INTRODUCTION. XXxlil 
geographical distribution of plants, in which he deals with both present and past con- 
ditions, more especially in relation to the theory of a general southward migration. 
He groups the Floras of the world into northern, tropical, and southern, and enters 
into particulars of the characters, relationships, connections, and dissimilarities of 
these groups. Drude groups his primary divisions in the same manner *; and this is 
perhaps the most philosophical method of dealing with them, though, on account of 
the greater differentiation the southern Floras present, it is preferable to consider 
them separately—that is to say to give the Australian, African, and South-American 
regions the same rank as the great northern region, and treat their tropical and 
temperate parts as subregions of so many regions, rather than subregions of a south 
temperate and a tropical zone respectively. Too close an adherence to climatal 
primary regions leads to unnatural combinations, as has been pointed out in regard to 
Drude’s Indian region. But before entering more fully into the limits of the primary 
botanical regions, Wallace’s zoological regions will be briefly discussed in relation to 
the distribution of plants. 
CoMPARISON OF THE ZOOLOGICAL WITH THE BOTANICAL REGIONS. 
Wallace based his zoological divisions (as tabulated, page xxviii) on the present 
distribution of mammals, having, after years of study and research, arrived at the con- 
viction that this class furnished the best foundation for the purpose. He also found 
that the distribution of birds and other groups harmonized sufficiently with such a 
division, and any anomalies or divergences in their distribution were capable of 
explanation by a study of the exceptional means of dispersal and conditions of 
existence. Still he is careful to emphasize the fact that any system of division must 
necessarily be more or less arbitrary and artificial, and not equally applicable to all 
classes of animals. Such mammals as the bats, which fly, and the oceanic mammals, 
which swim, possess exceptional means of dispersal, and therefore they are not taken 
into consideration. These exceptions and many other phenomena of distribution in 
the animal kingdom are paralleled in the vegetable kingdom, but no intelligible system 
of botanical division could be based on the distribution of any one group of plants 
less comprehensive than the Phanerogamia, because the large orders, such as the Com- 
posites or Leguminose, comprise plants of every size, habit, and duration, inhabiting 
every kind of situation. As an illustration of the wide dispersal of a very large pro- 
portion of the natural orders of plants, the reader may be referred back to the 
analysis of the flora of British India, pp. xv-xxii. Generally speaking, the smaller a 
Flora the larger the proportions of orders and genera to the total number of species t. 
* Petermann’s ‘ Geographische Mittheilungen,’ Ergiinzungsheft lxxiv. p. 43. 
+ The indigenous Phanerogamic Flora of the Bermudas, for example, is estimated at 120 species belonging 
to ninety-eight genera and forty-eight orders. See Botany of the ‘ Challenger’ Expedition, 1. p. 8. 
BIOL. CENTR.-AMER., Bot. Vol. I., October 1888. e 
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