ANIMAL COMMUNITIES AND SOCIAL LIFE 329 



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 gregaripusness is reduced to 

 a matter of slight or temporary convenience, possessing but 

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

 reindeer in the north, and of the bison or buffalo which 

 once ranged over the Western American plains, are examples 

 of a gregariousness in which mutual protection from ene- 

 mies, like wolves, seems to be the principal advantage gained. 

 The bands of wolves which hunted the buffalo show the 

 advantage of mutual help in aggression as well as in pro- 

 tection. In this banding together of wolves there is active 

 co-operation among individuals to obtain a common food 

 supply. What one wolf can not do that is, tear down a 

 buffalo from the edge of the herd a dozen can do, and all 

 are gainers by the operation. On the other hand, the vast 

 assembling of sea-birds (Fig. 200) on certain ocean islands 

 and rocks is a condition probably brought about rather by 

 the special suitableness of a few places for safe breeding 

 than from any special mutual aid afforded ; still, these sea- 

 birds undoubtedly combine to drive off attacking eagles 

 and hawks. Eagles are usually considered to be strictly 

 solitary in habit (the unit of solitariness being a pair, not 

 an individual) ; but the description, by a Russian naturalist, 

 of the hunting habits of the great white-tailed eagle (Hali- 

 cetos albicilla) on the Eussian steppes shows that this kind 

 of eagle at least has adopted a gregarious habit, in which 

 mutual help is plainly obvious. This naturalist once saw an 

 eagle high in the air, circling slowly and widely in perfect 

 silence. Suddenly the eagle screamed loudly. "Its cry 

 was soon answered by another eagle, which approached it, 

 and was followed by a third, a fourth, and so on, till nine 

 or ten eagles came together and soon disappeared." The 



naturalist, following them, soon discovered them gathered 

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