FROM AN OLD STANDPOINT. 



183 



changes of climate, bird, and plant have often gone on together, 

 the range of the bird being controlled by the climate and the 

 distribution of the plant being largely dependent on the bird. 

 The bird generalised in type that once ranged the globe is now 

 represented over its original area by a hundred ditferent groups 

 of tiescendants, each confined to its own locality. Climate, 

 oince so uniform, now so divorsitied, lias by restricting the 

 range of the bird favoured the pi-ocess of ditlerentiation, whilst 

 those plants that are dependent on the birds for their 

 distribution have in their turn responded to the changes. It 

 is not possible to deal larther with this su'ijeut here, but much 

 w.ll be found on these subjects in my bock on Plant-Dispersal, 

 sepacially in the last two chapters. 



The Tul'e Fqxctiox of the Agencies of Dispeksal. 



llegarding the study of plant-distribution as being almost 

 entirely concerned with continents, since islands cover a very 

 small proportion of the area of the globe, I am strongly 

 inclined to the view that the function of the dispersing agencies 

 has been chiefly limited to irregularly impeding the process of 

 differentiation that is itself primarily determined by the secular 

 changes in the climatic and surface conditions of the earth. 

 If the diversification of forms depended only on physical 

 conditions the earth's floras would be full of monutony. 

 Variety begins when the agencies of dispersal interfere. 

 Though naturally efficacious in stocking islands with their 

 plants, the dispersing agencies acquire quite a diiierent signifi- 

 cance in contniental regions. We are there brought lace to 

 face with problems concerned with station in its most com- 

 prehensive sense, with past changes in the history of climate 

 and in the arrangement of land and sea, and w^ith those 

 mysterious revolutions in plant-forms that have affected the 

 whole world. We cannot appeal to the dispersing agencies 

 for an explanation of the distribution of the great primitive 

 families, such as the Amentifene, the AracccC, the Coniferte, 

 the Palmaccie, and the Scitaniineai, that now in different 

 latitudes encircle the globe. Xor could they aid us in the case 

 of a genus like Acer that goes back to the Secondary epoch and 

 existed, as already shown, in early Tertiary times on both sides 

 of the Atlantic. 



The distribution of so many families in both the Old and the 

 Xew World, whether in tropical or in temperate latitudes, would 

 of itself suggest to us that in investigating means of dispersal 



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