38 The Life Story of the Fish 



experimental biology, the principal problem is how to put a 

 question to an animal in such a way that it can answer it. 

 There is no use in asking an animal what it sees, because it 

 cannot tell you, but you can ask what it is going to do about 

 what it sees, and it can answer you by its actions. 



Employing this well-known method, this scientist made 

 use of the fact, learned in his previous work, that in many 

 fish the color change takes a long time to complete. There is 

 a first, immediate change of small extent in the direction of 

 the new color, and then the rest of the adaptation proceeds 

 quite slowly. The explanation appears to be that some of the 

 chromatophore machinery is less sensitive and therefore 

 slower-acting than the rest, and also that there is a change 

 in the amount of permanent, or semipermanent, pigmenta- 

 tion. A fish kept for a long time on a white background loses 

 much of this pigmentation, and when it is moved to a black 

 background it takes some time to produce anew enough of 

 this semipermanent pigment to bring it back to its darkest 

 possible shade. 



A certain small killifish called Gambusia affinis, better 

 know as the "mosquito fish," a common and easily handled 

 species, was found to operate its color-changing machinery 

 along the desired lines. Fish kept in a black tank became 

 dark, fish kept in a white tank became pale. If now the pale 

 fish were placed in the black tank, they at once became appre- 

 ciably darker than they had been, but they remained for 

 some time very much lighter than the old-time residents of 

 that tank. 



The prey having been found, the next step was to find 

 the predator. Predator means in general a bird or beast of 

 prey, but it is used more specifically to refer to the particular 

 animal which preys on a particular species. The first predator 

 tried was a little Galapagos penguin which happened to be 

 living in a near-by zoo. Quantities of mosquito fish were 



