324 PLANTS, SEEDS, AND CURRENTS 



result of the varying differentiating influences of the hemispheres of 

 the east and the west. When the warm conditions returned, the 

 plants advancing northward met again in the common gathering- 

 ground around the pole, but modified by their different experiences 

 in southern regions lying oceans apart. There they mingled together, 

 the eastern and the western floras ; and when with the next climatic 

 change they began again to retreat to their ancient home in the 

 warm latitudes of the south, the east had borrowed from the west 

 and the west from the east. The secular climatic changes have, 

 therefore, tended in this way to mix together the floras of the 

 globe. 



(3) The Barriers athwart the Course of Migrating Plants. 

 — But another factor has intervened to disturb the effect of the 

 influence of the divergence of the land-masses and of the secular 

 fluctuations of climate on the operation of the differentiating agencies. 

 The contrast between the plants of the eastern and western hemi- 

 spheres may be, and has been, intensified in an irregular fashion by 

 the presence in one and the absence from the other of obstacles in 

 the line of retreat. During their sojourn in the north a huge Hima- 

 layan range, or a large Mediterranean sea, or a great Sahara desert, 

 may have been developed and lie athwart their line of march; or a 

 lofty Cordillera, running with the meridian almost from pole to pole, 

 may aid the migration between the north and the south. But 

 always there will have been the great preponderance of land in the 

 north, and always those two great diverging land-masses of the east 

 and the west, ocean-parted in the south and meeting in the north. 

 Thus the distribution of plants may now be expressed as the effect 

 of the operation of the differentiating agencies under the control and 

 direction exercised by the divergence of the land- areas from the 

 north and by secular fluctuations of climate, the resulting migration 

 of plants to and from the north being checked or aided in different 

 degrees by the surface- configuration of the continents. 



The Views of Sir W. T. Thiselton-Dyer. — How the author 

 came to recognise that distribution is something more than the 

 work of the differentiating and dispersing agencies is stated in a 

 later page. But it should be at once observed that the controlling 

 influences, as above described, are those which Thiselton-Dyer has 

 for years emphasised in his writings on distribution. Hooker's and 

 Bentham's well-known views on the spread of the Scandinavian 

 flora over much of the globe acquired increased significance in the 

 author's mind when employed by Thiselton-Dyer in his essay in 

 Darwin and Modern Science, as illustrating a general principle applic- 

 able to plant- distribution in geological time. 



Distribution, a Problem of the Northern Hemisphere. — In 

 the following pages the present author has endeavoured to give in 

 his own words the views stated by Thiselton-Dyer in this essay. In 

 the tropics, which have ever been the great home or " the area of 

 preservation " of plant-life, the continents are separated by broad 

 oceans. In the extreme north, where a permanent home has been 

 rendered impossible through climatic conditions, the continents meet. 

 With the land mostly in the north and the sea mainly in the south, 



