INTRODUCTION. lix 
local conditions. Some of the species of a genus having formally flat leaves are 
remarkable for their great likeness to a cypress, a lycopod, or a Salicornia. Develop- 
ment in this direction is exemplified in some of the New-Zealand species of Veronica. 
Another remarkable illustration of the almost universal dispersion of northern types is 
offered by the composition of the bulk of the vegetation of the central desert-region of 
Australia, where the Chenopodiacee number upwards of a hundred species, including 
thirty of Atriplex, fifteen of Kochia, twelve of Chenopodium, and seven of Salicornia, 
and associated with these are the European Sueda maritima and Salsola kali, and 
several endemic genera, mostly of one or very few species, except Rhagodia (12 species), 
which differs chiefly from Chenopodium in having a fleshy fruit. 
It has been shown (pp. xix-xxii) how few natural orders are unrepresented in any 
one of the large areas under consideration; and ninety-five or nearly half of the 
natural orders are represented in the Sandwich Islands. So far as we know, the 
African region is poorer than either the South-American or the Indian region, but 
further explorations may reveal the existence of several of the natural orders hitherto 
undiscovered. Those of relatively restricted areas are almost all small, and consist 
of one or few genera and few species, as may be seen on referring to the table on 
page xi; and most of them are more definitely characterized than some of the larger, 
generally-dispersed orders. Indeed, it is the absence, through destruction, dying 
out, or some other cause, of connecting links, that gives some at least of these small 
groups the status of natural orders. 
Turning to the groups intermediate in rank between orders and genera, a consider- 
able number of which are regarded by some botanists as distinct natural orders, we can 
better appreciate and estimate the amount of differentiation in development in different 
areas. As an illustration, a few of the more important in the thalamifloral Polypetale 
are noted. The Fouquieriese, Fremontiew, Limnanthee, Clusiee, Rhizobolez, Marc- 
graviee, Malpighiex, Gaudichaudiee, Cuspariee, and Luxembergiew are examples of 
distinct tribes or suborders restricted to America; and the Dilleniew, Dombeyee, 
Aurantiexw, and Phytocrenee are peculiar to the Old World. On the other hand, the 
Zanthoxylee is one out of many tribes that are generally spread in warm countries. 
The Lardizabalex are divided between Peru and Chili and Eastern Asia, from North 
India to Japan; the Hermanniee are African, with three or four representatives in 
Mexico, and the Colletieze are Andine and Australasian. 
Similar illustrations of the distribution of plants might be indefinitely multiplied. It 
is clear, as Sir Joseph Hooker suggests, that a classification of plants by Linneus or 
Jussieu would have been essentially the same had it been based entirely upon Chinese, 
Australian, South-American, or Mexican instead of mainly upon European plants. 
Whether the plants (and animals) of the earth had a common northern origin, as 
supposed by some writers, and the highly differentiated southern forms are descendants 
of northern ancestors which have undergone their great differentiation in the south, are 
h 2 
