DISTRIBUTION AND ADJUSTMENT 109 



fauna, are repelled by strong light. Such is the case with the 

 earthworm. An electric flash light will reveal in the garden or 

 forest at night many earthworms, their anterior ends projecting 

 from the burrows in search of decayed leaves, their chief article 

 of diet. The bright light causes their instant withdrawal. The 

 sow bug, woods cockroach, bark beetles, wood borers, slugs, and 

 many spiders prefer the dark or the dim light and are to be found 

 by day only in their retreats under bark or stones or in similar 

 situations. 



There is a whole host of water forms that, while minute, are 

 the chief source of fish food. They are so sensitive to light that 

 they migrate from the deeper waters to the surface and back 

 again as the Hght wanes or brightens. There are protozoa, 

 rotifers, and many tiny crustaceans in this association, together 

 with a variety of microscopic plants on which they feed. Alto- 

 gether this assemblage of living things, a floating or free swim- 

 ming population, is known as plankton. In Turkey Lake, Indiana, 

 at the time of its maximum, it constitutes more than one-half 

 per cent by volume of the water; in Lake Michigan scarcely a 

 thousandth as much. Such a density as that in Turkey Lake 

 means more than fifty million organisms to the quart. Kofoid , 

 found in the Illinois River at the State Biological Station about 

 five millions per quart, a million of which were animals. Since 

 tiny fish feed on these organisms they follow the plankton in its 

 diurnal migration, and the larger fish that are carnivorous go along 

 too. Any fisherman knows that trolling for bass is best in the 

 early morning or late afternoon, and that luck is better on a 

 cloudy day than a bright one. The plankton and the small 

 fry are near the surface in the dim light, and so, too, are the big 

 fellows that feed upon them and that may by mistake try the 

 fisherman's lure. 



Nesting sites and nest-building materials are factors that 

 help to determine animal distribution. The muskrat and his 

 northern relative, the beaver, are found along the streams, not 

 alone because his food is here abundant, but also because the 



