INTRODUCTION. XXVil 
constitute a prominent part of the vegetation, and specially characterize the dry districts. 
Such highly characteristic Mexican orders as the Cactacee, Labiate (Salvia), Amaryl- 
lidaceee (Agave and Furcrwa), Aroidee (Anthurium, Philodendron), and Liliacez 
(Yucca and Dasylirion) are as numerous in species as they are prominent in the 
scenery 
Turning to the Australian flora we find almost complete agreement between the 
numerical strength of the natural orders in species, and their degrees of domination 
in the composition of the vegetation. The Leguminose stand at the head with 
upwards of a thousand species, including the highly characteristic phyllodineous 
Acacias, numbering three hundred species, and prominently pervading the whole 
country. Next come the Myrtacee, to which belong Eucalyptus (120 species), 
Melaleuca (100 species), Verticordia, Calycothriz,and Darwinia, with thirty-five species 
each; followed by the Proteacee (Grevillea, Hakea, Banksia), Composite (Olearia, 
Helichrysum), Cyperacee, Graminez, Orchidew, Epacridee, Euphorbiacez, Goodeniacee, 
and Rutacez. . 
Similar conditions obtain in the Cape Flora, and a person possessing a fair knowledge 
of plants can, from statistics alone, form some conception of the nature and composition 
of the vegetation. 
Before leaving this part of the subject it may not be amiss to mention the fact that 
the vegetation of different districts of a country may present more striking diversities 
and much more abrupt transitions than does the whole flora of one country as compared 
with that of another. Within a few yards the whole character of the vegetation 
often changes, owing to differences in the substratum of the soil and other causes. 
Mr. Salvin was particularly impressed by this fact in Guatemala, where, in travelling, 
you often literally step from an oak-forest into a pine-forest, with the carpet and canopy 
(epiphytes, &c.) of vegetation equally as different in character as are the pines and 
oaks themselves. 
Tre Primary BoranicaL REGIONS OF THE WORLD CONSIDERED IN THEIR 
RELATIONS TO THE: ZOOLOGICAL REGIONS. 
Very various are the divisions and subdivisions of the world proposed by different 
botanists and zoologists who have written on the geographical distribution of plants 
and animals; but the comparisons instituted here will be with the zoological regions 
originally defined by Dr. Sclater *, and subsequently adopted, with slight modifications, 
by other eminent zoologists, notably by Mr. Wallace in his very elaborate treatise on 
the present distribution of animals, more especially of the mammals. For convenience, 
his table of regions t+ and subregions is reproduced here, as it is more intelligible than 
* Journ. Linn. Soc., Zool. vol. ii. 
+ The Geographical Distribution of Animals, 1876, i. p. 81. 
d 2 
