70 On ths Origin and Distribution of the British Flora.
however, I think, understand me when I say that climate
rather determines what shall not grow in a given locality—
what shall be exterminated if it attempt to grow—than what
shall grow ; if I remind you that plants do not always flourish
most in their native home, as witness the familiar instance of
the luxuriance of our English watercress and white clover in
New Zealand, and that many plants do not occur native in
climates admirably suited to them. In considering the causes
which have led to our British flora being what it is, we must
undoubtedly bear in mind that ours is an insular climate.
There being more moisture in the air the extremes of both
heat and cold are moderated, and our climate is better suited
to herbaceous perennials than to annuals; but the recent
separation of our islands from the continent causes their
flora to be in the main an extension of that of Germany,
altogether different from those "insular floras," rich in
endemic or peculiar types, which characterise "oceanic"
islands. The theory of evolution shows us that the real key
to geographical distribution is to be found in the community
of origin of allied forms, and their subsequent dispersal.
The subject was first reviewed from this standpoint by Mr.
Bentham in 1869.1 He then pointed out that the vegetation
of the globe must always have been separable into three
great latitudinal zones,—the northern, the tropical, and the
southern. The subsequent migrations of plants seem to
have tended rather from north to south and from east to
west than in the reverse directions.2 In explanation of the
1 In his Presidential Address to the Linnean Society.
2 Sir Joseph Hooker, in his Address to the Geographical Section of the
British Association at York, 1881, alludes to a lecture, by Mr. Thiselton
Dyer, "On Plant Distribution as a field of Geographical Research" ('Pro-
ceedings of the Royal Geographical Society,' vol. xxii., 415, 1878), which I
have not seen, wherein he argues that "the floras of all the countries of
the globe may be traced back at some time of their history to the northern
hemisphere." Sir Joseph also refers to Count Saporta's essay, entitled
"L'Ancienne Vegetation Polaire," in the 'Comptes Rendus', of the Inter-
national Congress of Geographical Science for 1875, which also I have not
seen. "Starting from Buffon's thesis, that the cooling of the globe having
been a gradual process, and the Polar regions having cooled first, these