Page:Popular Science Monthly Volume 52.djvu/452

434 specialization which arises in the higher or many-celled animals, certain alliances of cells or tissues are set apart for respiration alone, and certain others for digestion, while other functions of animal life are relegated to still other cell alliances. Each organ in turn is released from all functions except its own.

Irritability, or the response to external stimulus, is an attribute of all living organisms. In the method and degree of response variations occur. These variations favorable to the division of labor and the adaptation of the animal to its surroundings are seized and fixed by natural selection. In this way, on the basis of a diffused function, an organ is built up and the organ itself is specialized and perfected.

The mind and consciousness of man grow out from the irritability of the lower animals. They are developed through series of successive differentiations and integrations. All the higher animals are colonies of co-operating and co-ordinated cells. In such colonies of units the functions of sensation, thought, and motion are relegated to series of the most sensitive and most highly organized cells. This alliance of cells is adequate for the work it has to perform. The brain is always adequate for the mind, for the one is the organ, the other the function, and the development of the two must go on together.

The intellect of man can not be regarded as the crowning marvel of the "great riddles of life." A marvel is no greater for its bigness. Life is one continuous marvel, without break or end. The human mind is one of life's manifestations. The marvel appears in great or small psychic powers alike, for the great powers of the many-celled brain are produced by the co-operation and specialization of the small powers of the single cell. Nature knows neither great nor small. "God works finer with his hands than man can see with his eyes." The single cell is far from simple. The egg or germ cell carries within itself the whole machinery as well as the whole mystery of heredity. The simplest organism we know is far more complex than the Constitution of the United States. Its adjustments, checks, and balances are more perfect. It should in its changing relations be compared rather with the great unwritten constitution of civilized society. The laws of society spring from the laws governing the development of the single cell. If we knew the latter "all in all," as Tennyson says of the flower, "we should know what God is and man is."

If we could know all of any life problem to its uttermost detail, we should have the clew to all life.

Among the protozoa, as already stated, all activities are centered in the single cell which forms the animal unit. Each cell is