We now come to a third, and in itself a fully sufficient proof of the organic nature of society—not of the social organism as a useful figure, an illustration, an analogy, but as a literal biological fact. Here are a number of separate animal bodies. Each is a group of interactive organs, each does business for itself with no need of combination with another, save in the temporary union of sex with sex, and of mother with child. These creatures are individuals. Here again is a number of apparently separate animal bodies. But each has in his head an organ _which cannot perform its functions alone_; an organ which for its healthful use requires contact and exchange with similar organs lodged in other bodies. This organ is the brain. That degree of brain development which we call “human” is only found in creatures socially related; it is not individual brain power, but social. The human brain, for health and usefulness, for its normal life, requires a number of human beings with whom to feel, think, and act. We can, it is true, physically isolate a human animal, and maintain his animal life; but his human life—_i. e._, social life; his “feelings” and “thoughts,” the whole field of brain activity—is injured. The human brain is _the_ social organ; it is our medium of contact and exchange. Set a man in absolute solitude and his brain is affected at once. Cut off from the contact which enables it to freely receive and discharge its supply of social energy, its action becomes increasingly morbid. In proportion to the completeness and duration of the isolation the brain is injured, and ultimately ruined. We know the effect of solitary imprisonment, or of being cast away alone on some remote island. Short of this we know the progressive effect of degrees of isolation. The lighthouse keeper knows—they put two men in lighthouses most removed from social touch; and even that is a dangerously “short circuit” for the social organ to act in. The solitary shepherd knows, on the wide waste plains of Australia or Texas. The hermit or recluse of any age, the separate dwellers in old houses in the country, any human creature who lives alone, is injuriously affected in brain action. This is not saying that mere privacy is harmful—that is a necessity for the social brain; such temporary solitude as shall enable it to work out its special contribution to our common thought, and to rest from the forceful social currents. But however solitary the student or author, the product of his labour is for others, and must reach them; his brain must connect with the others, though at long range.