GEOLOGICAL AND GEOGRAPHICAL DISTRIBUTION 537 



revealed by the geological record. There are then to be determined 

 the factors which influence the distribution at the present time. 



Climate. First in importance in the distribution of plants is the 

 climate. The temperature and moisture of any district are of pri- 

 mary importance in determining the character of its flora. 



Migrations. The spreading of plants from one place to another 

 is dependent largely upon suitable land communication. Regions 

 separated by large bodies of water, high mountains, or extensive 

 deserts, differ much more from each other than do those where no 

 such barriers exist. Mountains, on the other hand, are important 

 highways for the migration of plants from high latitudes to lower 

 ones. Finally, plants are provided with special means of transport 

 through the agency of air and water, or by the aid of animals. 



Changes in Climate 



It is evident from a study of 'the Palaeozoic flora that it was essen- 

 tially the same all over the world, the same forms being found, for 

 example, in Africa, Australia, Europe, and North America. During 

 this period the predominant plants were Pteridophytes, although 

 some Gymnosperms were also present. The character of the Ferns, 

 mostly allied to the living Marattiacese, which are tropical, indicates 

 that the climate of the Carboniferous era was a warm and humid 

 one, such as now prevails only in the Tropics. 



The similarity in the flora over the surface of the earth continued 

 through the early Mesozoic ; but there are indications that the tem- 

 perature was beginning to lower somewhat, although the greater 

 number of plants of this period belong to types which are now either 

 tropical or subtropical, this being noticeably the case with the charac- 

 teristic Cycads. 



In the Cretaceous many living genera appear, and from that time 

 the number rapidly increases, this being especially the case with the 

 Dicotyledons. Among the Cretaceous genera may be mentioned 

 Cycas, Sequoia, Betula, Quercus, Juglans, Platanus, Liriodendron, 

 Acer, Eucalyptus, and numerous others. Some of these are still 

 widely distributed ; others, like Sequoia and Eucalyptus, have become 

 much more restricted in their range; but during the Cretaceous they 

 were all cosmopolitan, indicating a still uniform, probably subtropi- 

 cal, climate. 



Toward the end of the Cretaceous and the beginning of the Terti- 

 ary, certain of the types which now live in low latitudes began to 

 disappear from the more northern formations, indicating a cooling of 

 the northern regions, and the beginning of the differentiation of the 

 different climates with their more and more divergent floras. 



