Carpenter, in his physiology, reports the discovery, which we owe to German investigation, "that the whole structure originates in a single cell; that this cell gives birth to others, analogous to itself, and these again to many future generations; and that all the varied tissues of the animal body are developed from cells."[9] A more recent writer adds, "In the higher animals and plants, we are presented with structures which may be regarded as essentially aggregates of cells; and there is now a physiological division of labor, some of the cells being concerned with the nutriment of the organism, whilst others are set apart, and dedicated to the function of reproduction. Every cell in such an aggregate leads a life, which, in a certain limited sense, may be said to be independent; and each discharges its own function in the general economy. Each cell has a period of development, growth, and active life, and each ultimately perishes; the life of the organism not only not depending upon the life of its elemental factors, but actually being kept up by their constant destruction and as constant renewal."[10] Growth, health, and disease are cellular manifestations. With every act of life, the movement of a finger, the pulsation of a heart, the uttering of a word, the coining of a thought, the thrill of an emotion, there is the destruction of a certain number of cells. Their destruction evolves or sets free the force that we recognize as movement, speech, thought, and emotion. The number of cells destroyed depends upon the intensity and duration of the effort that correlates their destruction. When a blacksmith wields a hammer for an hour, he uses up the number of cells necessary to yield that amount of muscular force. When a girl studies Latin for an hour, she uses up the number of brain-cells necessary to yield that amount of intellectual force. As fast as one cell is destroyed, another is generated. The death of one is followed instantly by the birth of its successor. This continual process of cellular death and birth, the income and outgo of cells, that follow each other like the waves of the sea, each different yet each the same, is metamorphosis of tissue. This is life. It corresponds very nearly to Bichat's definition that, "life is organization in action." The finer sense of Shakspeare dictated a truer definition than the science of the French physiologist,-- "What's yet in this That bears the name of life? Yet in this life Lie hid more thousand deaths." _Measure for Measure_, Act iii. Scene 1. No physical or psychical act is possible without this change. It is a process of continual waste and repair. Subject to its inevitable power, the organization is continually wasting away and continually being repaired.