INTRODUCTION 95 



types during the Glacial period in their present areas. 

 The land-connection being pre-Glacial, the last con- 

 dition is an inevitable conclusion or corollary of the 

 first. The same applies to the argument as to the 

 occurrence of American plants in Ireland if the 

 existence of a former land-bridge (which in Tertiary 

 times no doubt did exist) is employed to explain their 

 present occurrence in this area, following their sur- 

 vival of the Glacial epoch if indeed they are so 

 ancient. 



Both these explanations {i. e. land-bridge theories) 

 are interesting, but there is hardly as yet sufficient 

 evidence to prove them. The balance appears to be 

 in favour of accidental means analogous to the present 

 colonisation, by such accidental means or by, for 

 instance, the various agencies of dispersal of plants 

 upon volcanic islands or areas where vegetation has 

 been destroyed by eruptions. 



