164 ANIMAL LIFE 
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 
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. 100) 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- 
etos albicilla) on the Russian 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 
