CHAPTER XXV 

 STIMULANTS, NARCOTICS, AND POISONS 



161. "Getting used." There are many kinds of fish that 

 live in salt water only, and there are many kinds that live in 

 fresh water only. There are some species, however, that can 

 be made to live in either salt or fresh water. Still, if we took 

 one of these fish out of the ocean and placed it in fresh water, 

 it would soon die. Or if we took a live one from fresh water 

 and put it into salt water, it would soon die. But if we slowly 

 increased the amount of salt in the fresh water, we could gradu- 

 ally bring the water to the composition of the ocean, and the 

 fish would remain alive. In the same way we could gradually 

 add fresh water to a tank of sea water, until there was a very 

 small proportion of salt in the mixture, then transfer our fish 

 to fresh water, and it would remain alive. 



In a case of this kind we say that the animal " gets used " 

 to living in the new conditions. This illustrates a pretty gen- 

 eral fact about protoplasm, or about living things. It is possible 

 for living things to get used to new conditions of temperature, 

 or of light, or of chemicals, or of food. This does not mean 

 that every living thing can come to live in any kind of sur- 

 roundings whatever ; we know that is not true. We know that 

 birds cannot get used to living in water, or that fish cannot get 

 used to living in the air ; we know that plants and animals cannot 

 get used to living without proteins or without salts. We under- 

 stand simply that we can change our conditions of living to a 

 certain degree or in certain directions, and still remain alive. 



Arsenic is a violent poison for all kinds of protoplasm. 

 It is used for killing animals as well as plants, as in fighting 

 many kinds of insects and many kinds of fungi. A very small 



