i88 THE BOOK OF NATURE STUDY 



(a) To explain the uniformity, Darwin referred to the way in 

 which birds and winds carry small creatures from one watershed 

 to another, and also to the fact that slight changes of land level 

 would often serve to bring different river systems into connection. 

 (b) When we take a case like that of the sponges, which are alto- 

 gether marine except one small family (Spongillidae), we are led 

 to infer that some of the freshwater forms must have gradually 

 migrated (actively or passively) from the sea, through brackish 

 water, into the fresh water. As the power of making and sur- 

 viving the transition depends on the constitution of the animal, 

 it is not surprising to find similar forms in different areas, (c) It 

 seems that some lakes are dwindling remnants of ancient seas. 

 If there was a fairly uniform pelagic fauna in these seas, e.g. before 

 cretaceous times, the conversion of the seas into lakes would have 

 similar sifting effects, hence similar survivals in different countries. 



In a lake we can distinguish littoral, surface, and deep-water 

 forms, just as in the sea. Near the shore we may find such 

 animals as freshwater mussels and smaller bivalves, freshwater 

 snails, numerous Crustaceans, water-beetles and other insects, 

 leeches and distant relatives of the earthworm, planarian worms, 

 the freshwater hydra, the freshwater sponge, and many Protozoa. 

 The surface fauna is quite different, and has a marked analogy 

 with the marine plankton ; it includes swarms of delicately 

 built water-fleas and Rotifers, and numerous Infusorians. The 

 deep-water lacustrine fauna has probably been derived from 

 the shore stocks, certain types having gradually followed the 

 sinking food material farther and farther into the depths. In the 

 depths of lakes we find Rhizopods related to the common Amoeba, 

 planarian worms, thread-worms, leeches, bristle-footed worms 

 (Chaetopods), numerous Crustaceans (Amphipods, Isopods, and 

 " water-fleas "), some water-mites, some insect larvae, various 

 molluscs, and so on. 



An account of many of the common freshwater animals will 

 be found in the chapter on Aquaria. All that we propose to do 

 here is to make a few suggestions in regard to possible ex- 

 cursions. 



Ponds. If a class excursion to a pond is to be successful 



