WHITEFISH OF THE LAKE OF GENEVA 



developed in the bosom of the waters and dispersed 

 among this dust of floating creatures. 



Their floating nature compels creatures like this to 

 submit themselves to the various conditions of the 

 medium which contains and supports them. The 

 slightest current carries them along, so they prefer 

 waters which are normally calm and relatively slow- 

 moving. Variations of temperature affect them; they 

 are more numer- 

 ous in spring and 

 summer than in 

 autumn, and 

 especially in 

 winter. Light 

 affects them un- 

 favourably, and 

 many of them try 

 to avoid it. They 

 hide themselves in 

 the hidden depth 

 of the lake 300 or 

 600 feet, some- 

 times even more, 

 below the surface, 

 for the Lake of 

 Geneva is over Fig 

 1000 feet deep in 

 places. They go 

 down during the brightest period of noon, when the 

 light is very strong upon the surface waters. At night 

 they come up again, and swim in waters they will leave 

 a few hours later. These movements are constant and 

 controlled; and they, in turn, govern the existence 

 of the fish which exploit these little creatures, which 

 are obliged to follow them in order to keep contact 

 with the prey which they make their food. 



But these prey, in turn, have to keep themselves 

 supplied with food. Hunted down by the fish whose 

 179 



34. — Leptodora kindti Focke, planktonic 

 crustacean of the order Cladocera. The 

 length is about half an inch. 



