178 PAST AND PRESENT 
plants, growing in different situations, should be 
brought thus from the Pyrenees and Mediterranean 
to our western coasts is a highly speculative sugges- 
tion. If we discard it, there is left the hypothesis 
that the plants migrated long ago overland, at a time 
when the western coastline of Europe was continuous 
and lay farther seaward. Such conditions have not 
occurred since the Ice Age; so we have to assume 
that the plants, arriving perhaps in Pliocene times by 
slow terrestrial dispersal, and subsequently cut off by 
invasions of the sea upon their line of advance, sur- 
vived the cold and ice of the Glacial Period within the 
limits of our islands. That appears, on consideration 
of the geological evidence of widespread glaciation, 
sufficiently improbable; but we must remember that 
the evidence supplied by the plants is buttressed for- 
midably by that of the corresponding animals, some 
of which, such as the Kerry Slug, are far less fitted 
for transmarine dispersal than are the seeds of plants. 
Also, we are faced with the problem of the American 
plants, and such organisms as the American Sponge, 
Heteromeyenia: a direct crossing of the ocean 
appears for them wholly impossible. Yet if they 
crossed over long-gone land surfaces, their arrival on 
this side of the Atlantic must be very ancient, and 
they must certainly have weathered successfully the 
Great Ice Age. The problem, it is clear, is an ex- 
ceedingly difficult one, upon which it would be rash 
to pronounce any hasty opinion. Students of the sub- 
ject have come to widely difficult conclusions: some 
holding with Edward Forbes that these Lusitanian 
and American organisms represent the very oldest 
element in our fauna and flora, having migrated over 
