‘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- 
