ORGANIC PARTNERSHIPS OR SYMBIOSIS 177 



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. 



According to the principle of natural selection nothing can arise 

 but that which is of use directly or indirectly to its possessor. 

 Nevertheless, there are cases in which it appears as if something had 

 arisen, which was of no use to the species in which the variation 

 appeared, but only to the species protected by it. This is the case 

 in the remarkable symbiosis between algse of the family Nostocacea? 

 and the floating, moss-like water-fern Azolla. This plant, in external 

 appearance almost like duckweed, has on the under surface of its 

 leaves a minute opening, leading into a relatively roomy hair-lined 

 cavity, and in this cavity there is always, enclosed in jelly, a bluish 

 green unicellular alga, Anahcena. The cavity is present in every 

 leaf, and the alga is present in every cavity, making its way in from 

 a deposit of alga-cells which is found on the incurved tip of every 

 young shoot. As soon as a young leaf of Azolla unfolds from the 

 bud it receives its Anabcena cells from this deposit, and no one has 

 yet found either twigs or leaves which were free from the alg}^. But 

 no one has succeeded in discovering any benefit derived by the Azolla 

 from this partnership. 



This looks like a contradiction of the theory of selection, but 

 there remains the possibility that there is some benefit rendered to 

 the Azolla by the alga, though we cannot see it as yet. There is 

 also the possibility that the cavity is an organ which was of use 

 to the plant at an earlier time, perhaps as an insect-trap, but has 

 now lost its significance, and is utilized by the alga as a dwelling- 

 place. This, however, is contradicted by the remarkable distribution 

 of the four known species of Azolla. Two of these are widely 

 distributed in America ; the third lives in Australia, Asia, and Africa ; 

 the fourth in the region of the Nile : all four have cavities in their 

 leaves, and in all these forms the cavity is inhabited by the same 

 species of Analcena. This indicates that the leaf-cavity and the 

 partnership with the alga must have originated in remote antiquity ; 

 the symbiosis must date from a time before the four modern species 

 of Azolla had split off" from a single parent-species. But no rudi- 

 mentary organ, that is to say, no organ not of use to the plant itself. 



