P aragraph 4.—Diffusion of plants by water.
I t is certain, that plants have been transferred from one
country to another, by means of currents in the ocean, and by
rivers; and it is probable, that water is one principal medium
of the dispersion of seeds. I t is well known, that plants of
the sea-coast are among those which are most extensively
spread in different regions.
The seeds of these plants are of such a character, that they
are not easily destroyed in the waters of the ocean, and of
those which appear less likely to be thus preserved, it probably
happens occasionally that a few escape out of the
great number which are exposed to the causes of injury. I t
is easy to collect facts of this description, many which
were known to Linnaeus, and are recorded in the Amsenitates
Academics* The seeds of the cassia fistula, anacardium
o ccidental, mimosa seandens, dolichos urens, guilandina
bonduc, and several other plants of Jamaica and the equinoctial
countries in America are occasionally collected in
the Hebrides, whither they are conveyed' by the .Gulf-
stream.+ Numerous instances of a similar descr^iontmay
be collected in the works of botanists and scientific travellers.;};
Streams of fresh water descending from Alpine regions bring
with them the seeds of plants, and it is common to find a
similar flora along the banks of rivers, and even the plants
of mountains occasionally reappearing where the waters have
deposited them.§ o Mr. Lyell cites a fact remarked by Keith
and others, that the southern shores of the Baltic are visited
by seeds which belong to the interior o f Germany, and
the western shores of the Atlantie by seeds which have been
generated in the interior of America, *§
* Amasn. Academic*. De Coloniis Plantarum.
+ Pennant’s Voyage to the New Hebrides, 1772. p. 23. Sloane in Philos.
Transactions. No. 223. p. 398. M. de Humboldt’s Travels, i. p. 59.
+ Amaen. Acad. De Telluris habitabilis incremento.
§ De Candolle. Diet, des Sc. Nat.
The greatest difficulty presented by the diffusion of plants,
when we adhere to the-theory of their propagation, and reject
that of equivocal production and of the spontaneous generation
of the same speeies in distant places, arises from those
not very uncommon instances in which aquatic plants occur
in the rivers and marshes of countries remote from each
other. Rudolphrisa^s^, that he possessed specimens of po-
tamogeton natans collected in St. Domingo., in nothing differ?
ent from that?pf Europe. The Nymphaea Lotus, supposed
formerly to have been peculiar to Egypt and India, grows also
Wildenowvremarked, that zanniehellia palus-
tris ,‘lemna^ minor, and polyrrhiza, and several other water-
plants, are indigenous both in Europe and in North Amp.-
rica. ■ ’
I t must be remarked, that many ofv these plants are very
generally and almost Tiniv^fsally spread, and their discovery
in #wo ffistantpjlaceS' is^so much the: lessiremarkable. In
pther instances we must haverecourse, in order to account for
them, to the-changes which the surfaces of various lands have
Undergone., and tor extensive inundations. We can only
thus - explain, as De Candolle observes, such facts as the discovery
.of the Aldrovanda in the basins of the Po and Of j the
Rhbne.' The same fishes and other fresh-Water animals are
also, found in. lakes_which have no communication. , The dispersion
of. these as well as -of .aquatic and even of , mountain
plants, can only be explained, by reference to inundations,
which in various times have covered extensive portions of our
continents.*
S ection TV .—Phenomena connected with the Vegetation o f
Islands, and o f opposite Coasts.
We have seen in a former Section, that the vegetation of
countries separated by wide oceans, or by great distances of
space from each other, consists in general of peculiar tribes;
and that when, in regions parallel in latitude and similar in
physical conditions, Nature reproduces analogous forms, or
* De Candolle, ubi supra.