40 LINKS WITH THE PAST [ch. 



imperfect record that we seek to discover the relative 

 antiquity of the several groups or genera of living 

 plants, and in the structure of extinct types we 

 endeavour to discover connecting links between 

 divisions of the plant kingdom which in the course 

 of evolution have retained little or no signs of a 

 common descent. 



Sir Joseph Hooker in a letter to Darwin in 1859 

 speaks of his 'conviction that we have not in a 

 fossilised condition a fraction of the plants that have 

 existed, and that not a fraction of those we have are 

 recognisable specifically '(12). Considering the nature 

 of the palaeontological documents the wonder is how 

 much they have taught us, and we may look with 

 confidence to the results of future research in a field 

 of which tlie importance has only recently been 

 appreciated. With the strata of sedimentary origin 

 are frequently associated igneous rocks, and in many 

 continental regions, as in the majority of oceanic 

 islands, the crust of the earth consists wholly of 

 volcanic material or of rocks produced by the gradual 

 solidification of molten magmas. Rocks composed 

 mainly of carbonate of lime, such as limestones and 

 chalk, bear witness to ocean beds or to sediments 

 deposited on the floors of inland seas beyond the 

 reach of land detritus where coral reefs were reared 

 or the shells and other calcareous skeletons of 

 animals supplied the material for future land. In 



