crease towards the equator, while they increase towards the
poles ; but monocotyledonous plants, in which the Ferns are
comprehended by M. De Candolle, undergo comparatively little
variation of number, and constitute every where about the
sixth part of the entire flora. If we compare by themselves
the two families of vascular plants, the monocotyledonous increase
regularly towards the pole and lessen towards the
equator. This applies chiefly to continents ; for in many islands
the proportion of monocotyledonous plants is greater
than according to the ratio of latitude, owing, as it appears,
to the abundant supply, of moisture.*
Paragraph 2. — Distribution of orders or families, and of
genera.
The application of the above-stated universal laws of proportion
in the distribution of vegetable tribes, belongs t ame
comparison of the three great classes. The'facts which relate
to families, more properly so called, or to the natural
orders of plants, are more limited ; but the distribution even
of these depends upon physical conditions, though often upon
such as are not manifest. This may be inferred frbm the
fact noticed by M. de Humboldt, that the proportional numbers
of plants belonging to particular families, in any given
region, have mutual relations. “ If we know in any country
under the temperate zone, the number of Cyperacèæ or of
Composite, it will be possible to estimate that of thé Gra-
minese or of the Leguminosae.”*f' ƒ
In many instances Nature seems to have deposited plaiits,
if we may use such an expression, upon different parts of
the earth, arranged according to their natural affinities ; particular
families having certain principal foci, wheve the plants
* De Candolle, Dictionaire d’Histoire Naturelle.
*{- Sur les loix que l ’on observe dans la distribution des formes végétales par
Alexandre de Humboldt. Dictionaire d’Hist. Naturelle__The following are the
expressions in which these remarkable inferences are conveyed in M. de Humboldt’s
Prolegomena.
u Disquisitiones istæ ex Arithmetica botanica petite leges nobis patefecerunt,
quarum imperio natura in quâvis zona subjecta est. Penitius autem cognoscentur,
quando major pars terræ a viris doctis accurate erit perlustrata. Ubicunque enim,
sub eodem parallelo, mirabilem harum legum consensum invenies : atque is est, in
that belong to them have been produced in the greatest
number of individuals and variety of species. It may be
said, that plants belonging to certain natural families are
placed in particular groupes,, each having a principal centre
around which the genera and species comprised in it are
more numerous, and display the characteristic 'form of the
order in the greatest perfection* At a distance from these
central points, the common type^pf the„order. becomes gra*
d uallfjp;ëvapsésfeen%
In some instances,: whole généra, and even entire families
of plants are confined to particular; régions; The. Hesper
riders arç peculiarj(to$India and China the Labiatiflorae belong
tOi South America.;* g the Epacrideæ' are nearly confined
to Terra Australis, where their species are vejy ,numerous a
few. being ;diétrtouted to the isles of the Pacifipj.^ In some to-
stances, a few sditory tribes, forming however distinct species.,
are-found at a' distance from their congeners, which tore
otherwise assembled to one irogion..* All the Passiflorae be***
long to Aiheripa, except one, which is in South Aftie£W s De
Candolle observes, that all the species; of Mesembryanthemum
inhabit th e Cape* of .Good Hope, except M. nodiflprum and
çppticW.; which hr# found to Corsica.and to Barbery. In
the same country .are, all the Ixiæ, except the Bulbpcodium ;
all the Gladioli, k except the Gladiolus communia found in
France ; all the Heaths, to numhejr, two or three hundred,
except five* or six fpund in JEurope ; and,nearly allth e Oxales,
'except three species which* are wild in France, and some
found in America. . . | | ||g x l
The manner in which tribes thus related to particular regions
are grouped is very r ema rk a b l e ;Mr . Brown has ob-
served, that in.Terra Australis, the foci of peculiar vegetation
quàlibet regione, nexus inter singulam tribùm et suminàm ' plantarum phane-
rogamarum, ut quofj.es Graminearum numerum cognoscas, inde probabiHtgr non
solum uniyersum numerum Phanerogamarum queas æstirpare, sed
positarum, Labiatarum et aliquarum tribuum. ’ Quippe vjdèînùs stirpium fapailias
modo ab oequatore polos versus numéro specâèrum aûgerî, ’niodo a polis versus
, æquatorem ; interdum etiam certiæ fàmilioe—v. c. Labiate, Umbelliferæ, Crudferæ—
in zona temperata numéro specierum m&xime abundant, polps et æquatorem versus
rarescunt.”—-De Distribution^ Geog. TPI. Prolegomena, p. 41.