remarkable degree (e.g. stimulated by exercise, the muscles and glands increase in size and in vigour: under intermittent pressure the skin thickens and hardens; as a consequence of the deep breathing caused by a venous condition of the blood, the lungs expand); but while the power of varying is clearly transmissible, there is nothing to show that the acquired variations themselves, which result from the exercise of it, are at all transmissible, whereas there is a mass of negative evidence tending to prove the converse; for instance, millions of people acquire callosities in their hands and feet, but none ever transmit them to their offspring—only the power to acquire the callosities anew is transmitted. Now this vitally important power, which can have arisen only by the accumulation of inborn variations (for instance, the accumulation of acquired variations can have had nothing to do with the evolution of the power the organism possesses of repairing a cut or abraded surface), is so great that, in a high multicellular adult organism such as a man, it is impossible at first sight to say what part of the whole development is due to the accumulation of inborn variations, and what to individually acquired and non-transmissible variations. For instance, since the muscles may be increased considerably in size if exercised, and may practically disappear if not exercised, we cannot say at first sight how much of the muscular development of a man is due to direct inheritance, and how much to exercise—i.e. to the power of varying to suit the circumstances, of developing under appropriate stimulation.
If the limb of an infant be paralyzed or so injured that the joints are locked, then, in consequence of the lack of stimulation, it (the limb) grows very little afterwards; muscles, bones, ligaments, blood-vessels, &c. all remain dwarfed and undeveloped. Again, if an adult limb be paralyzed, its component structures, no longer maintained by the stimulation of use, atrophy in their
