638 MAMMALIA. 



most of the Carnivora are carnivorous ; dolphins and seals 

 feed chiefly on fishes ; but in most cases the diet varies not 

 a little with the available food supply. 



The struggle for existence among Mammals is sometimes 

 keen among fellows of the same kind ; thus the brown rat 

 (Mus decumanus) tends to drive away the black rat (M. 

 rattus], but stress, due to over-population, is sometimes 

 mitigated by migration, as in the case of the lemmings. The 

 struggle seems to be keener between foes of different kinds, 

 between carnivores and herbivores, between birds of prey 

 and small mammals ; but combination for mutual defence 

 often mitigates the intensity of the conflict. Teeth and 

 claws, hoofs and horns are the chief weapons, while the 

 scales of pangolins, the bony shields of armadillos, the spines 

 of hedgehogs and porcupines, and the thick hide of the 

 rhinoceros may be regarded as protective armature. In 

 keeping their foothold some mammals are helped by the 

 harmony between their colouring and that of their surround- 

 ings ; thus, the white Arctic fox and hare are inconspicuous 

 on the snow, the striped tiger is hidden in the jungle, and 

 many tawny animals harmonise with the sandy background 

 of the desert. 



The majority of Mammals are gregarious, witness the herds 

 of herbivores, the cities of the prairie dogs, the packs of 

 wolves, the schools of porpoises, the bands of monkeys. 

 Combinations for attack and for defence are common ; senti- 

 nels are posted and social conventions are respected ; such 

 migrations as those of the lemming and reindeer are char- 

 acteristically social. In the beaver village and among 

 monkeys there is combination in work, and their communal 

 life seems prophetic of that sociality which is distinctively 

 human. 



Among Birds, mates are won by beauty of song and 

 plumage ; Mammals not less characteristically woo by force. 

 Rival males fight with one another, and are usually larger 

 and stronger than their mates. The antlers of male deer, 

 the tusk of the male narwhal, the large canine teeth of boars 

 illustrate secondary sexual characters useful as weapons. 

 But manes and beards, bright colours and odoriferous glands 

 are often more developed in the males than in the females, 

 and may be of advantage in the rough mammalian court- 



