CHAP. XVI THE BRITISH ISLES 



as ferns or L3Xopods ; 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 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 



^ The following remarks by the late Dr. Richard Spruce, who made a 

 special study of mosses aud especially of Hepaticse, are of interest. "From 

 what precedes, I conclude that no existing agency is capable of transport- 

 ing 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 hemi- 

 sphere partook of a tropical 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, -w^^ perennially entombed Mn thrilling regions 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 Norway at the pre- 

 sent 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 mon th 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 2nd ; by September 25th 

 they were re-covered with snow, and would not be again uncovered till the 

 following year. The mosses of Killarney might even enjoy a longer summer 

 than this ; for the gulf-stream laves both sides of the south-western angle 

 of Ireland, and its tepid waters would exert great melting power on the ice- 

 bound const, 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 Hepaticse in a paper on '*A New Hepatic from Kil- 

 larney," in the Journal of Bofmiy, 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. , 



