In this is another side of the proof of our mental collectivity. The poet feels for humanity, the student studies for humanity; the discoverer, inventor, all work for humanity. (This does not refer to the pay they expect, and their attitude toward it, but to the work itself.) All through our history we see the great-brained men who thought for the world, moved by a quenchless impulse to transmit this thought to the others, to pour out into the common stock the product of their brains. This they did because they _must_—even when loss and injury, ostracism or martyrdom followed. It is the compelling functional necessity of the brain to discharge into other brains, as well as to seek from them its vast and varied stimulus. In more immediate and commonplace instances we see the same law. The difficulty of “keeping a secret,” _i. e._, of voluntarily retaining stimulus; the necessity of “relieving one’s mind”—a perfectly fit phrase, as much so as its familiar physiological analogue; the value of the confessional; and, commonest of all, the vivid interest of each human brain in the affairs of the others; all these show the collective nature of that organ. The most ordinary woman, gossiping with her neighbours, manifests this social necessity for contact and exchange, however low. “Mind your own business!” we cry, and cry in vain. No brain advanced enough to be called human can possibly find full use and exercise in contemplation of one person’s business. It must concern itself in the business of the others, their common business. The human brain is a social organ. Human thought is a social function. Approach this fact along lines of evolution. The brain, like all other organs, is called for by conditions and developed by exercise. Simple conditions, simple exertions—low brain. Enlarge and elaborate the conditions—increase the exercises—and the brain develops. Observe here, within human history, how we have developed the brain of the dog by such change of condition and action. In every form of animal life you find an exact relation between the range of activities of the creature and his degree of brain development. This is necessarily so, as the increase of activities is what produced that degree of development. The simple activities of the clam need no brain, and have none. The complex activities of the fox need a complex brain, and have it. Everywhere this exact proportion is found until you reach the human animal.