28 LINKS WITH THE PAST [ch. 



formed a continuous forest : he attributes the present 

 discontinuous distribution, in part at least, to the 

 effects of a warmer succeeding a colder climate. The 

 less favourable conditions drove the vegetation of 

 the lowlands to seek more congenial habitats at 

 higher altitudes. In this connexion it is interesting 

 to find that in Algeria the cedar is confined to the 

 higher ground where the snow lies long in the 

 spring(2i). 



The Tulip Tree of North America and Central 

 China afibrds one of many examples of existing 

 flowering plants which illustrate the close connexion 

 between present distribution and past history. The 

 genus Liriodendron, often cultivated in the south 

 of England, is now represented by two species, the 

 best known of which — the Tulip Tree, Liriodendron 

 tidipera — extends from Vermont to Florida and 

 westwards to Lake Michigan and Arkansas. The 

 leaves . bear a superficial resemblance to those of 

 the Sycamore, but are as a rule easily distinguished 

 by the truncated form of the apex ; the specific name 

 was suggested by the tulip-like form of the flowers. 

 Fossil leaves of Liriodendron are not uncommon in 

 the Cretaceous rocks of Disco Island in latitude 70° N., 

 where they occur Avith other flowering plants which 

 bear striking testimony to the mildness of the Cre- 

 taceous climate in high northern latitudes. One 

 of the associated flowering plants is a species of 



