CHAP. XVI.] 
THE BRITISH ISLES. 
345 
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 adap- 
tation 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 remarks on the Peculiarities of the British Fauna and 
Flora. — The facts, now I believe for the first time, brought to- 
gether 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 Hepaticse 
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 ? It is true that numerous species have been first dis- 
covered in Britain, and, subsequently, on the continent ; but we 
have many species which have been known for twenty, thirty, 
or forty years, some of which are not rare with us, and yet have 
