THE HAUNTS OF LIFE 115 



it is also very important for small animals. A diagrammatic 

 instance may be found in the occurrence of a freshwater 

 sponge in a pond in the middle of the sandy Sable Island 

 which lies out in the Atlantic, a hundred miles from Nova 

 Scotia. 



From Land or Air back to Water. — There is a 

 certain contingent of the freshwater fauna that has arisen 

 by a sort of turning back of terrestrial and aerial forms. 

 Just as whales and dolphins are in all probabiUty the 

 descendants of terrestrial mammals which took secondarily 

 to the sea, so some freshwater animals, such as aquatic 

 insects, the water-shrew and the water-vole, the otter and 

 the beaver, are doubtless the descendants of terrestrial forms. 



In this connexion it may be noted that many water 

 animals are not so much wetted as one might think. In 

 some water-beetles, such as the whirligig {Oyrinus) and the 

 water boatman {Notonecta), the body is very partially 

 wetted. In the water-spider {Argyroneta) considerable 

 areas of the hairy body refuse to become wet. In the family 

 of Hydrophilid beetles, some hardly wet at all, some keep 

 considerable parts of the body dry, and some become wholly 

 wet. The wetting or not wetting depends on capillary 

 phenomena, which depend on the structure of the surface 

 of the body and its hairs or setae. There can be little 

 doubt that the differences are finely adaptive to slight 

 differences in habit. 



The Water- Spider. — ^In illustration of the interesting 

 habits of freshwater animals we may take the case of 

 the water-spider, Argyroneta naians, of which Dr. Wagner 

 has made a fine study. It is remarkable as an air-breather 

 which spends most of its fife under water, and it is remark- 

 able among spiders inasmuch as the male is much larger 



