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20 THE WORLD OF LIFE CHAP.
tributed than those whose seeds are not winged. These
facts certainly prove that the dispersal of seeds by wind or
by birds has been brought about for the purpose of securing
ample means of reproduction within the area to which the
whole plant has become specially adapted, not to facilitate
its transmission to distant lands or islands which, only in
a very few cases, would be suited for its growth and full
development. Very extensive dispersal must, therefore, in
most cases be looked upon as an adventitious result of general
adaptation to the conditions in which a species exists.
De Candolle's work also treats very fully the subject of
the comparative preponderance of the various natural orders
of plants in different regions or countries. This mode of
studying plant-distribution was introduced by our greatest
English botanist, Robert Brown, and it is that most generally
used by modern botanical writers on distribution. It con-
sists in the characterisation of the vegetation of each region
or district by the proportionate abundance in species belong-
ing to the different natural orders.
This is used in many different ways. In one the
minimum number of orders whose species added together
form one-half of the whole flora are given. Thus, it was
found that in the Province of Bahia (Brazil) the 1 1
largest natural orders comprise
half the whole number of
species. In British Guiana 12 orders are required, and in
in British India 17. Coming to temperate regions, in
Japan there are 16, in Europe 10, in Sweden 9, in Iceland
and in Central Spain 8. The general result seems to be
that those regions which are very rich in their total number
of plants require a larger number of their preponderant
orders to make up half the total flora ; which implies that
they have a larger proportion of orders which are approxi-
mately equal in number of species.
Another mode of comparison is to give the names of the
first three or four, or even ten or twelve, of the orders which
have the greatest number of species. It is found, for
example, that in equatorial regions Leguminosa:: usually
come first, though sometimes Orchids are most abundant ;
in temperate regions the Composites or the Grasses ; and in
the Arctic, Grasses, followed by Cruciferae and Saxifrages.