128 GEOGRAPHICAL DISTRIBUTION. 



which are as transparent as the medium in which they live have 

 held their own." 



The lacustrine pelagic animals perform daily vertical migrations 

 of the same character as has been noted in the case of the oceanic 

 pelagic fauna, descending to the regions of obscurity during the 

 day-time, and ascending by night. The animals appear to shun 

 the light of the sun, and even of the moon, and hence retire to a 

 depth probably not far from the limits of light penetration ; the 

 fauna is, therefore, one of darkness. The greatest depth whence 

 specimens were obtained by Forel in Lake Geneva was about one 

 hundred and fifty metres ; but at this depth only Diaptomus was 

 found. At a depth of fifty metres, in the Lago d'Orta, Pavesi 

 found a very profuse fauna, represented by seven species ; in the 

 Lago d'Iseo, at five, fifteen, and thirty metres, the catch appears to 

 have been exceedingly abundant (" la pesca fu prodigiosamente ab- 

 bondante ") ; but, at one hundred metres, where the temperature of 

 the water was 19 C., as compared with a surface temperature of 

 23 C., the fauna was decidedly scanty, although five distinct forms 

 were obtained. 



To what extent the downward extension of the pelagic fauna is 

 governed by conditions of temperature, or in how far this limitation 

 is dependent principally upon the presence or absence of light as a 

 determining factor in the evolution of plant life, still remains to be 

 ascertained. Forel, in 1874, found that paper sensitised with chlo- 

 ride of silver was still acted upon by the diffused light of the Lake 

 of Geneva at a depth of about forty-five metres in summer and one 

 hundred metres in winter, while ordinary shining objects disap- 

 peared from view at a depth of sixteen to seventeen metres. Asper, 

 in August, 1881, obtained positive results through the use of plates 

 sensitised with an emulsion of bromide of silver at a depth some- 

 what exceeding ninety metres in the Lake of Zurich ; and more re- 

 cently (1884-'85) Fol, Sarasin, Pictet, and others, have been able 

 to detect the penetration of light in Lake Geneva to a maximum 

 depth (in winter) of two hundred metres. In summer the penetra- 

 tion is considerably less. Fol and Sarasin 51 have also demonstrated 

 that, in the Mediterranean, the solar rays penetrate to a depth nearly 

 double that to which they were found to descend in the Swiss lakes, 

 or to four hundred metres, and that at a depth of three hundred 

 and eighty metres the intensity of light is as great as in Lake 



