Lviii INTRODUCTION. 
dispersal of plants than are those of animals, the divergences in distribution are no 
greater than might be expected. 
Beyond this, many natural orders most widely separated from each other in floral 
structure repeat the same modifications of their vegetative organs under similar circum- 
stances, and possess equal capabilities for adaptation to external conditions. 
The present distribution of orders, genera, and species, and all that is known of the 
past, seems to point to one original centre of creation and development, and to such 
physical conditions at different periods as permitted, or even favoured, the general 
spread of all the principal types of plants and the wide migrations of many of the same ~ 
forms or species. Assuming this to have been so, any system of regional division is 
arbitrary, and only useful in proportion to its agreement with the present distribution 
of plants, inasmuch as it is merely a foundation on which to build a knowledge ‘of 
botanical geography, not a representation or classification of the facts. An exami- 
nation of the extensions and isolated remains of extensions of the types characteristic 
of each region throws more light on the subject than can otherwise be obtained. 
Professor Huxley seems to have been so convinced of this in the Animal Kingdom 
that he even goes so far as to say that he thinks it would not be difficult to show that 
the whole surface of the globe should be primarily divided into a northern and a 
southern region in order to display best the geographical distribution of Birds and 
Mammals*. And he further remarks, in connection with his proposed four primary 
zoological regionsf (1, Arctogea; 2, Austro-Columbia; 3, Australasia; 4, New 
Zealand), that the three latter are in some respects less unlike one another than they 
are unlike the first. The same might be said of the plant-regions, taking tropical and 
South Africa and the Indian region out of Huxley’s Arctogea. 
Even the most highly specialized Floras exhibit merely a local development of species, 
of genera, or groups of genera, belonging to orders of universal dispersion, often differing 
more strikingly in their vegetative organs (roots, stems, and leaves) than in their repro- 
ductive organs (flowers and fruits) from the usual character of the order. The nature of 
the medium in which plants grow, combined with the climatal conditions, determine the 
character the development assumes, and similar phenomena in development are repeated 
in widely-sundered areas, where the prevailing physical conditions are the same or similar. 
Familiar examples of this kind of parallelism are offered by the Cactacez of Mexico 
and certain African species of the genus Euphorbia, some of which so strongly resemble 
the branching Cerei, some the spheroidal Melocacti, as to deceive any but the most 
experienced eye. Such American genera as Yucca, Agave, and Dasylirion are replaced 
in Africa by Aloe, and in Australia by Xanthorrhwa. Some of the species of a genus 
develop tuberous roots, like many of the Australian Sundews, and some thick fleshy 
stems, as South-African species of Pelargonium and Vitis, thus adapting themselves to 
* Proc. Zool. Soc. Lond. 1868, p. 313. T See ante, p. Xxxiv. 
