380 EVOLUTION AND ANIMAL LIFE 



these points is an effective protection against the encroachments of 

 the leaf-cutters, as courage and eagerness to fight other ants is already 

 characteristic of the Aztecs. Not all candelabra trees live in symbiosis 

 with ants or possess this special protection against the ravages of the 

 leaf-cutter species. Schimper found in the forests of Brazil several 

 species of Cecropia which never shelter ants in the chambers of the 

 hollow trunk. Now these species do not develop the curious special 

 food-producing organs at the bases of the leaf petioles. These species 

 lack the means of attracting and retaining the ant guests. Only one 

 species of candelabra tree, Cecropia peltata, has develojDed this arrange- 

 ment, and it is plainly of no direct use for the tree except through the 

 bringing to it of the j^rotecting ants." 



There are, of course, numerous other examples known of the 

 symbiotic association of plants and animals; and if we were to 

 follow the study of symbiosis into the plant kingdom we should 

 find that in one of the large groups of plants, the familiar 

 lichens which grow on rocks and tree trunks and old fences, 

 every member lives symbiotically. A lichen is not a single 

 plant, but is always composed of two plants, an alga (chloro- 

 phyll-bearing) and a fungus (without chlorophyll) living 

 together in a most intimate, mutually advantageous associa- 

 tion. But we must devote no more space to the consideration 

 of this fascinating subject. 



The simplest form of social life, or the living together of 

 several to many individuals of the same sjDecies, is shown among 

 those kinds of animals in which many individuals of one species 

 keep together, forming a great band or herd. In this case there 

 is not much division of labor, and the safety of the individual 

 is not wholly bound up in the fate of the herd. Such animals 

 are said to be gregarious in habit. The habit undoubtedly is 

 advantageous in the mutual protection and aid afforded the 

 individuals of the band. This mutual help in the case of 

 many gregarious animals is of a very positive and obvious 

 character. In other cases this gregariousness is reduced to 

 a matter of slight or temporary convenience, possessing but 

 little of the element of mutual aid. The great herds of rein- 

 deer in the north, and of the bison or buffalo which once' ranged 

 over the Western American plains, are examples of a gregari- 

 ousness in which mutual protection from enemies, like wolves, 

 seems to be the principal advantage gained. The bands of 



