FRESH WATER ANIMALS 211 



is the bark of a cherry tree in my back yard. The water in 

 the vases holding flowers if it stands long enough will often 

 show pecuKar creatures not so common elsewhere. Dead grass 

 or leaves from under snow in winter will surprise you with its 

 productivity. In the summer if you place the cultures out of 

 doors the insects hnd them, and young mosquitoes very soon 

 appear. 



The most impressive lesson that there is in the whole field 

 of nature study is to be learned from hay and water. In such 

 a culture we see on a small scale an epitome of the hideous 

 struggle for existence of each animal type at the expense of all 

 competitors; while at the same time we are forced to an ap- 

 preciation of the fact that wherever a food reserve is brought 

 together some animal type is ready to consume it. Keep 

 the culture for a few weeks. You will find the balance changes 

 constantly. Some types wane and disappear, others become 

 abundant, different conditions favor different types. 



One other group of animals characteristic of fresh waters 

 should be mentioned, consisting of those vertebrates that breed 

 or lay their eggs on land, but feed in water. Chief among 

 these are the crocodiles, caymans, gavials and alligators, such 

 striking features of warm lakes and rivers, which in fresh water 

 largely take the place of sharks. But it should be noticed that 

 one of our crocodiles is marine, or at least chiefly so, as is the 

 largest of the crocodiles, inhabiting the lands about the Indian 

 Ocean and the Malayan and north Australian region. River 

 and pond turtles are common almost everywhere, and some are 

 very large, the alligator turtle of the Mississippi reaching 140 

 pounds in weight. Another type of turtle is marine. There 

 are numerous more or less aquatic snakes and lizards which 

 eat fish and frogs. One group of snakes and a single lizard are 

 marine; of the former, which are viviparous, all but one or two 

 are helpless out of water. The hippopotamus occurs only in 

 fresh water. 



Several animal types chiefly represented in the sea occur also 

 in fresh water. In some of the great rivers of South America, 



