380 NATURAL SCIENCE [December 
Caspian and Aral with the White Sea and the Baltic, thus forming 
an effectual barrier to the westward course of the mammals. The 
existence of such a sea is supported by the presence of arctic forms 
of life in the Caspian, and the occurrence of the Caspian molluse 
Dreyssensia polymorpha in the Lower Boulder Clay of Germany. As 
this central European sea was replaced in part by a land surface, 
the way was opened for the Siberian mammals to pass on into 
western Europe. Now we are confronted with the startling fact that 
the British deposit in which these mammals first appear is the 
Forest Bed, usually considered the newest member of the Pliocene 
series. Are we to suppose, Dr Scharff asks, that the animals made 
their way into England by Asia Minor, Greece and Southern 
Europe, and so reached our shores before Central Europe was open 
to them? That part of the older southern fauna—the ‘South- 
Central’ section—travelled into Western Europe by this route from 
Siberia during Pliocene times he does believe. But, he argues, it 
is impossible that the true ‘Siberian’ animals could have passed 
that way, seeing that their remains are entirely absent from South 
European, as well as from Irish, Scottish and Scandinavian, deposits. 
He is therefore driven to the conclusion that the Forest Bed and 
other British deposits usually classed as Newer Phocene must be 
_ considered as rather later than the Lower Continental Boulder Clay, 
and reckoned to be of Pleistocene age. In support of this correla- 
tion he also brings forward the presence of arctic shells in the newer 
crags.! 
Having thus fixed the period when these Siberian mammals 
appeared in England, Dr Scharff believes that he has obtained the 
latest possible date for the ‘last link’ of the land-connection be- 
tween England and Ireland, For if the way into Ireland remained 
open long after these mammals reached English territory, what can 
have prevented their onward course to the western island? The 
wide range of the mammals as compared with the restricted range 
of the invertebrates of the same faunistic section has been dwelt 
upon in the opening part of this paper. It is certain that the vast 
number of widespread invertebrates that inhabit Ireland as well as 
Great Britain must have passed over the Irish Sea when it was a 
lake and river valley, or crossed the later northern isthmus which 
joined northern Ireland to south-western Scotland. But as the 
Siberian mammals were kept out of Scotland by the Pleistocene sea, 
this northern isthmus may be left out of reckoning as far as they 
are concerned, If the slowly-moving army of spiders, beetles, snails 
1 The reader is referred to Dr Scharff’s paper for the numerous references supporting 
these positions. It will be seen that the editorial statements of Dr Scharff’s views 
(supra, p. 224, ‘‘that the lower continental boulder clay is Pliocene . . . that the 
Siberian mammals migrated into Western Europe to the south of this sea’’) convey the 
exact reverse of the opinions really advocated by the author. 
