MUTUALISM. 19 



Owing to the length of the corolla-tube, which is furnished 

 inside with a ring of hairs pointing towards the centre, only 

 long " tongued " insects, as some kinds of bees, can reach 

 the honey ; in other words, the arrangements are such that 

 only those insects that are adapted for effecting fertilization 

 by removing the pollen from one flower and depositing it 

 on the stigma of another can obtain the honey by legitimate 

 means. But certain insects that could not effect the fertiliza- 

 tion of the plant are conscious of the existence of honey, 

 and not being able to reach it by the usual channel, bite 

 through the corolla-tube from the outside at the point where 

 the honey is secreted, and thus obtain a meal by sur- 

 reptitious means. On the other hand, instances of protection 

 afforded by members of the animal kingdom to certain 

 plants has gradually evolved into mutualism, or that con- 

 dition of things where two living organisms, originally 

 independent of each other in every way, become so modified 

 as to live in each other's society with mutual advantage. 



A striking illustration of mutualism is described by Belt 

 (The Naturalist in Nicaragua) as follows. "A species of 

 Acacia, belonging to the section Gummiferce, with bi-pinnate 

 leaves, grows to a height of fifteen to twenty feet. The 

 branches and trunk are covered with strong curved spines, 

 set in pairs, from which it receives the name of ' Bull's-horn 

 Thorn,' they having a very strong resemblance to the horns 

 of that quadruped. These thorns are hollow, and are 

 tenanted by ants, that make a small hole for their entrance 

 and exit near one end of the thorn, and also burrow through 

 the partition that separates the two horns, so that the one 

 entrance serves for both. Here they rear their young, and 

 in the wet season every one of the thorns is tenanted, and 

 hundreds of ants are to be seen running about, especially 



