IN THE NORFOLK BROADS DISTRICT. 
77 
If then there is in such a district as that of the Norfolk 
Broads a sub-district distribution, perhaps mainly dependent 
upon available shelter, there are undoubtedly factors which 
further localise the species within the sub-districts. Some 
species for instance, such as Agabus paludosus, Ilybiiis 
fuliginosus, and Deronectes depressus, apparently occur most 
commonly in clear running water ; others again, such as 
Agabus nebulosus, prefer stagnant ponds. Others, such as 
Ilybius guttiger, Hydroporus umbrosus, and oblongus, are perhaps 
commonest on mossy, marshy ground where water is scarcely 
visible above the vegetation. But even so, a habitat to all 
appearances eminently suitable for a certain species does not 
necessary hold that species. For instance, Hydroporus 
scalesianus occurs not uncommonly in certain patches of 
mossy marsh at Sutton and at Catfield Fen, but not, so far 
as I have been able to find, on similar ground most suitable 
in appearance in other places. 
The factors which produce such fine distinctions in 
localisation are for the most part unknown. Many factors 
such as light, temperature, salinity, or other chemical con- 
ditions of the water, as well as the presence or absence of 
certain species of animals or plants combine to produce 
a suitable habitat, but the absence of any one of these is 
probably sufficient to make a habitat unsuitable. 
One extremely interesting case of localisation has occurred 
at the station known as the Palling Brick Pits. The collecting 
place is a large pit sunk below the level of the surrounding 
ground. In this pit is a series of moderately sized ponds in 
the gravelly sand where clay has been dug for the brick- 
making. These ponds are mostly separate units in the 
summer, but in the winter many of them are connected 
owing to the rise of the water. The ponds in the eastern 
part of the pit are however always a separate series from 
those in the western part. The western series are rather 
deeper than the eastern, but otherwise there is nothing 
striking to distinguish them. The water is clear, and numerous 
plants such as Hottonia, Anacharis, Callitriche, Potamogeton 
natans grow in various ponds in both series. Yet the distri- 
