344 
ISLAND LIFE. 
[part II. 
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 Ilepaticse 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 con- 
stitution, 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 
always such as afford the peculiar 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. 
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 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 
