procuring prey, and certainly gives an opportunity for co-operation 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 AMPHIBIANS and REPTILES may be more or less gregarious, but little of interest is known about them in

the present connection, so that we may pass on to the more intelligent animals included in the two highest vertebrate classes, i.e. Birds and Mammals. A comparatively large and complex brain is here associated with sagacity that may make itself manifest in social developments.

SOCIAL BIRDS (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

VOL. IV.

103

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 explained: 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

CRESTED PENGUINS OR ROCK-HOPPERS

(Eudyptes chrysocome)

Penguins are more thoroughly aquatic than any other existing birds, their wings having been converted into paddles of great efficiency, though entirely useless for purposes of flight. The group is characteristic of the Southern Hemisphere, and practically limited to the shores of the Antarctic Ocean. The species represented is the Crested Penguin or Rock-hopper (Eudyptes chrysocome), which ranges from the Falkland Islands to New Zealand. It is a handsome black-and-white bird, with an orangecoloured crest on either side of the head. Like so many sea-birds the Rock-hoppers are social in habit. Their favourite breedinggrounds are boulder-strewn slopes at some little distance from the sea, and near fresh water, in which they are fond of bathing. The nest is often a mere hollow scratched in the earth, though stems and leaves may be roughly drawn together to form it. The Rockhopper, like Penguins generally, broods over its eggs in an erect or semi-erect position, bringing them into contact with a bare patch on the under side of the body, an arrangement which secures a maximum of heat.