CONDITIONS OF EXISTENCE 27 



reference to certain stones or pebbles under or near which they live 

 (e.g., mayfly nymphs) and thus work out simple homing paths. 



As has been stated, in streams the rate of flow is determined by 

 volume of water and slope of stream bed. In a comparatively 

 straight stream the current is swiftest in the center at the top and 

 least swift at the sides near the bottom; the center of the stream 

 bed has a current intermediate between the two. Thus sluggish 

 portions of streams like the Fox River (Illinois) may be swift 

 enough at the bottom of the center to support some swift stream 

 animals such as Hydropsyche and Heptageninae. There are back 

 eddies about stones and other obstructions so that currents in 

 streams are somewhat irregular. 



In lakes circulation is determined by wind and differences in 

 temperature. A lake which is equal in temperature throughout 

 has a complete circulation (Fig. 4 A). The wind indicated by the 

 arrow (W) tends to pile the water up on one side. To compensate 



FIG. 4. 



The circulation of the water (A) in a lake of equal temperature; (5) in a lake of unequal tempera- 

 ture. W represents the direction of the wind; E, epilimnion; T, thermocline; H, hypolimnion. 

 (After Birge.) 



for this currents are started downward along the shore and a cir- 

 culation across the bottom and upward on the other side is initiated. 

 Very shallow lakes and deeper lakes in the cold months of the year 

 have a complete circulation. Lakes of unequal temperature are 

 very different. For example a deep lake has a uniform tempera- 

 ture for a time in the spring just after the ice melts, complete cir- 

 culation takes place and the bottom waters are aerated. As the 



