OF THE ENGLISH LAKE DTSTKICT. 433 
entrance of salt water, immigration into Ennerdale was possible. I must 
freely admit tliat such a supposition is not supported b}^ the geological 
evidence, but it does not appear to me to be an altogether impossible 
assumption in view o£ the following facts. 
There is evidence of a subsidence of land immediately after the withdrawal 
of the ice, and of a later elevation at a time when the submerged peat-beds 
and forests were formed. But the amount of the latter elevation is uncertain, 
and there is no positive evidence of a rise of more than 60 feet. On the 
other hand, the occurrence in Ireland of the Red Deer and Reindeer seem to 
necessitate the supposition that there was some land connection between 
England and Ireland in neolithic time, and, as Jukes-Browne has pointed out, 
firstly it is possible that this elevation niaj^ have been greater than has been 
supposed, and secondly it is very probable that the floor of the Irish Sea, 
which was thickly covered by drift, has been greatly eroded and re-modelled. 
It may consequently have been at so much higher a level that a comparatively 
small elevation may have sufficed to effect a land connection with partial or 
complete damming off of the sea. 
Dr. Scharff has assumed the existence of a great lake occupying the 
trough of the Irish Sea to explain the present distribution of the genus 
Coregonus in Britain and Ireland, and if such a lake could have originated 
by freshening of a Glacial sea, not only might L. gvimaldii have been therein 
isolated and transformed, but also Mysis oculata could in this way have been 
changed into M. 7-elicta and have reached its present station in Lough Neagh. 
There are two obvious objections, namely that M. relicta does not live in 
Ennerdale and L. macrurus does not occur in Lough Neagh. Further, that 
conditions which would allow the latter to reach Ennerdale should also allow 
it to reach Wastwater and perhaps other lakes. To these objections not 
much importance need be attached until more is known of the conditions 
which determine the distribution of fresh-water Crustacea. Not only does 
the plankton of Lough Neagh differ radically from that of the English lakes 
in general, but these lakes differ unaccountably among themselves, and 
Wastwater and Ennerdale, alike as they seem to be in physical conditions, 
contain quite different plankton. Hence it cannot be assumed that a species 
would establish itself in every lake to which it had access. 
It is equally difficult to explain the distribution of Limnocalanus in North 
America. Its distribution follows rather closely that of Mysis relicta, as 
shown in Table 7, page 434. 
L. macrurus therefore occurs in all the great hikes and in a number of 
lakes belonging to the same drainage system, but not, so far as I can find, in 
any lakes outside this region. 
So far as the origin of these specjes is concerned, the view once held that 
the Great Lakes were themselves at one time invaded by the sea and that 
M. relicta is a true relict within them has been generally abandoned, but at 
the same time there appears to be no doubt that the sea did, at a late stage 
