196 ASSOCIATION OF ORGANISMS 



are a matter of trainmg; and training rests upon high 

 brain development. There is here the element of choice, 

 or the power to direct the activities of the body along 

 any one of several lines, whereas the ant and other lower 

 animals are little more than dumb machines whose 

 structure determines within narrow limits their lifetime 

 duties. 



Symbiosis. — The examples cited to illustrate coopera- 

 tion and mutual aid have thus far been concerned with 

 individuals of a single species. There are, however, many 

 cases where one animal enters into a partnership of mu- 

 tual benefit (symbiosis) with another totally different 

 kind of animal, or one plant becomes associated with 

 another plant, or a plant and animal may merge in- 

 terests. In the lichens, for example, that incrust fences, 

 old roofs and similar structures, two plants are in inti- 

 mate union, a green one-celled alga and a fungus. It 

 is assumed that nutritive products manufactured by the 

 green plant are utilized by the fungus, which repays the 

 debt by furnishing moisture and offering protection. 

 Unicellular plants are also present in large numbers in 

 many animals, such as the green hydra, sea anemones, 

 and certain worms, where, it is believed, a beneficial 

 interchange of food materials takes place. 



Many species of ants feed upon a sweet liquid that 

 issues from the body of certain aphids or plant lice, which 

 are small insects feeding upon plant juices. In return the 

 ant protects the aphid from enemies, transports it to new 

 feeding grounds, or carries it underground, where food is 

 provided. Several species of hermit crabs, living in the 

 empty shells of snails, carry sea anemones attached to 

 their houses (Fig. 47). The crab is protected by the 

 stinging cells of the anemone, which enjoys a plenteous 

 food supply in consequence of being carried about. 



CoMMENSALisM, OR Messmattsm. — There are many 

 examples of the association of two different species 

 of organisms where one partner only derives a benefit. 



