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Show 344 ISLAND LIFE. (PAR'!' 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 Hepaticre 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 b~en 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 como 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 fa~t, that the phenomena of distribution actually presented by t~1s group do. not essentially differ from those presented by the higher fiowermg plants which have apparently far less diffusive OHAP. XVI.] TilE BRITISH ISLES. 345 power, as we shall find when we como 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 arc 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. Concluding rem.adcs on the Peculiarities of the British Fauna and Plm·a.-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 a field as one of the most recent of continental islands. Looking at the general bearing of these facts, they prove, that the idea so generally entertained as to the biological identity of the British Isles with the adjacent continent is not altogether correct. Among birds we have undoubted peculiarities in at least three instances; peculiar fishes are much more numerous, and in this case the fact that the Irish species are all different from the British, and those of the Orkneys distinct from those of Scotland, renders it almost certain that the great majority of the fifteen peculiar British fishes are really peculiar and will never . be found on the European Continent. The mosses and Hepaticre also have been sufficiently collected in Europe to render it pretty certain that the more remarkable of the peculiar British forms are not found there ; why therefore, it may be well asked, should there not be a proportionate number of peculiar British insects 1 It is true that numerous species have been first discovered in Britain, and, subsequently, on the continent; but we have many species which have been known fo;. twenty, thirty, or forty years, some of which are not rare with us, and yet have |