D IS TRIB U TION. 2 1 7 



pathetic coloring and adaptation to the transparent clear- 

 ness of the water. 



Distribution of Fresh-water Animals. In fresh 

 water two groups of animals must be distinguished, of 

 which the one comprises mainly the more highly organized 

 forms, the mollusks, fishes, and higher crabs, the other the 

 lower animal world. The mode of distribution of the 

 former is mainly determined by the same factors which are 

 at work in the separation of terrestrial forms ; the mode of 

 distribution of the latter, however, is a cosmopolitan one. 

 The same infusorians and rhisopods, copcpods, fresh-water 

 sponges and polyps which occur in America seem to be 

 distributed over the entire earth. This is connected with 

 the fact that all these animals have resting conditions 

 in which they endure desiccation. The resting stage, be it 

 as a hard-shelled egg or as an entirely encapsuled animal, 

 may be borne about by the wind as dust, or may be car- 

 ried with the mud by aquatic birds, and upon again reach- 

 ing the water the animal resumes its active state. 



