from a one-sided commensalism, since polyps establishing themselves on mollusc shells which were often made use of by hermit-crabs would be better fed than those which settled down on stones. There are still species which make use of both modes of settlement. Then followed the adaptation of the crustacean to the polyp, for, first, those hermit-crabs would thrive best which tolerated the presence of the polyp; then those which sought its presence, that is to say, which gave a preference to shells covered with polyps; and, finally, those which would take no others, and even themselves fixed the sea-anemone upon it, if it chanced to be removed. Intelligence need not be taken into account in the matter at all, not even in the hermit-crab's case. We have only to recall the complex instincts, exercised only once in a lifetime, which compel the silkworm and the emperor moth to elaborate their effective cocoons. The elaboration of the spinning-instinct can only be due to natural selection, for the insect can have had no idea of the utility of its performance, and the same is true in the case of the sea-anemones or the hydroid polyps and the hermit-crab. The sea-anemone is quite unconscious that it is defending its partner, the hermit-crab, when it lashes out its stinging acontia on any disturbance, and the hermit-crab is equally unaware that the sea-anemone is contributing to its safety; both animals act quite unconsciously, purely instinctively, and the origin of these instincts, on which the symbiosis is based, must be due, not to intelligent activities which have become habitual, but only to the survival of the fittest.