246 BIOLOGY AND HUMAN LIFE 



selves— people who wear different kinds of clothes, who attend 

 a different church, or who speak with a different dialect or a 

 different accent; or we may have the habit of feeling kindly 

 toward strangers. Our feeling habits show themselves in the 

 attitudes that we assume in various kinds of situations. 



Our habits become so fixed and constant that they may be 

 relied upon under nearly all circumstances. This is what is 

 meant by character: it is the whole combination of habits of 

 feeling and thinking and doing which distinguishes one person 

 from another, or a mature person from a child. We differ very 

 much from one another in thinking power, in strength of mus- 

 cles, in endurance, and in depth of feeling ; but all can acquire 

 habits to make a character that can be depended upon, to the 

 extent of our abihties, in all emergencies. 



4. Physiological habits. A different class of habits is illus- 

 trated by an experiment performed on rabbits^ome years ago. 

 A very small quantity of arsenic, which is a violent poison for 

 all kinds of protoplasm, will kill a person or a rabbit.^ In this 

 experiment arsenic was given to rabbits in a fraction of the 

 amount that would ordinarily kill them. After a few days the 

 rabbits were given a little more. The dose was gradually in- 

 creased until the animals had become so accustomed to the 

 poison that they could stand several times the ordinary fatal 

 dose. The arsenic acts upon the protoplasm of the nerves and 

 muscles in such a way as to put the animal in a state of tonus, 

 that is, the way one feels when one is "all on edge," ready to 

 jump or scream on the slightest provocation. The rabbits thus 

 treated became extremely sensitive to the least disturbance; 

 they would jump on hearing the faintest sound or on the pass- 

 ing of a shadow. But after the animals had been treated with 

 the poison in this way for a considerable time, it was impossible 

 for them to live without it. If the drug was omitted from their 

 daily rations, they quickly died. 



Instead of establishing new nerve connections the arsenic 

 feeding established a new condition of the nerves. This illus- 



1 Strangely enough, a child can stand larger doses of arsenic than an adult. 



