DIVERGENCE AND ADAPTATION 367 



harbor multitudes of unicellular algae, which grow and 

 multiply within the tissues of their hosts. The algae derive 

 food from the carbon dioxide and probably other products 

 of excretion from the animal's body, while they give off 

 oxygen which is of value to the animal. Each member 

 of the partnership thus profits by its association with the 

 other. Another striking case of symbiosis is afforded by a 

 species of hermit crab whose shell usually carries a specimen 

 of sea anemone, Adamsia. The anemone secures the 

 advantage of being carried about in situations where it can 

 obtain more food, while by means of its nettling organs 

 it affords a certain degree of protection to the hermit 

 crab. If for any reason the shell of the hermit crab should 

 be deprived of the anemone the crab hunts for another 

 specimen which it manages to work loose from its attach- 

 ment and fasten to its shell. 



One of the most striking cases of mutual adaptation is 

 afforded by the relations of insects and flowers whereby the 

 insects are enabled to secure food and the flowers to obtain 

 the advantages of cross fertilization. It has been abun- 

 dantly shown by the investigations of Darwin in his inter- 

 esting work on the effect of cross and self-fertilization of 

 flowers that the seeds of flowers which have been cross 

 fertilized generally produce plants of greater vigor than 

 those resulting from self-fertilization. In higher plants 

 fertilization is accomplished by means of pollen grains; 

 in many plants these are blown by the wind from one flower 

 to another, but in many others they are carried by insects. 

 The insects attracted by the honey of the flowers become 

 dusted with the pollen as the result of their visit and when 

 they fly to another flower some of this pollen may be 

 rubbed off against the surface of the stigma and thus effect 

 cross fertilization. Many flowers show remarkable adap- 

 tations for bringing the pollen in contact with the body of 



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