The Problems of Plant Distribution 247 



which the silk-oak (Grevillea) is the best known, 

 are examples of characteristic austral families. 



Floras of the Old and New World. The main 

 elements of the north-temperate floras, of both the 

 Old and the New World, are evidently derived from 

 the Tertiary flora of the ancient northern continent, 

 and many families and genera are still common to 

 the Eurasian continent and North America. In 

 the tropics, as we have already pointed out, the dif- 

 ferences between the Old and New World are very 

 marked. Probably the two richest botanical regions 

 in the world are the Indo-Malayan region and 

 tropical South America. A comparison of these two 

 regions shows very few genera in common, and 

 there are even many families which are peculiar 

 to one region or the other. For instance, the palms 

 of the Old and New World belong almost without 

 exception to different genera, and the same is true 

 of the vast majority of the orchids and other large 

 families. The whole family of the Screw-pines is 

 confined to the Old World, and the no less marked 

 Pineapple family is peculiar to America. Where 

 there are genera common to the tropics of both 

 hemispheres, they are usually widespread ones, with 

 representatives in the temperate zones between, and 

 usually they are genera provided with very favora- 

 ble means of distribution, such as certain Compositae 

 like Vernonia and Senecio. The great differences 

 in the character of the floras of the two great trop- 

 ical regions are easily understood, since these are 



