MUD-SKIPPERS 379 



danger, and that they clearly adopt the best course 

 for escaping it by immediate flight to the deeper 

 parts of the stream. But, in places where no 

 fishermen are present, no special danger attends 

 farther progress up the sloping banks of mud, a 

 course which must be most effectual as a means 

 of avoiding all inconvenience from the temporary 

 disturbance of the water, and here we find the 

 fish almost all running upwards. The differences 

 in the behaviour of the fish in connection with the 

 differences of environment are very striking, and 

 at first sight might be taken to imply the exercise 

 of highly evolved intelligence. They are, however, 

 probably merely the outcome of processes of natural 

 selection. 



It is probable that, from the outset, there was 

 a dislike to the disturbance attending any con- 

 siderable agitation of the surface of the water, and 

 a corresponding tendency to try to avoid it, but 

 that originally the line of flight was unspecialised 

 and directed indifferently either upwards or down- 

 wards over the surface of the banks. But, in places 

 where no special danger attends an upward course, 

 those fish who naturally tended to follow it 

 would certainly be more likely to escape injury 

 than those who descended and thus ran a risk of 

 being knocked about by the waves. In consequence 

 of this, the former class of fish would almost 

 inevitably come to predominate, and in course 



