OF THE TONGA OR FRIENDLY ISLANDS. 209 
of other species of wide range which very probably owe their 
presence to human agency, direct or indirect. Out of a total of 
290 species, 105, or more than a third, are confined to Polynesia ; 
and on examination it will be found that a large number of these 
species are peculiar to the Fiji, Navigator, and Tonga groups 
of islands, fewer extending eastward to the Society Islands, or 
westward to the New Hebrides. Considering the area and 
elevation of the islands and the extent of cultivation in bygone 
times, the number of indigenous vascular plants still existing 
is considerable; and, as in most insular floras, the number 
of natural orders and genera represented is large in proportion 
to the number of species. Out of the 202 natural orders 
recognized in Bentham and Hooker’s ‘ Genera Plantarum,’ 79 
are represented by apparently indigenous species in the Tonga 
Islands; and the 290 species comprise no fewer than 202 genera. 
From calculations I have published elsewhere *, based upon the 
numbers of the ‘Genera Plantarum,’ the proportions for the 
whole world are: 37°50 genera to an order, and 12°65 species to 
a genus. In the Tonga Islands the proportions are: 2°55 
genera to an order, and 1°43 species to a genus. The proportion 
of species to genera is about the same as in the remote insular 
flora of St. Helena, calculated from the probably indigenous 
species. It may be interesting to repeat here the figures for 
some distant dissimilar and larger areas :— 
Average number Average number 
of Genera to of Species to 
an Order. a Genus. 
India ...... cece eee cece eee 13°0 6:0 
MexiC0... oo... cece eee eee ees 11:0 64 
America, north of Mexico... 9°6 6:2 
Australia... .........c0ccee eee eee 8:7 6°4 
In further illustration of the general distribution of natural 
orders, it may be added that 174 are represented in British India, 
163 in Mexico and Central America, and 144 in Australia. 
The object of giving the foregoing figures is to demonstrate 
the composite character of the Polynesian flora; to demon- 
strate that natural orders have, with very few exceptions, a wide 
distribution ; to demonstrate that there must have been general 
migrations ; to demonstrate that the vegetation of the world must 
be the evolution of the same elements throughout, and not of dif- 
ferent elements in different parts of the world. In illustration of 
* ‘Biologia Centrali-Americana, Botany,’ vol. i. Introduction, p. xxiii, 
