430 
MR. ROBERT GURNEY ON THE CRUSTACEA 
fresh -water.* It seems possible that the increasing intrusion 
of marine types may perhaps be connected with a progressive 
uniformity of climate and water temperature. 
It may be objected that there are littoral and estuarine 
species which have accustomed themselves to unstable 
conditions of salinity and temperature — why should they not 
go further and establish themselves in fresh water ? So far 
as concerns salinity it seems that, though undoubtedly 
a limiting factor, it is not an absolute barrier. On the other 
hand, I do not know of observations on the thermal conditions 
in tidal rivers, but I should imagine that, where a large volume 
of water is oscillating to and fro in a comparatively deep 
channel, the conditions will be more or less uniform. It 
seems that the range of the marine species with which I have 
been dealing generally stops short at, or before, the point 
where the stream becomes more or less sluggish. None, 
except Neomysis vulgaris and Tachidius littoralis, range into 
the Broads themselves. If it were only a question of salinity 
it is probable that our Broads might be tenanted by species 
such as Corophium grossipes, but that low salinity and variable 
temperature combined prove, at present, an insuperable 
barrier. 
Another factor, which must be of some importance in this 
connection is the density of the water. Animals accustomed to 
water of a high density would lose buoyancy when transferred 
to water of low density and vice versa, so that floating marine 
species could not maintain themselves at the surface in fresh 
water, and all fresh-water species would have a tendency to 
float on water of high density. It is a fact that the marine 
immigrants are (with the exception of Eurytemora affinis) 
either large vigorous species, or creeping bottom forms, and 
that the fresh-water species are rarely found alive in the 
river in water of high salinity. 
It seems probable, therefore, that the main causes which 
have contributed in the first place to drive out the old marine 
fauna, and in the second place to check immigration, are 
firstly differences and fluctuations of salinity, and secondly 
fluctuations of temperature. The latter influence would come 
* Weber. Zool. Ergeb. einer Reise in Niederlandisch Ost Indien, 1892. 
