162 Browne: The Aquatic Coleoptera of the Isle of Man. 
“(Ecology of the Mollusca of Lonsdale,’ mentions Unio mar- 
garitifey and Neritina fluviatilis as associated species, while in 
Ireland, Mr. Stelfox tells me, the former is entirely calcifuge 
and the latter purely calciole. 
Among the water beetles there are similar examples— He- 
dochares punctatus is chiefly a fresh-water marsh species in East 
Norfolk, while in the Isle of Man, south west of Scotland, and 
the west of Ireland, it is purely oxylophile. The same is true of 
Hydroporus granularis and Philhydrus nigricans, which are 
helophiles in E. Norfolk, and oxylophiles in the Solway. district, 
and I have already referred to the case of Ilybius subeneus. 
These facts all illustrate the control exercised by the biolo- 
gical factor, and Dr. Scharff’s cases of the failures of invading 
or introduced species to establish themselves, are only further 
examples of the same thing. 
In these remarks I have criticised Dr. Scharff’s evidence, and 
not his contention as to the necessity for land-connections to 
account for insular faunas and floras. Whether there is a case 
for proving a general principle I am not at present in a position 
to judge, but it seems to me that some other kind of evidence 
than that which he has brought forward is necessary. In the 
case of the Isle of Man, geologists can give us no definite as- 
sistance, but the fact that the flora, as I learn from Mr. 
Praeger, the land and fresh-water mollusca—I am _ told, 
and the water beetles, as I have endeavoured to show, are more 
closely related to those groups in the north west of England 
than to those in S.W. Scotland or N.E. Ireland, seems to provide 
us with evidence of another kind. 
If on the disappearance of the ice-age, the Isle of Man was 
an island, then, being virgin soil, it was open to attack from all 
directions. Now under such circumstances the prevalence of 
westerly winds would undoubtedly have favoured species 
coming from Ireland. The ocean currents also would have 
brought Irish species, and would have kept out English ones. Dr. 
Dakin has kindly supplied me with details of tides and currents 
in the Irish Sea, and there is, apart from the tides, a general 
drift from south to north, so that bottles thrown from steamers 
leaving Liverpool always appear on the coasts of Lancashire 
or Cumberland, and shingle also tends to move northward. 
The best proof of this northerly drift is afforded by detailed 
observations on salinities and temperatures, which show that 
a current of warm and more saline water passes northward as 
far as the Isle of Man, and then the bulk of this current turns 
eastward so that there is no great drift between the Isle of Man 
and Ireland. The south tidal current runs up St. George’s 
Channel, and bends round Anglesey, turning east into Liver- 
pool Bay. The north tidal current sweeps down the north 
channel, divides at the point of Ayre into two, a larger part 
Naturalist, 
