boulder-clay does not, therefore, offer any serious difficulty to the
supposition that some of the so-called Newer Pliocene Crags of the east
coast of England were laid down at the same time by the same sea.

This would also explain how the Arctic species come to inhabit the
Caspian, as the old Aralo-Caspian Sea could have had some communication
(Fig. 12, p. 156) with the North European Sea. And this again offers an
explanation of the otherwise mysterious occurrence of the Caspian
_Dreyssensia polymorpha_ in the lower continental boulder-clay.

The climatic reasons for the supposition that the boulder-clay is a
marine deposit have already been given (p. 66). However, it may be asked
what about the glacial flora which has been proved to have existed all
over the plains of Northern Europe?--what about the relics of this same
flora which still linger on in a few localities to the great delight of
the systematic botanist? They have been spoken of as indications of a
former Arctic climate in Europe. The presence of an Arctic species such
as _Dryas octopetala_ in any of the pleistocene deposits is often looked
upon as an absolute proof of a very severe climate having prevailed at
the time they were laid down. Professor Geikie tells us that the South
of England was clothed with an Arctic flora, when the climate became
somewhat less severe than it had been during the climax of the glacial
cold (p. 398). Relics of such a flora have been detected at Bovey
Tracey, in Devonshire, the Arctic plants found comprising _Betula nana_
and _B. alba_, _Salix cinerea_ and _Arctostaphylos uva-ursi_.

Now three of these four species of plants are still natives in the
British Islands, and all are forms which probably came to us with the
Arctic migration which I described in Chapter IV. They travelled south
with the reindeer, or before it, and may have covered large tracts of
country at the time. With the increased struggle for existence on the
arrival of the Siberian and Oriental migrants, they have probably been
evicted by these more powerful rivals. A discovery of their remains does
not necessarily indicate that a great change of climate has taken place
since they lived in the country. And certainly these Arctic plants
cannot be taken as indicating a low temperature, for it has been shown
that Alpine plants are mostly intolerant of very low temperatures.
"Arctic and Alpine species in the Botanical Gardens at Christiania,"
says Professor Bl