Pelagic Fauna of Freshwater Lakes. 323 
100 and even 150 metres; at these great depths, however, I 
have only found Diaptomus. 
On account of these migrations Weismann regards them as 
nocturnal animals which keep at the extreme limit of light ; 
their optic nerve would suffer under the influence of too bright 
a light, and they therefore descend into the deep water so 
soon as the light of the sun or moon becomes too strong. 
Nevertheless they must still see in order to be able to pursue 
their prey; and they therefore only descend to the point where 
their generally well-developed eyes enable them to find their 
nourishment. Weismann justly remarks that in these migra- 
tions they traverse daily a colossal stratum of water, in which 
they may find sufficient nourishment, sparingly as this may 
be distributed in the comparatively clear water of the fresh- 
water lakes. 
But what is the limit of light in freshwater lakes? I 
demonstrated in 1877 that the transparency varies with the 
seasons of the year; in the Lake of Gehieve a shining object 
immersed in the water disappears (when the conditions of 
illumination and transparency are most favourable) when it is 
in a stratum of water of a depth of 16-17 metres. Photo- 
graphic investigations with paper sensitized with chloride of 
silver had proved to me in 1874 that the limit of absolute - 
darkness in the Lake of Geneva lies at a depth of 45 metres 
in summer and of 100 metres in winter. Asper, using much 
more sensitive plates (with emulsion of bromide of silver), in 
August 1881, found that the rays are still efficacious at 90 
metres and more in the Lake of Zurich. All this, however, 
tells us nothing as to the limit of absolute darkness for the 
retina and especially the visual nerves of the low animals. 
What is the origin of this pelagic fauna? Does it depend 
upon a local differentiation? Have the palustrine or fluvia- 
tile Entomostraca, those of the littoral region of the lakes, 
become transformed in each lake into pelagic species or 
varieties? ‘T'o this last question we can with certainty 
answer in the negative. The remarkably wide distribution 
of this fauna, the almost complete identity of the pelagic En- 
tomostraca in all the Kuropean lakes, from the Scandinavian 
to the Swiss, Italian, and Armenian, speak in favour of a 
common origin and distribution. 
But how has this distribution been effected? Active mi- 
gration from one lake to another is not admissible, both on 
account of the difficulty of communication between the diffe- 
rent lakes, and because of the slowness and inactivity of the 
pelagic Entomostraca. On the other hand, passive migration 
in the state of resting-eges which may have attached them- 
