GEOLOGICAL AND GEOGRAPHICAL DISTRIBUTION 229 



which had preceded it. While the comparative scarcity 

 of Monocotyledons has been explained by the assump- 

 tion that they are a later development than the Dicoty- 

 ledons, a more plausible explanation is probably that the 

 small number of types in which the tissues were firm 

 enough to have been preserved, accounts for their 

 scarcity in a fossil state. 



The geological history of the Dicotyledons shows, as 

 might have been expected, that the earlier types are all 

 Choripetalse largely the primitive Amentacere, willows, 

 poplars, etc. These may have been preceded by herba- 

 ceous forms like the living Piperinese, but of these no 

 traces have been found. The SympetalcC, which are 

 the most specialized and numerous group at present, 

 do not appear until much later, and the fossil record, 

 so far as it goes, is quite in accord with the conclu- 

 sions based upon comparative morphology. 



GEOGRAPHICAL DISTRIBUTION 



In considering the distribution of terrestrial plants, as 

 they at present exist, many factors must be taken into 

 account. First of all, we must examine the original 

 distribution of the ancestors of the existing flora, as 

 revealed to us by the study of fossil forms. There are 

 next to be considered the factors affecting the distri- 

 bution of these forms as they are found at the present 

 time. The most obvious of these factors are climate, 

 i.e. temperature and rainfall ; direction of mountains 

 and distribution of arid districts ; currents of air and 

 water ; animals, including man. 



The distribution of plants during the Tertiary period, 



