70 On the Origin and Distnbution of the Ihitinh Flora. 



however, I tliink, understand me when I say that cHmate 

 rather determines what shall not grow m 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 thek 

 flora to be in the main an extension of that of Gennany, 

 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 1.869. -^ He then pointed out that the vegetation 

 of the globe must always have been separable into three 

 gi'eat 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.^ In explanation of the 



1 In liis Presidential Address to the Linnean Society. 



2 Sir Joseph Hooker, in liis Addi'ess 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 Eesearch " ('Pro- 

 ceedings of the Koyal 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," Sh Joseph also refers to Count Saporta's essay, entitled 

 " L'Ancienne Vegetation Polaire," in the ' Comptes Eendus', of the Inter- 

 national Congress of Geographical Science for 1875, which also I have not 

 seen. " Starting from Buflon's thesis, that the cooling of the globe having 

 been a gradual process, and the Polar regions having cooled first, these 



