174 



FRESHWATER FAUNA 



the colder zones. With the increase of temperature the 

 populousness of the fresh waters increases, but is still 

 limited in the sub-tropical zone by partial desiccation. 

 In the tropical zone the conditions of temperature of the 



iv v \T vn vm cv x xi xn. i n ui iv 



FIG. 50. Diagram to show the increase and diminution of 

 the Freshwater Planctone in the course of the year. 

 The months are indicated by Roman numerals. Curve 

 a a, given by Apstein for the Dobersdorf lake. In April, 

 at the beginning of the curve, a column of water 1 square 

 metre in section and 20 metres high contained 530 cubic 

 centimetres of planctone; in October, near the culmination 

 of the curve, 3,136 cubic centimetres. Curve b &, con- 

 structed from data given by Zacharias for the Ploen lake. 

 In April, 43 cubic centimetres, and in August, 509 cubic 

 centimetres of planctone were observed in 40 cubic metres 

 of water. 



fresh waters approach most nearly to those of the sea, 

 and with them their populousness." * 

 The effect of a temperate climate is well illustrated 



* " On the Occurrence of Marine Animal Forms in Fresh Water," 

 by E. von Martens, Ann. and Mag. Nat. Hist., 1858, vol. i. p. 50. 



