CHAPTER XV 



DISTRIBUTION 



The differentiation theory could of itself explain distribution only 

 where a continuous land-area not affected by unstable climatic 

 conditions is concerned. As our globe presents itself, three factors 

 control and direct the operations of the differentiating agencies : 

 (1) the divergence of the land-masses from the north; (2) the secular 

 fluctuations of climate; (3) the barriers lying athwart the line of 

 march of migrating plants. 



(1) The Divergence of the Land-masses. — Although the differ- 

 entiation theory explains the diversity of plant-forms, it does not of 

 itself account for their present distribution. It might do so if this 

 were a comparatively orderly world with stable climatic conditions, 

 and if the plants had differentiated in situ over a continuous land- 

 surface. But a uniformly constituted plant- world of this descrip- 

 tion does not exist, since differentiation is intensified as one recedes 

 from the northern polar area, until in the southern lands of South 

 America, South Africa, and Australia it displays its most pronounced 

 effects. That floras become more and more dissimilar with distance 

 from the pole is the result of the continuity of the land-masses in the 

 north and their disseverment by broad oceans in the south. In 

 other words, the continuity of the floras of the north and their dis- 

 continuity in the south represent the response of the plant-world 

 to the arrangement of the great land-masses. 



(2) The Secular Fluctuations of Climate. — If the first factor 

 alone prevailed, distribution would not be such a very complex 

 matter, since we could express it as the effect of the differentiating 

 process controlled by the divergence from a common centre in the 

 north of the great land-masses of the Old and the New World. But 

 in so doing we should be ignoring a very important disturbing factor, 

 namely, the secular climatic changes. During much of geological 

 time there have been fluctuating conditions of climate which have 

 produced a series of advances and retreats to and from the north 

 polar area of the plants of the warm regions of the globe, regions 

 that have ever been the great home of plant-life. 



At a time when a genial climate prevailed over the northern or 

 land hemisphere the plants now represented in type in the warm 

 latitudes occupied the regions beyond the Arctic circle. When this 

 period gave place to cooler conditions the retreat to the south began ; 

 and the plants, as the diverging continents pulled them more and 

 more asunder, became more and more distinct from each other as a 



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