THE FRESH WATERS 143 



ticular form is tending whether it is becoming 

 more of a fresh-water animal or less ? The 

 process of change in an animal race may go on 

 so very slowly that at a given point we cannot 

 detect it at all. But that is not to say that it 

 is not taking place. It has been said that if a 

 clock could be invented that would go so slowly 

 that it would only tick once in thirty years, we 

 should not believe that it was going at all. Yet 

 even that rate is fast compared with the rate at 

 which Nature works out some of her wonderful 

 changes. 



But though we may not hope to detect Nature 

 actually at work, there are various ways by 

 which those who study her closely can trace out 

 some of the changes that have taken, and are 

 still taking, place. One of these is by compar- 

 ing one kind of animal with another closely 

 related to it, and trying to make out the meaning 

 of the differences between them. Sometimes so 

 many kinds of animals, with only slight differ- 

 ences between each kind, are found that they 

 can be arranged in a regular series, and it is 

 possible to be fairly certain of the path along 

 which the race has travelled. 



Another way is by studying the growth of a 

 particular animal from the time that it begins to 



