134 ASSOCIATION OF ORGANISMS—THE WEB OF LIFE 
be regarded as natural “labels”, which indicate the presence of 
other specific differences that may be much more difficult to detect. _ 
The theory of recognition marks is strongly supported by the 
fact that animals which turn white in winter commonly do not 
do so completely, but retain small dark patches. The Ptarmigan 
(Lagopus mutus, fig. 1105), for example, retains black markings 
on the side of the tail, while the male in winter plumage is further 
distinguished by black streaks near the eye. 
SoctaL Mammats (Mammatia).—A large number of mammals 
affect the gregarious habit, and some examples of the benefits 
Fig. r105.—Male Ptarmigan (Lagopus mutus) in Winter Plumage 
accruing have elsewhere been given. Wolves, for example, hunt 
in packs, and sometimes secure their prey by very ingenious 
devices (see vol. ii, p. 16). The herding together of various 
Hoofed Mammals is distinctly of advantage in defence, and many 
such forms post sentinels to warn them of approaching danger 
(see vol. ii, p. 365). Troops of Baboons and Monkeys make 
well-organized raids, while their tactics during retreat from foes 
are often decidedly skilful, and have reference to the well-being 
of the community (see vol. ii, p. 363). The cubs of the Fur- 
Seal benefit by the mode of life in their “rookery” (see vol. iii, 
p. 492). To consider in detail all the groups of mammals in 
regard to social habits would be an almost endless task, and the 
present purpose will perhaps be sufficiently served by taking one 
or two typical and interesting cases. 
