II] GEOGRAPHICAL DISTRIBUTION 29 



Artocarpiis, described by Dr Nathorst as Artocarpus 

 Dicksonl which bears a close resemblance to ^1. incisa 

 the bread-fruit tree of the southern tropics of the 

 Old World. Without attempting to deal fully with 

 the past history of Liriodendron, it may be confidently 

 stated that the records of the rocks are consistent 

 with the idea of antiquity suggested by the present 

 distribution of the two surviving species. 



Islands such as Great Britain and Ireland, situated 

 a short distance from a continent, contain many plants 

 which are widely spread in different parts of the 

 world, together with a very small number peculiar 

 to the British Isles though closely allied to species 

 on the neighbouring , continent or to plants farther 

 afield. The occasional recognition of species pre- 

 viously believed to be confined to Britain tends to 

 reduce the short list of our endemic types. 



An enquiry into the origin of an island flora 

 involves a consideration of the data in regard to 

 changes in level and relative distribution of land 

 and water in the course of geological evolution. It 

 is generally agreed that at no distant date, in a 

 geological sense. Great Britain and Ireland were 

 united to the continent. There is, however, another 

 fact to reckon with, namely the prevalence of Arctic 

 conditions in northern Europe when a thick sheet of 

 ice spread over the greater part of the British Isles. 

 There can be no doubt that the severity of the climate 



