eculiar conditions of climate and soil best suited to them. The repeated
changes of temperature and other climatic conditions, which, as we have
seen, occurred through all the later geological epochs, combined with those
slower changes caused by geographical mutations, must have greatly affected
the distribution of such ubiquitous yet delicately organised plants as
mosses. Throughout countless ages they must have been in a constant state
of comparatively rapid migration, driven to and fro by every physical and
organic change, often subject to modification of structure or habit, but
always seizing upon every available spot in which they could even
temporarily maintain themselves.[86]

Here then we have a group in which there is no question of the means of
dispersal; and where the difficulties that present themselves are not how
the species reached the remote localities in which they are now found, but
rather why they have not established themselves in {370} many other
stations which, so far as we can judge, seem equally suitable to them. Yet
it is a curious fact, that the phenomena of distribution actually presented
by this group do not essentially differ from those presented by the higher
flowering plants which have apparently far less diffusive power, as we
shall find when we come to treat of the floras of oceanic islands; and we
believe that the explanation of this is, that the life of _species_, and
especially of _genera_, is often so prolonged as to extend over whole
cycles of such terrestrial mutations as we have just referred to; and that
thus the majority of plants are afforded means of dispersal which are
usually sufficient to carry them into all suitable localities on the globe.
Hence it follows that their actual existence in such localities depends
mainly upon vigour of constitution and adaptation to conditions just as it
does in the case of the lower and more rapidly diffused groups, and only
partially on superior facilities for diffusion. This important principle
will be used further on to afford a solution of some of the most difficult
problems in the distribution of plant life.[87]

_Concluding Remarks on the Peculiarities of the British Fauna and
Flora._--The facts, now I believe for the first time brought together,
respecting the peculiarities of the British fauna and flora, are sufficient
to show that there is considerable scope for the study of geographical
distribution even in so apparently unpromising