286 MR. 0. V. APLIN ON THE DISTRIBUTION IN GREAT BRITAIN 
makes it more clear, that ever since our climate became sufficiently 
mild to allow of the existence of our present fauna and flora, many 
ot the river-basins of Britain have formed isolated areas. Each of 
the mountain tarns has probably obtained its inhabitants gradually 
through the accumulation of accidents, and not by wholesale 
migration along a continuous water-way. In the Eastern Counties, 
however, there is evidence of a former continuity of water-ways 
greater even than that now existing. That continuity is probably 
the reason of the more prolific aquatic fauna and flora of Norfolk 
when compared with other districts, like the Severn valley, which 
have always been more completely isolated. 
To guard against misapprehension, it may be observed that this 
paper deals only with a single group of animals and plants — that 
found in isolated ponds. These species are apparently most often 
transported on the feet of birds. There are other equally large 
groups which are dispersed in quite different ways, and it must 
not be thought that any attempt is here being made to account in 
the same way for the whole of our fauna and flora. One branch 
only of a wide subject is here dealt with. 
IV. 
ON THE DISTRIBUTION IN GREAT BRITAIN AND 
IRELAND OF THE RED-BACKED SHRIKE. 
(LANIUS COLLURIO, LINN.). 
By 0. V. Aplin. 
Read 23rd February, 1892. 
Introductory Remarks. 
The Red-backed Shrike ( Lanius collurio ) is a bird in which I have 
always taken a peculiar interest, and its curious and remarkably 
local distribution in Great Britain lias suggested to me that it 
would be worth while to trace carefully its range throughout the 
kingdom. 
