onship, with very little identity, between the productions of
North America and Europe." (Pages 333, 334.) He showed that this could
be accounted for by their migration southwards from a common area, and
he told Wallace that he "doubted much whether the now called Palaearctic
and Neartic regions ought to be separated." ("Life and Letters", III.
page 230.) Catkin-bearing deciduous trees had long been seen to justify
Darwin's doubt: oaks, chestnuts, beeches, hazels, hornbeams, birches,
alders, willows and poplars are common both to the Old and New World.
Newton found that the separate regions could not be sustained for birds,
and he is now usually followed in uniting them as the Holartic. One
feels inclined to say in reading the two pages, as Lord Kelvin did to
a correspondent who asked for some further development of one of his
papers, It is all there. We have only to apply the principle to previous
geological ages to understand why the flora of the Southern United
States preserves a Cretaceous facies. Applying it still further we can
understand why, when the northern hemisphere gradually cooled through
the Tertiary period, the plants of the Eocene "suggest a comparison of
the climate and forests with those of the Malay Archipelago and Tropical
America." (Clement Reid, "Encycl. Brit." (10th edition), Vol. XXXI.
("Palaeobotany; Tertiary"), page 435.) Writing to Asa Gray in 1856
with respect to the United States flora, Darwin said that "nothing has
surprised me more than the greater generic and specific affinity with
East Asia than with West America." ("More Letters", I. page 434.) The
recent discoveries of a Tulip tree and a Sassafras in China afford fresh
illustrations. A few years later Asa Gray found the explanation in
both areas being centres of preservation of the Cretaceous flora from
a common origin. It is interesting to note that the paper in which this
was enunciated at once established his reputation.

In Europe the latitudinal range of the great mountain chains gave the
Miocene flora no chance of escape during the Glacial period, and the
Mediterranean appears to have equally intercepted the flow of alpine
plants to the Atlas. (John Ball in Appendix G, page 438, in "Journal of
a Tour in Morocco and the Great Atlas", J.D. Hooker and J. Ball, London,
1878.) In Southern Europe the myrtle, the laurel, the fig and the
dwarf-palm are the sole representatives of as many great tropical
families. Another great tropical fa