222 AMERICA : GEOGRAPHICAL DISTRIBUTION 



the whole question to which he had himself contributed so much 

 exact material from every quarter of the globe. It had been 

 his earliest as it was his most constant interest. Like Darwin, 

 he saw that the secret of Distribution must throw light on the 

 origin of existing plants and animals ; it was reciprocally 

 true that a valid theory of the Origin must reflect light on the 

 dark places of Distribution. 



When Humboldt developed Linnaeus' analysis of the 

 habitats of plants into a richly furnished Botanical Geography, 

 and further strengthened this with numerical data, he founded 

 the science of Geographical Distribution, of which Forbes 

 was to be the reformer and Darwin ' its latest and greatest 

 lawgiver ' ; but he, no more than his predecessors who noted 

 the most obvious instance of Distribution in the succession of 

 plants in ascending the mountains, could realise its full bearing. 

 It was left for later investigators to show ' that the parallelism 

 between the floras of mountains and latitudes was the result 

 of community of descent of the plants composing the floras, 

 brought about by physical causes.' Not otherwise could the 

 existence of representative types, so utterly perplexing to 

 earlier naturalists, be explained. 



Historically it had first to be shown (and this was Lyell's 

 work) that our continents and oceans had experienced great 

 changes of surface and climate since the introduction of the 

 existing assemblages of plants and animals ; that there had 

 been a glacial period, as betokened by the Arctic survivals 

 and fossils found in N. temperate lands ; and, long before, a 

 warm Arctic period, attested by the abundant fossils brought 

 back by Arctic travellers, of plants belonging to a warm 

 temperate zone. 



Stirred by results so largely due to himself and Asa Gray 

 (though these, to be sure, were barely touched upon in the 

 address) new research and new interpretations had pushed the 

 history still further. As Forbes had traced successive plant 

 immigrations into the British Isles, so Blytt traced in the 

 Norwegian peat bogs the succession of five different waves of 

 plants following wet or dry periods. And as for the world- 

 tides of migration so clearly worked out in the Northern hemi- 



