286 DARWINISM AND THE PROBLEMS OF LIFE 



certain species of lice on them, and it is easy to see 

 that the lice have the advantage of being protected from 

 many enemies by the presence of these stalwart com- 

 panions. On the other hand, the ants are fond of the 

 faeces of the lice, which are as sweet as honey.^ 



Symbioses between plants and animals, or between 

 plants and fungi, are much more common than between 

 two species of animals. That is easily explained. 

 Animals are rivals in the struggle for life, but plants 

 have a different kind of food and respiration ; in fact, 

 the latter difference makes the proximity of plants 

 healthy, and often indispensable, for animals. The 

 fungi are in the same position as the animals. 



Hence besides the hydra, certain protozoa are 

 inhabited by green algse ; and the best known instance 

 of a symbiosis between fungi and algae is found in the 

 lichens, which are not single organisms, as one would 

 infer from their appearance. 



^ There are also symbioses between higher plants and animals. Thus 

 in South America the trees are threatened by the leaf-cutter ants. 

 They cut off the leaves with their shear - like jaws, and gather them 

 into heaps, on which they grow fungi. Certain plants, the imbauba, or 

 chara, are protected from these enemies by harbouring fighting ants of 

 a different species, which drive off the leaf-cutters when they come. 

 But in order to induce the tree-ants to remain outside the tree, so as 

 to perceive the approach of the enemy — which would often be impossible 

 if they were always inside — the tree has developed special nutritious 

 pods at the threatened points — the stalks of young leaves — which the 

 ants constantly gather for their young inside, and so they are always 

 found at these spots. 



This case, again, cannot be explained on the Lamarckian principle. 

 The nutritious pods that are formed by the plant for the ants, and that 

 are wanting on trees related to the imbaubas which have no ant-guests, 

 cannot have been produced either by the will of the plant or the 

 repeated bites of the insects. Hence the explanation of symbiosis as 

 an inherited habit does not hold in this instance. 



