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 



