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 aU aquatic animals can be divided into 

 marine and fresh-water types, for this would exclude 

 forms hke 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 ia 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 aU 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 



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