south-east coast. I have
shown in a previous essay that the former presence of a freshwater lake
between England and Ireland is indicated by the distribution of the
Charrs and also by the various species of British Coregonus. There are
three British species of Coregonus, viz., _C. clupeoides_, _C.
vandesius_, and _C. pollan_. These are confined to the lakes of North
Wales, North-western England, South-western Scotland, and Ireland. All
but the latter communicate at present directly with the Irish Sea. The
lakes of the latter country, however, must have done so at a time when
the west of Ireland stood at a higher level than it does now. The
ancestors of the three Coregonus species, and also those of the Charrs,
then lived in the large freshwater lake indicated on the map (p. 60),
and when the sea gradually crept up the river valley and finally
converted the lake into a gulf, the freshwater fish took refuge in the
rivers which supplied it with water.

Now as for the continuous sea-shore between the coast of Brittany and
the south-west of Ireland, zoological distribution again aids us in
proving that such must have actually existed at no very distant
geological date. Most of our common shore forms of life migrate along
the coast exactly as land animals do--step by step. Their eggs are
carefully attached to fixed objects, so as not to be carried away by the
waves, whilst the young often remain and grow old in some particular
little pool, rarely venturing farther than a few yards from the spot
where they first saw the light of day. A number of such shore forms are
found on the west coast of France, the same species recurring again on
the south-west coasts of England and Ireland, thus clearly indicating a
former continuity of coast-line between these points, now separated by
deep sea. A very familiar example to British zoologists is the purple
rock-boring Sea-urchin (_Strongylocentrotus lividus_), but there are a
great many others, such as the semi-marine Beetles _Octhebius Lejolisii_
and _AEpophilus Bonnairei_, the Crustaceans _Achaeus Cranchii_, _Inachus
leptochirus_, _Gonoplax angulata_, _Thia assidua_, _Callianassa
subterranea_, the Fishes _Blennius galerita_ and _Lepadogaster
Decandollii_, and the Molluscs _Otina otis_, _Donax politus_, and
_Amphidesma castaneum_.

Before concluding this chapter, a few words as to my views on the
conditions prevailing during the Glacial period will not be out of
place. They do not diff