ORGANIC PARTNERSHIPS OR SYMBIOSIS 165 



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 1 

 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 persona3 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. But it is obvious 

 that some such colonies must have had among their members some 

 with a stronger external skeleton, and therefore less easily crushed 

 than the rest, and this would lead to their more frequent survival. 



No adaptation seems to have taken place in the hermit-crab in this 

 case, but that is probably only apparently the case ; the probability 

 is that it would not tolerate the presence of the polyp-colony on the 



