402 
THE FKESH-WATER LOCHS OF SCOTLAND 
enormous part the plankton plays as fundamental food- material for 
the animal life in the water. To assume that the waters of the past 
did not contain the same fundamental food-material would lead to 
the most absurd consequences. 
The reason for this food-material having on the whole remained 
unchanged from the oldest times until now is, in part, that it has been 
in a less degree than any other community exposed to complete 
extinction and new formation consequent upon the great convulsions 
of the earth ; in part also, because of its powers of spreading, by 
means of which it would be able to recapture the areas from which it 
had been driven at any time. 
That this community has contributed much towards the formation 
of the crust of the earth (limestone, petroleum, etc.) is certainly incon- 
testable ; that its particular forms have not been able to persist 
permanently owing to their delicate skeletons, upon which their 
occurrence as plankton organisms always depends, is by no means 
unnatural. In several cases, however, it has been supposed that 
either the plankton organisms or forms nearly related occurred in 
very old deposits. Thus, the blue-green algag Girivanella problem- 
atica (Silurian), which together with other Cyanophyceas are said to 
form the oolitic structure of rocks (Ordovician age in the Girvan 
strata) ; the Peridinium pyrophorum described by Ehrenberg from 
cretaceous rocks, which seems hardly to be distinguished from Peri- 
dinium divergens of the present day ; CoccospJieres and Rhahdo spheres 
in the Lias deposits ; the large Diatom deposits from the tertiary 
and cretaceous series, but curiously enough not from earlier deposits 
(see Seward, 1898, p. 117) — all these indicate that it is only to the 
lack of investigations and the nature of the material that our slight 
knowledge of fresh-water plankton in earlier times is due. 
Further, it must be remembered that numerous forms of the 
botbom and littoral fauna and flora to which the plankton organisms 
are related are extremely old forms. We need only mention the 
Ostracoda of the palaeozoic times. Nothing in the structure of the 
Daphniae justifies us in considering these as much younger than the 
Ostracoda, their nearest relatives. That the Daphnias appear only 
in the upper Miocene mud deposits from the old lakes in the Egerer 
and Falkenauer basin is certainly due, in part, to the preservation of 
these forms being much more difficult, and in part to the ephippia 
having been misunderstood (see also Brehm and Zederbauer, 1906b, 
p. 477). 
If future researches raise the hypothesis that the fresh-water 
plankton of the world is collectively a cosmopolitan community to a 
scientific theory, the explanation of this should be sought for in the 
immense age of this community. With this view in mind, we may 
