324 Prof. F. A, Forel on the. 
selves to the feathers of migratory birds (ducks, grebes, gulls, 
&c.) perfectly explains the transference from one lake to 
another (A. Humbert, Forel). Pavesi has urged against this 
common origin and mode of distribution the irregularity of 
the pelagic peopling of the different Italian lakes, as many 
species are deficient in certain lakes while they occur in 
neighbouring lakes; but this very irregularity seems to me 
to be in favour of the occasional and accidental mode of dis- 
tribution that we have just indicated. If we accept this mode 
of distribution, the differentiation of the pelagic species is no 
longer necessarily limited to the lake mm which we find the 
animals or to the present geological epoch. This fact is of 
great importance in the explanation of the pelagic fauna of 
certain lakes of comparatively recent origin; in the case of 
our Swiss lakes the glacial period forms an absolute limit, 
which entirely prevents our accepting a local differentiation 
of the old Tertiary species and their transformation into the 
existing species. The pelagic faunas of many Italian lakes 
of volcanic origin are of a much later date still. But ds we 
are no longer confined to a local differentiation of the autoch- 
thonous species, we have more time and space at our disposal 
for this differentiation. ; 
I believe we must find the cause of the differentiation of 
the pelagic fauna in the combination of two different pheno- 
mena—namely, the daily migrations of the Entomostraca, and 
the regular local winds of the great lakes. It is well known 
that on the borders of great masses of water two regular winds 
prevail, one of which blows at night from the land towards 
the water, the other by day from the water to the land. The 
nocturnal animals of the shore-region which swim at night at 
the surface are at this time driven towards the middle of the lake 
by the surface-current of the land-wind, sink during the day, 
being driven away by the light, into the deep water, and thus 
escape the surface-current of the lake-wind, which would 
otherwise have carried them again to the shore. Constantly 
driven further every night, they remain confined to the pelagic 
region, as they are not carried back again during the day. 
Thus a differentiation takes place by natural selection, until 
at last, after a certain number of generations, there remain 
only the wonderfully transparent and almost exclusively 
swimming animals which we know. When this differentia- 
tion has once taken place, the pelagic species is conveyed by 
the migratory water-birds from one country to another and 
from one lake into another, where it reproduces its kind if 
the conditions of existence of the medium are favourable. In 
this way we may find the pelagic Entomostraca in lakes 
which are too small to possess the alternation of winds, the 
