More permanent adaptive changes than these also take place as the result of prolonged use of certain parts. If the muscles work against powerful resistance, they become larger after several days or weeks, and are capable of doing more work than at first. Conversely, when any group of muscles is not used, it becomes smaller than the normal and capable of doing less work. It would be a nice point to decide whether this latter change is also an adaptation. If so it is one in a somewhat different sense from that usually employed. The result is of no direct advantage to the animal, except possibly in saving a certain amount of food, but since the same change will take place when an abundance of food is consumed, the result is, under these conditions, of no use.
The thickening of the skin on those parts of the body where continued pressure is brought to bear on it is a change in a useful direction. The thickening on the soles of the feet and on the palms of the hands is a case in point. Not only is the skin thicker at birth in these parts, but it becomes thicker through use. In other parts of the body also, the skin hardens and becomes thicker if pressure is brought to bear on it. We may regard this as a general property of the skin, which is present even in those parts where, under ordinary circumstances, it can rarely or never be brought into use.
Even as complicated and as much used an organ as the eye can become adaptively improved. It is said that the lateral region of the field of vision can be trained to perceive more accurately; and every one who has used a microscope is familiar with the fact that if one eye is habitually used it becomes capable of seeing more distinctly and better than the other eye. This seems to be due, in part at least, to the greater contraction of the iris.
Another phenomenon, which, I think, must be looked upon as an adaptation, is the immunity to certain poisons that can be gradually brought about by slowly increasing the amount introduced into the body. Nicotine is a most virulent poison, and yet by slowly increasing the dose an animal can be brought into a condition in which an amount of nicotine, fatal to an ordinary individual, can be administered without any ill effects at all resulting.