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 The Industries 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 calling 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 



