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 Hepaticse 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 



