iv TROPICAL FLORAS 61 



It may be noted that of the 1 2 most abundant orders, 

 8 are the same in these two very widely separated parts 

 of the earth. But even this table greatly exaggerates the 

 actual difference between the two very distinct floras. 

 There are 175 natural orders in British India, and of these 

 only 20 are absent from the Mexican region. Of these 20 

 orders, 18 have less than 10 species (5 of them having only 

 i species), so that, judging from the great types of plants, 

 the difference is wonderfully small. We can therefore 

 understand Sir Joseph Hooker's view, that there are only 

 two primary geographical divisions of the vegetable kingdom, 

 a tropical and a temperate region. 



It must be remembered, however, that even when the 

 series of orders in two remote areas are nearly identical, 

 there may be a very marked difference between their floras. 

 Orders that are very abundant in one area may be very 

 scarce in the other ; and even when several orders are 

 almost equally abundant in both, the tribes and genera 

 may be so distinct in form and structure as to give a very 

 marked character to the flora in which they abound. Thus 

 the Urticaceae include not only nettles, hops, and allied 

 plants, but mulberries, figs, and bread-fruit trees. Even 

 with so much identity in the natural orders, there is often a 

 striking dissimilarity in the plants of distinct or remote areas, 

 owing to the fact that the genera are very largely different, 

 and that these often have a very distinct facies in leaf and 

 flower. Thus, though the Myrtaceae are found in hot or 

 warm countries all over the world, the Eucalypti, so abundant 

 in Australia, give to its vegetation a highly peculiar character. 

 So the Onagraceae are found in all the temperate regions, 

 yet the Fuchsias of South-temperate America are strikingly 

 different from the Willow-herbs of Europe or the CEnotheras 

 of North America ; and there are thousands of equally 

 characteristic genera in all parts of the world. 



In Mr. Hemsley's elaborate table of the General Distri- 

 bution of Vascular Plants, he gives, in Central America, the 

 number of species of each order in Nicaragua, Costa Rica, 

 and Panama respectively, these three states constituting the 

 tropical section of the whole area, and the same for six sub- 

 divisions of the rest of the area. But the numbers added 



