370 EVOLUTION AND ANIMAL LIFE 



or a herd of bison, to the extreme!}^ speciahzed, interdependent 

 and unified community of the honeybee, or agricuhural ant. 

 Before taking up this series of stages in true social or communal 

 development among the lower animals, however, we may profit- 

 ably give some attention to the conditions of animal association 

 commonly known as commensalism or symbiosis in which in- 

 dividuals of one species are associated to their mutual advan- 

 tage with individuals of different species. 



In the relations of parasite and host, discussed in the last 

 chapter, all the advantages of the association lie with the para- 

 site. The other animal involved, the host, suffers inconveni- 

 ence, injury, often untimely death. But in commensalism and 

 symbiosis both associating kinds of animals reap advantage, or 



Fig. 227. — Remora Echeneis remora, with dorsal fin modified to be the sucking plate 



by which the fish attaches itself to a shark. 



at least neither suffers in any serious way from the effects of 

 the other's presence. The two kinds live together in harmony 

 and usually to their actual mutual advantage. The term 

 commensalism may be applied to denote a condition of loose 

 and often not obviously equally mutual advantageous asso- 

 ciation, while symbiosis is used to refer to a more intimate and 

 persistent association with maybe marked cooperation and 

 mutual advantage. A few examples of each are given in the 

 following pages. Of com*se, no marked line of demarcation 

 can be really drawn between the two conditions, any more 

 than we can establish a sharp distinction between the preda- 

 tory and parasitic modes of life. 



A curious example of commensalism is afforded by the 

 different species of Remoras (Echenididse) which attach them- 

 selves to sharks, barracudas, and other large fishes by means 

 of a sucking disk on the top of the head (Fig. 227). This disk 

 is made by a modification of the dorsal fin. The Remora thus 

 attached to a shark may be carried about for weeks, leaving 

 its host only to secure food. This is done by a sudden dash 



