The Senses and the Nervous System 93 



These arguments are not without logic, but they are the- 

 oretical. They are based on differences in the structure of 

 fish and human visual machinery, and such a base is not 

 always a safe one. Rather than reply theoretically, with the 

 same lack of safety, let us see what factual evidence we can 

 find on the other side. Controlled experiments being few in 

 number and limited in scope, we shall fall back on observa- 

 tion. What can- we tell about how much fish see from what 

 they do in the course of their daily lives? 



Star witness for the defense is Toxotes jaculator. This is 

 a little fish which lives in Siam. It earns its living by knock- 

 ing insects off twigs above the surface with a drop of water 

 which it shoots from its mouth. How it does this is an in- 

 teresting matter for speculation, but we are trying to shun 

 theory now. The fact is that it does succeed in shooting down 

 insects as much as three feet above the surface. This, it 

 seems to me, is good proof of three things: that Toxotes 

 sees objects in the air with considerable distinctness, or he 

 could not tell the insect from the twigj that he has an 

 excellent sense of direction 5 and that his judgment of dis- 

 tance is not at all bad. 



So much for what the fish can see in the air. Now for some 

 light on what he can see under water. The witness with 

 which I happen to be best acquainted is the jewel-fish al- 

 ready mentioned in Chapter III. This fish is unpopular with 

 tropical-fish fanciers on account of his bad temper, but 

 popular with seekers after information because he lives and 

 breeds unconcernedly in an aquarium and can thus be easily 

 observed. He is among the few fishes which feel some re- 

 sponsibility about their children. He has several hundred at 

 a time, each less than one-quarter of an inch long when they 

 begin to swim, and he and his mate ride herd over their 

 flock, driving off intruders, and picking up any stragglers in 

 their mouths and spitting them out unharmed into the mid- 



