The Problem of Adaptation 13 
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 circum- 
stances, 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. 
The same phenomenon has been observed in the case of 
other poisons, not only in case of other alkaloids, such as 
morphine and cocaine, but also in the case of caffein, alcohol, 
and even arsenic. There is a curious phenomenon in regard 
to arsenic, which appears to be well established, viz., that a 
person who has gradually increased the dose to an amount 
great enough to kill ten ordinary men, will die if he sud- 
denly ceases altogether to take arsenic. He can, however, be 
gradually brought back to a condition in which arsenic is not 
necessary for his existence, if the dose is gradually decreased. 
It is a curious case of adaptation that we meet with here, 
since the man becomes so thoroughly adjusted to a poison 
