ime existed." Besides the groups
referred to, I claim that particular attention should be devoted to
Amphibia, which, contrary to Wallace, I hold do not possess special
facilities for dispersal; and also to spiders and to all wingless
animals leading a subterranean life, such as some of the wood-lice,
planarian worms and apterous beetles.

A thorough knowledge of the changes in the distribution of land and
water is desirable in order to appreciate the extent and variations of
former migrations. A study of the British fauna, for example, teaches us
that the British Islands were once connected with one another and with
the continent of Europe between England and France. It was Professor
James Geikie, I believe, who first pointed out, many years ago, that the
area now covered by the Irish Sea was formerly in all probability a
fresh-water lake. This had its outlet at the southern extremity in the
form of a stream into which most likely flowed the smaller rivers from
the south-east of Ireland, and which was joined from the east by the
Severn, and finally debouched into the Atlantic (Fig. 4). The range in
the British Islands of those species which have migrated to them from
the south, indicates that whilst the Atlantic Ocean had gradually crept
up and flooded the area between Ireland and Wales, and had turned the
fresh-water lake into a bay, communication between Scotland and Ireland
was still possible. The occurrence of many Scandinavian species in
Scotland which are absent on the continent of Europe, indicates that
these two countries also were united formerly. Most geologists hold that
such a connection, if it existed, must have broken down in Pliocene
times. Professor Judd, however, has expressed his belief (p. 1008) that
it still existed until after the appearance of man in Northern Europe,
and that our forefathers might have been able to walk dry foot from
Scotland to Norway.

[Illustration: Fig. 4.--Map of the British Islands and surrounding area
at a time when the earlier members of the southern migration reached
England. (Only some of the rivers have been indicated. The shaded parts
represent water, the light land.)]

I shall also show on distributional evidence, in the fourth chapter,
that until recent geological times Scandinavia was continued northward,
by way of Bear Island, with Spitsbergen and probably Franz Josef Land,
which islands again were joined with North Greenland and Arctic North
America, and that t