THE FRESH WATERS 143 



ticular form is tending whether it is be- 

 coming 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 com- 

 pared with the rate at which Nature works 

 out some of her wonderful changes. 



But though we may not hope to detect Na- 

 ture 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 

 comparing 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 differences 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 be- 



