nstance, where it appears around the hot-springs in the Azores,
although it is neither an inhabitant of the Canaries nor Madeira.
Doubtless its microscopic sporules are everywhere present, ready to
germinate on any spot where they can enjoy throughout the year the
proper quantity of warmth, moisture, light, and other conditions
essential to the species.

Almost every lichen brought home from the southern hemisphere by the
antarctic expedition under Sir James Ross, amounting to no less than 200
species, was ascertained to be also an inhabitant of the northern
hemisphere, and almost all of them European.

_Agency of rivers and currents._--In considering, in the next place, the
instrumentality of the aqueous agents of dispersion, I cannot do better
than cite the words of one of our ablest botanical writers. "The
mountain stream or torrent," observes Keith, "washes down to the valley
the seeds which may accidentally fall into it, or which it may happen to
sweep from its banks when it suddenly overflows them. The broad and
majestic river, winding along the extensive plain, and traversing the
continents of the world, conveys to the distance of many hundreds of
miles the seeds that may have vegetated at its source. Thus the southern
shores of the Baltic are visited by seeds which grew in the interior of
Germany, and the western shores of the Atlantic by seeds that have been
generated in the interior of America."[855] Fruits, moreover, indigenous
to America and the West Indies, such as that of the _Mimosa scandens_,
the cashewnut and others, have been known to be drifted across the
Atlantic by the Gulf stream, on the western coasts of Europe, in such a
state that they might have vegetated had the climate and soil been
favourable. Among these the _Guilandina Bonduc_, a leguminous plant, is
particularly mentioned, as having been raised from a seed found on the
west coast of Ireland.[856]

Sir Hans Sloane states, that several kinds of beans cast ashore on the
Orkney Isles, and Ireland, but none of which appear to have naturalized
themselves, are derived from trees which grow in the West Indies, and
many of them in Jamaica. He conjectures that they might have been
conveyed by rivers into the sea, and then by the Gulf stream to greater
distances, in the same manner as the sea-weed called _Lenticula marina_,
or Sargasso, which grows on the rocks about Jamaica, is known to be
"carried by the winds and current towards the coast of Fl