198 BOTANY. [CHAP. vu. 



pletely obliterated during the latter part of the Creta- 

 ceous period, when all the land was submerged and 

 covered by a deep sea, in the bed of which the chalk was 

 slowly deposited. The comparatively uniform warm 

 temperature of the globe, indicated by the world-wide 

 diffusion of the same species in Palgeozoic and less con- 

 spicuously in Mesozoic time, at the commencement of 

 the tertiary period merged into the modern phase of 

 graduated and often extreme temperatures, and this 

 change of climatic conditions determined a corresponding 

 change in life. During the first two periods individuals 

 predominated ; that is, the luxuriant vegetation of early 

 times consisted of numerous individuals belonging to 

 comparatively few groups, whereas at the present day 

 groups or genera predominate, the comparatively homo- 

 geneous nature of the ancient flora being broken up 

 into numerous minor sections depending on the various 

 lines of departure taken by the ancient types in their 

 endeavours to accommodate themselves to the new 

 conditions. 



At the close of the Cretaceous period great geographi- 

 cal changes took place, the bed of the Cretaceous sea 

 was gradually and irregularly elevated, and the present 

 site of England was once more dry land. During the 

 Eocene period the temperature of England, as of Europe, 

 was tropical, and the flora of our country during that 

 period has its nearest living representatives in the hotter 

 parts of India, Australia, Africa, and America. As 

 examples of the flora of that period may be mentioned 

 palms, aroids, cactuses, figs, eucalyptus, etc. All these 

 have long since disappeared from Europe, but their fossil 

 remains are abundant in eocene rocks. To this period 



