To bring, however, this part of my observations to a close, I will take leave of the nervous system, and glance hurriedly at other phases of organised matter, where we meet with the same powers of reproduction, but in simpler guise. Daily experience teaches us that a muscle becomes the stronger the more we use it. The muscular fibre, which in the first instance may have answered but feebly to the stimulus conducted to it by the motor nerve, does so with the greater energy the more often it is stimulated, provided, of course, that reasonable times are allowed for repose. After each individual action it becomes more capable, more disposed towards the same kind of work, and has a greater aptitude for repetition of the same organic processes. It gains also in weight, for it assimilates more matter than when constantly at rest. We have here, in its simplest form, and in a phase which comes home most closely to the comprehension of the physicist, the same power of reproduction which we encountered when we were dealing with nerve substance, but under such far more complicated conditions. And what is known thus certainly from muscle substance holds good with greater or less plainness for all our organs. More especially may we note the fact, that after increased use, alternated with times of repose, there accrues to the organ in all animal economy an increased power of execution with an increased power of assimilation and a gain in size. This gain in size consists not only in the enlargement of the individual cells or fibres of which the organ is composed, but in the multiplication of their number; for when cells have grown to a certain size they give rise to others, which inherit more or less completely the qualities of those from which they came, and therefore appear to be repetitions of the same cell. This growth, and multiplication of cells is only a special phase of those manifold functions which characterise organised matter, and which consist not only in what goes on within the cell substance as alterations or undulatory movement of the molecular disposition, but also in that which becomes visible outside the cells as change of shape, enlargement, or subdivision. Reproduction of performance, therefore, manifests itself to us as reproduction of the cells themselves, as may be seen most plainly in the case of plants, whose chief work consists in growth, whereas with animal organism other faculties greatly preponderate.