THE ANIMALS OF LAKES AND RIVERS 183 
ing that all the latter live only in the ocean, and that 
lakes and rivers contain only the former; nor can we 
assume that all aquatic animals can be divided into 
marine and fresh-water types, for this would exclude 
forms like the brine-shrimps, characteristic of salt 
lakes and brine pools. But, at the same time, it is 
convenient to recognize a general distinction between 
the animals of the oceans, the marine forms, and those 
characteristic of lakes and rivers, which are for the 
most part fresh-water animals. 
Using the terms in this general sense, we may note 
that the poverty of the fresh-water fauna is due to the 
relatively unfavourable conditions. Among these un- 
favourable conditions we probably need not include 
the absence of considerable amounts of salt dissolved 
in the water, for the presence of an appreciable amount 
of salt, e.g. in the Caspian, does not give rise to a 
marked increase in the fauna as compared with fresh- 
water lakes, and the presence of a large amount of 
salt, e.g. in the Great Salt Lake, is distinctly inimical 
to animal life. 
Before proceeding to discuss the especially unfavour- 
able conditions, we may note that it is generally 
believed that life originated in the sea, not in fresh 
water, and that therefore all the animals now found 
in rivers and lakes must have originated in one of two 
ways: (1) they may have been derived directly from 
marine forms, or (2) they may have been derived from 
land animals which have reacquired the aquatic habit 
of their remote ancestors. 
Of the first group, fresh-water fishes may be taken 
as examples. They have certainly had marine ances- 
tors, and in some cases, e.g. the eel and the salmon, 
the existing forms have retained the power of living in 
