SOCIAL BACKBONED ANIMALS 129 
procuring prey, and certainly gives an opportunity for co-opera- 
tion to the benefit of the community. On the other hand, living 
in shoals must tend very greatly to increase mortality among 
ill-defended species, which thus are bound to attract the notice 
of their foes. The well-being of the individual is here, as 
generally, subordinated to the interests of the species, the matter 
apparently being determined by the exigencies of spawning. 
Though some of the Ampnipians and Reptites may be more 
or less gregarious, but little of interest is known about them in 
Fig. 1102.—Part of a Shoal of Herrings (Clupea harengus) 
the present connection, so that we may pass on to the more 
intelligent animals included in the two highest vertebrate classes, 
ze. Birds and Mammals. A comparatively large and complex 
brain is here associated with sagacity that may make itself 
manifest in social developments. 
SociaL Birps (Aves).—Many Birds are eminently social in 
their habits, and migrant forms may be associated in vast 
numbers when making their long journeys (see p. 61). But we 
have yet much to learn regarding the way in which the living 
together of numerous individuals results in division of labour, or 
in concerted action. Of two closely related birds the one may 
Vou, IV. 103 
