The habit of coming together for dancing and of decorating the places where the birds habitually perform their dances is, of course, well known from the pages that Darwin gave to this subject in The Descent of Man (ch. xiii).  Visitors of the London Zoological Gardens also know the bower of the satin bower-bird.  But this habit of dancing seems to be much more widely spread than was formerly believed, and Mr. W. Hudson gives in his master-work on La Plata the most interesting description, which must be read in the original, of complicated dances, performed by quite a number of birds:  rails, jacanas, lapwings, and so on.

The habit of singing in concert, which exists in several species of birds, belongs to the same category of social instincts.  It is most strikingly developed with the chakar (Chauna chavarris), to which the English have given the most unimaginative misnomer of “crested screamer.”  These birds sometimes assemble in immense flocks, and in such cases they frequently sing all in concert.  W.H.  Hudson found them once in countless numbers, ranged all round a pampas lake in well-defined flocks, of about 500 birds in each flock.

“Presently,” he writes, “one flock near me began singing, and continued their powerful chant for three or four minutes; when they ceased the next flock took up the strains, and after it the next, and so on, until once more the notes of the flocks on the opposite shore came floating strong and clear across the water—­ then passed away, growing fainter and fainter, until once more the sound approached me travelling round to my side again.”

On another occasion the same writer saw a whole plain covered with an endless flock of chakars, not in close order, but scattered in pairs and small groups.  About nine o’clock in the evening, “suddenly the entire multitude of birds covering the marsh for miles around burst forth in a tremendous evening song....  It was a concert well worth riding a hundred miles to hear."(27) It may be added that like all sociable animals, the chakar easily becomes tame and grows very attached to man.”  They are mild-tempered birds, and very rarely quarrel”—­we are told—­ although they are well provided with formidable weapons.  Life in societies renders these weapons useless.

That life in societies is the most powerful weapon in the struggle for life, taken in its widest sense, has been illustrated by several examples on the foregoing pages, and could be illustrated by any amount of evidence, if further evidence were required.  Life in societies enables the feeblest insects, the feeblest birds, and the feeblest mammals to resist, or to protect themselves from, the most terrible birds and beasts of prey; it permits longevity; it enables the species to rear its progeny with the least waste of energy and to maintain its numbers albeit a very slow birth-rate; it enables the gregarious animals to migrate in search of new abodes.  Therefore, while fully admitting