130 ASSOCIATION OF ORGANISMS—THE WEB OF LIFE 
be solitary, and the other markedly gregarious, the Raven and 
the Rook affording a good instance of this. Each is adapted to 
its surroundings in a different way, and both adaptations are 
admirable of their kind, though possibly the social habit gives a 
better chance in the struggle for existence, and it certainly has 
a tendency to promote the development of comparatively high 
mental qualities. As elsewhere remarked (p. 107), the remarkable 
caste-system which distinguishes social insects has a serious 
penalty attached to it, for extreme specialization involves a loss 
of plasticity which, if surroundings change quickly, may mean 
extinction. But in social Birds and other Vertebrates improved 
mental powers may be expected to confer increased ability to cope 
with changing surroundings, and a community of the kind does 
not suggest a complicated machine easily thrown out of gear, as 
a nest, say, of Termites, irresistibly does. 
As an example of a common social bird we may take the Rook 
(Corvus frugilegus), where there is abundant evidence to show 
that individuals may render services to the community, and that 
there may be co-operation to bring about certain common ends. 
We must, however, receive with caution some of the accounts 
that have been given of these crafty birds, and which would endow 
them with almost human attributes. It would appear that when 
raiding cultivated fields they commonly set sentinels on adjoining 
trees, and these worthies promptly give warning in raucous tones 
of the approach of danger in the shape of an agriculturalist. They 
certainly seem to have acquired knowledge, based on painful 
experience, of the lethal properties of firearms. Bernard observed 
Rooks co-operating to hunt field-voles, and his observations are 
thus summarized by Houssay (in Zhe /ndustries of Animals):— 
“His curiosity was excited by the way in which numerous rooks 
stood about a field cawing loudly. In a few days this was ex- 
plained: the field was covered with rooks; the original assemblage 
had been caHing together a mouse-hunt, which could only be 
successfully carried out by a large number of birds acting in 
conjunction. By diligently probing the ground and blocking up 
the net-work of runs, the voles, one or more at a time, were 
gradually driven into a corner. The hunt was very successful, 
and no more voles were seen in that field during the winter.” 
The social nesting-habits of Rooks are familiar to all, for the 
cheerful sights and sounds of the rookery lend to the country a 
