CHAP. XVI THE BRITISH ISLES 36^ 



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 geograph- 

 ical 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 Avhere 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 



^ The foUowiug remarks by Dr. Richard Spruce, who has made a special 

 study of mosses and especially of hepaticte, are of interest. ' ' From what 

 precedes, I conclude that no existing agency is capable of transporting the 

 germs of our hepatics of tropical type from the torrid zone to Britain, and 

 I venture to suppose that their existence at Killarney dates from the remote 

 period when the vegetation of the whole northern hemisphere partook of a 

 trojjical character. If I am challenged to account for their survival 

 through the last glacial period, I reply that, granting even the existence of 

 a universal ice-cap down to the latitude of 40° in America and 50"' in Europe, 

 it is not to be assumed that the whole extent, even of land, was perennially 

 entombed ' in thrilling legions of thick-ribbed ice.' Towards the southern 

 margin of the ice the climate was probably very similar to that of Greenland 

 and the northern part of Xorway at the present day. The summer sun would 

 have great power, and on the borders of sheltered fjords the frozen snow 

 would disappear completely, if only for a very short period, and I ask only 

 for a month or two, not doubting the capacity of our hepatics to survive in 

 a dormant state under the snow for at least ten months in the year. I have 

 gathered mosses in the Pyrenees where the snow had barely left them on 

 August -^nd ; by September 25th they were re-covered with snow, and would 

 not be again uncovered tiil the following year. The mosses of Killarney 

 might even enjoy a longer summer than this ; for the gulf-stream laves botli 

 sides of the south-western angle of Ireland, and its te])id waters would exert 

 great melting power on the ice-bound coast, preventing at the same time 

 any formation of ice in the sea itself" This passage is the conclusion of a 

 very interesting discussion on the distribution of hepaticre in a paper on 

 "A New Hepatic from Killarney," in the Journal of Botany, vol. 25, 

 (Feb. 1887), pp. 33 — 82, in which many curious facts are given as to the 

 habits and distribution of these curious and beautiful little plants. 



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