162 THE EVOLUTION THEORY 



great simplicity, and which seem to have brought about almost no 

 change in the two associated species. 



We shall take our first examples from the Animal Kingdom. 



The partnership between certain sea-anemones (Actiniae) and 

 hermit-crabs (Paguridse) had been noticed long before any particular 

 attention was devoted to it. Many species of hermit-crab frequently 

 carry a large sea-anemone about with them on the mollusc shell 

 which they use as a protecting-house ; indeed, two or three of these 

 beautiful many-tentacled polyps are often attached to them, and this 

 is not at all a matter of chance, but depends upon instinct on the part 

 of both animals ; they have the feeling of belonging to each other. If 

 the sea-anemone be taken away from the hermit-crab and put in a 

 distant part of the aquarium, the crab seeks about till it finds it, then 

 seizes it with its claws and sets it on its house again. The instinct to 

 cover itself with Actiniae is so strong within it that it loads itself 

 with as many of these friends as it can procure, sometimes with more 

 than there is room for on the shell. The sea-anemone on its part 

 calmly submits to the crab's manipulations — a fact very surprising to 

 any one who is aware of the anemone's ordinarily extreme sensi- 

 tiveness to contact, and knows how it immediately draws itself 

 together on any attempt to detach it from the ground, and will often 

 let itself be torn in pieces rather than give way. The mutual instincts 

 of the two creatures are thus adapted to each other ; but it does not 

 at first sight seem as if any structural changes had taken place in 

 favour of the partnership. This is true, indeed,, as regards the hermit- 

 crab, but not as regards the sea-anemone, although the nature of the 

 adaptation on the sea-anemone's part only becomes apparent when 

 the two animals are closely observed in their life together. 



We owe our understanding of this adaptive change in the sea- 

 anemone, and, indeed, our knowledge of this whole case of Symbiosis, 

 to the beautiful observations of Eisig. Starting from the hypothesis 

 that the mutual relations could only be the outcome of natural 

 selection, Eisig pointed out that this partnership must offer some 

 advantage not to one partner only, but to both ; otherwise it could not 

 have arisen through selection. The advantage to the sea-anemone is 

 obvious enough ; since of itself it can only move very slowly, and is 

 usually firmly fixed in one place, it is easy to see that it would be 

 useful to it to be carried about on the floor of the sea by the hermit- 

 crab, and to get its share of the hermit-crab's food. But the service 

 yielded to the hermit-crab by the sea-anemone in return is not nearly 

 so apparent. Eisig made an observation in the Zoological Station at 

 Naples which solved this riddle. He saw an octopus attack the 



