urope.

The distribution of the non-European genera of Hepaticae is as follows:--

CHASMATOCOLIA. South America and Ireland.

ACROBOLBUS. A small genus found only in New Zealand and the adjacent
islands, besides Ireland.

{368} PETALOPHYLLUM. A small genus confined to Australia and New
Zealand in the southern hemisphere, Algeria, and Ireland in the
northern. We have also one of the Hepaticae--_Mastigophora
Woodsii_--found in Ireland and the Himalayas, but unknown in any part
of continental Europe. The genus is most developed in New Zealand.

These are certainly very interesting facts, but they are by no means so
exceptional in this group of plants as to throw any doubt upon their
accuracy. The Atlantic islands present very similar phenomena in the
_Rhamphidium purpuratum_, whose nearest allies are in the West Indies and
South America; and in three species of Sciaromium, whose only allies are in
New Zealand, Tasmania, and the Andes of Bogota. An analogous and equally
curious fact is the occurrence in the Drontheim mountains in Central
Norway, of a little group of four or five peculiar species of mosses of the
genus Mnium, which are found nowhere else; although the genus extends over
Europe, India, and the southern hemisphere, but always represented by a
very few wide-ranging species except in this one mountain group![85]

Such facts show us the wonderful delicacy of the balance of conditions
which determine the existence of particular species in any locality. The
spores of mosses and Hepaticae are so minute that they must be continually
carried through the air to great distances, and we can hardly doubt that,
so far as its powers of diffusion are concerned, any species which fruits
freely might soon spread itself over the whole world. That they do not do
so must depend on peculiarities of habit and constitution, which fit the
different species for restricted stations and special climatic conditions;
and according as the adaptation is more general, or the degree of
specialisation extreme, species will have wide or restricted ranges.
Although their fossil remains have been rarely detected, we can hardly
doubt that mosses have as high an antiquity as ferns or Lycopods; and
coupling this antiquity with their great powers of dispersal we may
understand how many of the genera have come to occupy a number of detached
areas scattered over the whole earth, but {369} always such as afford the
p