THE FACTORS OF EVOLUTION 183 



sea-anemones, and close up the entrance to the shell, the crab 

 will make the most desperate efforts to remove the obstacle. 

 Only when it is convinced of the futility of its endeavours it will 

 go in search of another domicile. Having found a suitable shell, 

 it examines it carefully with its claws in order to find out 

 whether any enemy may be hidden within. When the examina- 

 tion has proved satisfactory the hermit-crab settles down within 

 its new armour with every possible speed. Then begins a most 

 attractive spectacle. Quickly it returns to its old domicile and 

 attempts by means of its claws gently and carefully to remove 

 the sea-anemone from the shell and induce it to settle down 

 on the new house. Very soon its endeavours are crowned with 

 success. Sometimes it happens that for some reason the new 

 home does not please the sea-anemone. In such case it has 

 been observed that the crustacean will untiringly change its 

 domicile until it meets at last with a shell which pleases its 

 friend the sea-anemone. 



With many hermit-crabs the settling on their houses of sea- 

 anemones has become a perfect mania. I have myself seen 

 hermit-crabs which carried about with them three and even four 

 Actinias, all of them of the same size as the crustacean itself, 

 and their combined weight so heavy as to make it almost 

 impossible for the crab to drag its dwelling along (see coloured 

 plate 6). 



A similar alliance was observed by Sluiter between a ' sea- 

 anemone ' and a fish. On the coral-reefs near Batavia are found 

 numerous large yellow Actiniae with long tentacles thickly 

 covered with stinging-cells. Between these weapons, which are 

 carefully avoided by other animals, there swims calmly a little 

 defenceless fish which is thus completely protected against all 

 attacks. In return for the protection the fish faithfully shares 

 its food with its friend by placing pieces into the mouth of the 

 Actinia. The ' sea-anemones/ which as a rule reply to even the 

 slightest stimulus from outside with a fierce attack, quietly 

 submit even to being hustled and pushed by these little fishes. 



The symbiosis between Algae and Actiniae I have already 

 mentioned. We saw that in that case adaptation had advanced 

 to such a degree that the animals had lost their ability to feed 

 themselves. 



